AHIT: Heating Systems

Draft (induced)

(1) the process of using negative pressure created by a fan to pull combustion gasses through a heat exchanger; draft inducers do not place combustion gasses in a chimney or vent under positive pressure; most medium efficiency gas furnaces are induced draft appliances. (2) the process of using negative pressure created by a fan to pull combustion gasses through a vent system; this vent system may be used when the vent system for a natural draft appliance cannot be installed to operate using the stack effect; mechanical draft inducers are installed at the vent termination.

Vent Clearances

-Category I appliance vent clearances to components such as windows and sidewalls do not apply to Category III and Category IV vents. Clearances for these vents are based on ANSI Z223.1 / NFPA 54 or CAN/CSA-B149.1. You will see a version of the illustrations and charts in almost all manufacturer's instructions. -The first important thing to know about these clearances is that there are different illustrations and charts for direct vented and direct exhaust appliances, and that there are different requirements for the USA and Canada. The second thing is that different manufacturers have different clearance requirements. Finally, clearances depend on the Btu input of the appliance. -The requirement that vents and combustion air pipes should terminate at least 12 inches above grade or above the local snow accumulation height (whichever is higher) is consistent among manufacturers as is the clearance around gas meter regulators. Clearance to operable windows and doors depends on the Btu rating of the appliance and on whether the appliance is configured as a direct or direct exhaust. Clearance to non-operable windows is recommended by some manufacturers to avoid condensation on the window. Clearance to inside and outside corners and to soffits varies between manufacturers.

Vent Materials

-Category III direct vented appliances use manufacturer recommended metal pipe. The pipe often consists of two concentric pipes, one inside the other, with the vent in the center and combustion air drawn through the outer pipe. These pipes must be sealed because the vent is under positive pressure. The pipes are often made from stainless steel. -Most Category IV appliances use plastic pipe for venting and combustion air intake. Manufacturers usually recommend Schedule 40 PVC, ABS, or CPVC. The pipe should be solvent cemented using compatible cement and primer, if required. Manufacturers recommend pipe support in the 3 to 5 foot range. Some manufacturers do not recommend using cellular core PVC pipes as the vent material. -A new material for venting Category IV appliances is polypropylene. Polypropylene vents come in rigid and flexible types. Like all manufactured vent products, manufacturer's installation instructions rule. A home inspector may wish to jot down the manufacturer's information and do some research about manufacturer's instructions, although this is not required.

Combustion Air Ducts

-Combustion air ducts should usually be constructed using at least 28 gauge galvanized steel, although other equivalent materials may be used if approved. Stud and joist cavities may also be used if not more than one fireblock is removed. The minimum duct dimension is 3 inches. Flexible duct and duct board may not be used. Combustion air ducts should have only one opening. The same duct may not serve as the upper and lower duct and one duct may not open into different rooms. A screen should be installed on combustion air intake openings that terminate outside. A screen should not be installed where a combustion air intake opening is in an attic. -Remember that the screen reduces the net free opening area. Combustion air ducts should terminate at least 12 inches above grade or above the highest anticipated snow accumulation level. -Rooms or spaces where combustion air ducts from the outside terminate are unconditioned spaces. This includes ducts from attics and crawl spaces. These rooms or spaces should be treated like exterior walls; exterior walls have insulation and solid weather-stripped doors. These rooms or spaces are frequently uninsulated and the doors are not weather-stripped; thus, they allow large amounts of unconditioned air into the house. Home inspectors should consider reporting the energy efficiency penalty inherent in this configuration.

Gas Appliances, Combustion Air from Inside the House

-Combustion air from inside the house may be drawn from the same room where the appliances are located. The same room includes adjacent rooms when there is no door between the rooms. -Combustion air may also be drawn from other adjacent rooms on the same story through openings between the rooms and the room where the appliances are located. There should be two openings, one commencing within 12 inches from the floor and the other within 12 inches from the ceiling. Each opening should have at least 1 square inch of net free opening area for every 1,000 Btu/h input of the appliances in the room. -Combustion air may also be drawn from different stories. There should be one or more openings having a total combined net free opening area of at least 2 square inches for every 1,000 Btu/h input of the appliances in the room. The opening may be a louvered door or a similar opening.

Oil Appliances, Combustion Air from Inside the House

-Combustion air from inside the house may be drawn from the same room where the appliances are located. The same room includes adjacent rooms when there is no door between the rooms. Drawing combustion air from the same room requires at least 50 cubic feet of volume in the room for every 1,000 Btu/h input of the appliances in the room. -Combustion air may also be drawn from other rooms on through openings between the rooms and the room where the appliances are located. There should be two openings, one near the floor, and the other near the ceiling. Each opening should have at least 1 square inch of net free opening area for every 1,000 Btu/h input of the appliances in the room.

Oil Appliances, Combustion Air from Outside the House

-Combustion air from outside the house may be provided by two openings in the wall or by two ducts. One opening or duct should be near the floor and the other near the ceiling. The openings and ducts must communicate directly with the outdoors. Outdoors includes ventilated crawl spaces and attics. -The net free opening area for wall openings and vertical ducts is at least 1 square inch for each 4,000 Btu/h input of the appliances in the room. The net free opening area for horizontal ducts is at least 1 square inch for each 2,000 Btu/h of the appliances in the room. -Combustion air may be provided by one opening in the wall, floor, or ceiling. The net free opening area is at least 1 square inch for each 5,000 Btu/h input of the appliances in the room.

Gas Appliances, Combustion Air from Outside the House

-Combustion air from outside the house may be provided by two openings in the wall or by two ducts. One opening or duct should commence within 12 inches from the floor and the other within 12 inches from the ceiling. The openings and ducts must communicate directly with the outdoors. Out-doors includes ventilated crawl spaces and attics. -The air net free opening area for wall openings and vertical ducts is at least 1 square inch for each 4,000 Btu/h input of the appliances in the room. The net free opening area for horizontal ducts is at least 1 square inch for each 2,000 Btu/h of the appliances in the room. -Combustion air may be provided by one opening in the wall or ceiling commencing within 12 inches the ceiling. The net free opening area is at least 1 square inch for each 3,000 Btu/h input of the gas appliances in the room.

Heating System Capacity Determination

-The heating (and air conditioning) loads for a house should be determined using the protocol set forth in the Air Conditioning Contractors of America (ACCA) Manual J. The loads from Manual J are used to find the recommended system capacity in ACCA Manual S. These calculations should be performed when the house is built and when a new system is installed. These calculations are now performed using computer software. -The Manual J calculation estimates sensible heating loads for a house. Unlike air conditioning loads, the latent load is not a factor when estimating heating loads because heating performs no dehumidification. Factors that determine the sensible load for a house include geographic location, orientation (north, south, east, west), insulation amount, window type and size, and air infiltration rate. -The Manual J and Manual S protocols do not apply to hot water heat and steam heat. Capacity determination and equipment sizing for these systems should be performed using equipment manufacturer's recommendations, or accepted engineering practices.

Gas Decorative and Oil Decorative Fire-Places (VENTED)

-Vented gas fireplaces and vented oil fireplaces include appliances such as freestanding appliances, inserts for installation into masonry fireplaces, and vented logs intended for installation in factory-built and masonry wood-burning fireplaces. These appliances are not really fireplaces; the more accurate term is decorative gas (or oil) appliance. These appliances have nothing in common with solid-fuel-burning systems. Most of these appliances are gas; oil appliances are available, but are uncommon. These appliances distribute heat by radiation and convection; some use a fan to assist in heat distribution. Some appliances are vented using a Type B (gas) and some are direct vented. These appliances may be found in any house and are primarily decorative but may be used as supplemental heating. They are not intended for use as the sole heating source in a house or in a habitable room.

Gas Furnace

A fossil fuel appliance that is designed to heat air and supply it to the structure. has several safety sensors (switches). Many are not visible, but some are. The silver-colored buttons near the burners are the flame-rollout sensors. These switches shut off the furnace if they sense excess heat. The high limit (fan limit) sensor also shuts off the furnace if it detects excess heat. This switch may be contained in a small box in some furnaces, or it may be a small round button located on the burner compartment wall. The draft sensor is connected by a tube to the draft inducer fan. This switch stops the ignition sequence if the draft inducer is malfunctioning and creating inadequate draft in the heat exchanger.

Heat Source

A heat source capable of maintaining at least 68° F. should serve every habitable space in houses in almost all regions. The temperature measurement is made 3 feet above the floor and 2 feet from an exterior wall. The heat source does not need to be in the room if a heat source in another room can maintain 68° F in the room. Central heat is not required; however, portable heat sources may not provide the required heat, nor may heat sources that are not designed to provide comfort heating. Examples of appliances that may not be the required heat source include unvented room heaters and most vented gas-burning and oil-burning fireplaces.

Masonry Chimney Used as a Category I Vent

A masonry chimney is frequently used in a Category I gas appliance vent system. This occurs in one of two ways. The chimney can serve as the vent itself, or the chimney can serve as an enclosure (a chase) for a vent such as a Type B vent or a chimney liner. In both cases, the chimney should be sealed below the place where the vent or chimney liner leaves the chimney, and any fireplace should also be blocked or labeled to make it clear that it may not be used.

Reportable Gas Flame Defect

A yellow or orange colored flame; defects include an improperly adjusted burner or gas valve, or inadequate combustion air.

Combustion Air

Air necessary for complete combustion of a fuel including theoretical air and excess air. -All fuel-burning appliances require combustion air to ensure complete and safe fuel burning, to ensure that the combustion byproducts are safely expelled through the chimney or vent and out from the house, and to help cool the appliance. Combustion air is often subdivided into different types depending on its function. The names for these types vary depending on whom one asks. -Combustion air should be drawn from outside of the house, including ventilated attics and crawl spaces. Combustion air (usually) may be drawn from inside the house; however, this is not best practice, and may not be allowed in new construction and in houses that have been air sealed.

Heat Distribution Method

Another way to classify heating systems. Each distribution method relies on one or more of the heat transfer methods previously discussed. -The most common distribution method in many markets is the forced-air distribution system. These systems use a fan to move heated air through ducts. Forced-air distribution systems rely mostly on convection to distribute heat. Convection can be a problematic heat distribution method because of uneven distribution of heated air (it tends to rise near the ceiling), and because movement of air over the skin (even heated air) evaporates moisture from the skin and creates a cooling effect.

Category I

Category I appliance vent gas is hot (around 300° F.) and rises through the vent by being less dense and more buoyant than the surrounding air (stack effect). Category I gas appliance vents include the Type B vent, masonry chimneys, and single-wall vents. Common examples of Category I appliances include low and medium efficiency furnaces and boilers, and draft hood-equipped water heaters. -Some direct vented gas appliances are Category I appliances. Common examples include some decorative gas fireplaces. These appliances must use manufacturer-recommended vent components, and may not be connected to vent systems for other Category I appliances. Clearance between these manufacturer-recommended vents and combustible materials may be different as well. Clearance between the top side of these vents and combustible materials can be 2 or 3 inches.

Category II

Category II appliance vent gas is rather cool (around 140° F.) and rises through the vent by being less dense and more buoyant than the surrounding air (stack effect). The problem is that it is difficult to get cooler vent gasses to rise in a natural draft chimney or a vent without either backdrafting or cooling to the point that condensation occurs in the vent. No examples of Category II appliances were known to exist as of the publication date of this book.

Category III

Category III appliance vent gas is as hot or is hotter than Category I vent gas. Category III vents are under positive pressure. Some direct vented gas appliances are Category III appliances. Some demand (tankless) water heaters are in this category. Category III appliances must use manufacturer-recommended vent components, and may not be connected to vent systems for Category I appliances. Category III appliances are not considered condensing appliances; however, condensation can be a problem. Condensate collection and disposal systems are often required in cold climates, and may be required in some vent configurations (e. g., multiple elbows and vertical vent installations).

Category IV

Category IV appliance vent gas is rather cool (around 140° F.). Category IV appliance heat exchangers are very efficient (90+ %) at drawing heat from the combustion products. The resulting vent gas is so cool that it must be exhausted under positive pressure, and uses plastic pipe, usually PVC, as the vent. Common examples of Category IV appliances include high efficiency, condensing furnaces and high efficiency water heaters (both storage and demand types). -Category IV appliances are called condensing appliances because some of the water vapor produced by the combustion process is condensed into liquid in the furnace instead of going up the vent. As discussed in the Cooling System chapter, heat is released when matter changes state from gas to liquid. Heat extraction by condensation is why condensate collection and disposal in the furnace is an issue for Category IV appliances. -Condensation in the vent system is also an issue for all Category IV appliances. Some manufacturers recommend that the vent slope back toward the appliance so that condensate will flow into the collection system. Other manufacturers recommend sloping horizontal vents toward the discharge point to let the condensate drain through the wall cap. -Manufacturers often recommend insulating Category IV vents that run through uninsulated spaces in cold climates (design temperature below 32° F). This is to help prevent the condensate from freezing and blocking the vent.

Solid Fuel, Combustion Air

Combustion air from outside should be provided for solid fuel-burning appliances; however, it is uncommon to find combustion air from the outside for masonry fireplaces, for older factory-built fireplaces, and for wood stoves. -Home inspectors should consider how to report this lack of combustion air from outside. Reporting the lack of combustion air from outside and recommending an upgrade to current standards is one option for older houses. Reporting the lack of combustion air from the outside as a defect is another option for older houses; however, this may be problematic if there is no evidence of harm caused by lack of combustion air. -Reporting lack of combustion air from outside as a defect may be appropriate for newer houses because this is recommended by current accepted standards. -Combustion air for factory-built solid fuel-burning appliances such as fireplaces and wood stoves should be installed according to manufacturer's instructions. Home inspectors may consider recommending evaluation based on manufacturer's instructions when the only source of combustion air is from inside the house. -A masonry fireplace should have a combustion air duct that terminates outside or in a ventilated crawl space. The duct exterior termination should be at or below the firebox. The duct interior termination may be on the back or sides of the firebox or within 24 inches of the firebox opening. The duct should have an area of at least 6 square inches and not more than 55 square inches. The duct should be closable.

Standard Method

Drawing combustion air from the same room is called the and requires at least 50 cubic feet of volume in the room for every 1,000 Btu/h input of the appliances in the room.

Floor Furnace Components

Floor furnaces are simple devices consisting of a cabinet covered by a grille, a burner and controls, a heat exchanger, and a vent. They are controlled by a thermostat located on a wall. Thermostat location is important with floor furnaces. The thermostat should be located in the same room as the floor furnace grille or in an adjacent room with no door between the rooms. Areas further away from the furnace will be cool if the thermostat is located too close to the furnace grille. Areas near the furnace are often too hot regardless of the thermostat location. -Gas floor furnaces are Category I furnaces. Most are low efficiency draft hood equipped units and such units are considered obsolete by current standards.

Floor Furnace Installation

Floor furnaces should be installed according to manufacturer's instructions. These instructions may be different from the general guidelines discussed here. Instructions are rarely available for old units, but may be available for newer replacement units. Home inspectors should consider recommending evaluation based on manufacturer's instructions for installations that are significantly different from the general guidelines discussed here. Installations that do not follow these guidelines are not necessarily deficient. -Floor furnaces get very hot while operating. Clearances to combustible materials are important, as is providing safe walking paths around the furnace grille. Occupant burns from walking on hot grilles are not uncommon, especially for children. -The furnace grille should be at least 6 inches away from walls. The grille should be at least 12 inches from materials such as drapes and curtains, and from a door when open in any position. The grille should be at least 5 feet below projecting combustible materials like wood shelves. If the furnace is located in a corner, the distance to at least one wall should be at least 18 inches. Floor furnaces should not be installed in bedrooms and bathrooms. -Floor furnaces should not project down into habitable space. The bottom of a floor furnace that projects into a crawl space should be at least 6 inches above the crawl space floor. A pit may be dug under the furnace to provide this space. There should also be at least 6 inches between the draft hood and combustible materials such as wood floor joists. -Cuts to floor joists should be framed like any other opening in the floor joists. A single or double header between adjacent joists should be installed depending on the width of the cut space. Refer to the floor system section in the Structural Components chapter for more about openings in floor joists. -The vent and vent connector for gas floor furnaces should be Type B or an appropriate chimney. Single-wall vent connectors may not be used in crawl spaces. Refer to the Category I gas vent systems section for more about installing these vents and vent connectors. The vent and vent connector for an oil floor furnace should be a Type L or an appropriate chimney.

Prohibited Gas Appliance and Oil Appliance Locations

Gas-burning and oil-burning appliances should not be located in a bedroom, bathroom, closet, or in a space that opens only into these spaces. Several exceptions exist. This does not apply to direct vented appliances that obtain all combustion air from outside. This does not apply to a space with a solid door that is weather-stripped and all combustion air is obtained from outside.

Gas Appliance Categories

Gas-burning appliances, including gas forced-air furnaces, are classified as Category I - IV appliances. The classifications are based on vent gas temperature, and on whether the vent is under positive or natural pressure. Most furnaces and boilers are either Category I or Category IV appliances. Inspection procedures and typical defects vary somewhat based on the type of furnace being inspected.

Heat Exchanger

Heart of the furnace. The gas fire heats the interior of the heat exchanger, making the heat exchanger walls very hot. The fan pushes air across the exterior of the heat exchanger. The air absorbs heat and continues across the air conditioner evaporator coil (if any) into the ducts. The evaporator coil is, of course, not operating when the furnace is on. The combustion gasses remain inside the heat exchanger and are expelled out through the vent. -Home inspectors should inspect the heat exchanger for cracks and other damage, if possible. The heat exchanger is rarely visible in an induced draft furnace. About 20 percent of the heat exchanger may be visible in a draft hood furnace. Home inspectors should use an inspection mirror and flashlight to inspect a visible heat exchanger. Home inspectors should report whether or not the heat exchanger was visible and inspected.

Serpentine Heat Exchanger

Heat exchangers in newer furnaces have many curves that increase the heat exchanger surface area. These heat exchangers are sometimes called serpentine heat exchangers because the curves look like a snake. The air flowing across this increased surface area absorbs more heat; this is one way that newer furnaces achieve their higher efficiency. The serpentine structure is why a draft inducer fan is required to pull the combustion gasses through the curves.

Service Access Inspection

Heating systems are subject to the same service access requirements as cooling systems. Refer to the service access inspection section of the Cooling System chapter for more information.

Determining Heating System Capacity

Home inspectors are not required to determine or confirm if the capacity of the heating system is appropriate; however, home inspectors should know some basic information about how heating system capacity is determined in order to understand how to interpret the results of a heating system inspection, and how to identify systems that may be significantly oversized or undersized.

Types of Category I Furnaces

Home inspectors may encounter two types of Category I gas furnaces. The draft hood type has not been produced since the 1970s. These are low efficiency furnaces (about 65%) and are usually beyond the end of their expected service life. These furnaces often have three to six ribbon burners. The gas is burned along the length of each ribbon burner. The induced draft type is the most common Category I furnace. These furnaces are considered medium efficiency (about 80%). These furnaces often have two to six inshot burners. The gas is pulled into the heat exchanger from one burner opening in each inshot burner.

Inspecting Category I Furnaces

Inspecting a Category I furnace begins with observing the physical condition of the furnace, the fuel and electrical connections, and the vent system to verify that the system is not shut down and appears safe and prudent to operate. The cabinet should be well-supported and substantially free from rust and debris. All cabinet components, such as the access covers, should be present and intact. There should be no scorching or staining that indicates possible flame rollout. The pilot light (older systems) should be lit and steady. Internal wiring should be intact and undamaged. Automatic safety controls should not be bypassed and should be in their factory installed position. Bypassed controls are usually visible as wires spliced together with wire nuts. These controls include the high (fan) limit switch, the flame rollout (limit, spill) switches, and the draft pressure switch. The electrical cables, thermostat cables, and fuel connections should be properly installed and undamaged. Electrical cables and thermostat cables should enter the cabinet through a bushing, grommet, or similar protection and should be installed inside any covered box provided by the manufacturer. The vent system should be intact and complete from the appliance to the visible termination point.

Inspecting Category IV Furnaces

Inspecting a Category IV furnace is similar to inspecting an induced draft Category I furnace. Most inspection techniques and typical defects apply to both types. The primary differences involve the condensate collection and vent systems of Category IV furnaces. Refer to special gas vent systems section for more about Category IV vent systems. -The condensate collection system in a Category IV furnace usually consists of one or two tubes that collect the condensate and deposit in a collection tank which is connected to the primary evaporator coil condensate drain. -These tubes should not be crimped and should be sloped to drain to the collection tank. The tank and discharge pipe should be sloped to drain into the condensate drain. The condensate from a Category IV furnace is usually acidic, so disposal should not be where the condensate could cause damage.

Inspecting Floor Furnaces

Inspecting a floor furnace begins with observing the physical condition of the furnace, the fuel connections, and the vent system to verify that the system is not shut down and appears safe and prudent to operate. It is not uncommon to find that a floor furnace has been abandoned when forced-air heating is installed. The cabinet should be supported by framing and the interior should be substantially free from rust and debris. Debris in floor furnaces is common. Internal wiring should be intact and undamaged. The vent system should be intact and complete from the appliance to the visible termination point. Inspect the appliance for black soot. This could indicate inadequate combustion air or flame rollout. Inspect the pilot light (if any and if on). It should be steady and large enough to touch the thermocouple. -If the home inspector elects to operate the furnace, it should be activated using the thermostat. Home inspectors should attempt to listen to the ignition sequence. The ignition sequence for a gas floor furnace consists of a click when the gas valve opens, and burner ignition. Ignition sequence failure or successful ignition followed by shut down and repeated ignition are reportable defects. -Home inspectors should observe the burners and the gas flame, if possible. A properly burning gas flame is blue, sometimes with a slight yellow tip. A yellow or orange colored flame is a reportable defect that could indicate problems such as an improperly adjusted burner or gas valve or inadequate combustion air. The flame should be strong and steady.

Inspecting Gas Fireplaces and Oil Fireplaces (Vented)

Inspecting a vented gas fireplace or a vented oil fireplace begins with observing the physical condition of the unit and the fuel connections to verify that the appliance is not shut down and appears safe and prudent to operate. These appliances are frequently shut down. Home inspectors are not required to, and usually should not, operate any appliance that requires manual ignition using an open flame. -Inspect the logs, glass, and the vent for black soot. This could indicate improper log placement or inadequate combustion air. Inspect the pilot light (if any and if on). It should be steady and large enough to touch the thermocouple. -If the home inspector elects to operate the unit, it should be activated using the normal operating controls. Ignition sequence failure or successful ignition followed by shut down and repeated ignition are reportable defects. -Home inspectors should observe the burner and the gas flame. A properly burning gas flame in a vented gas fireplace is blue. A yellow or orange colored flame is a reportable defect could indicate problems such as an improperly adjusted burner or gas valve or inadequate combustion air. The flame should be steady, but not too high or strong; it should look natural.

Inspecting wall Furnaces

Inspecting a wall furnace begins with observing the physical condition of the furnace, the fuel connections, and the vent system to verify that the system is not shut down and appears safe and prudent to operate. The cabinet should be substantially free from rust and debris. Internal wiring should be intact and undamaged. The vent system should be intact and complete from the appliance to the visible termination point. Inspect the appliance for black soot. This could indicate inadequate combustion air or flame rollout. Inspect the pilot light (if any and if on). It should be steady and large enough to touch the thermocouple. -If the home inspector elects to operate the furnace, it should be activated using the thermostat. Home inspectors should attempt to listen to the ignition sequence. The ignition sequence for a gas wall furnace consists of a click when the gas valve opens and burner ignition. Ignition sequence failure or successful ignition followed by shut down and repeated ignition are reportable defects. -Home inspectors should observe the burners and the gas flame, if possible. A properly burning gas flame is blue, sometimes with a slight yellow tip. A yellow or orange colored flame is a reportable defect that could indicate problems such as an improperly adjusted burner or gas valve or inadequate combustion air. The flame should be strong and steady.

Inspecting Unvented Appliances

Inspecting an unvented appliance begins with observing the physical condition of the appliance and the fuel connections to verify that the appliance is not shut down and appears safe and prudent to operate. Home inspectors are not required to, and usually should not, operate any appliance that requires manual ignition using an open flame. -Inspect the appliance for black soot. This often indicates inadequate combustion air or an improperly adjusted flame. Inspect the pilot light (if any and if on). It should be steady and large enough to touch the thermocouple. -If the home inspector elects to operate the appliance, it should be activated using the normal operating controls. Ignition sequence failure or successful ignition followed by shut down and repeated ignition are reportable defects. -Home inspectors should observe the burner and the gas flame, if possible. A properly burning gas flame in an unvented appliance heater is blue. A yellow or orange colored flame is a reportable defect could indicate problems such as an improperly adjusted burner or gas valve or inadequate combustion air. The flame should be strong and steady.

Makeup Air

Makeup air replaces air removed from the house by appliances, such as clothes dryers, kitchen exhaust hoods, and bathroom exhaust fans. These exhaust appliances compete for air with fuel-burning appliances. This is especially true if the exhaust appliance and the fuel-burning appliance are in the same space. This competition can cause incomplete combustion and backdrafting of chimneys and vents. Home inspectors should look for evidence of backdrafting when competing appliances are in the same space and should consider testing for backdrafting by operating both appliances simultaneously. -The term makeup air is frequently used when combustion air should be used. Makeup air and combustion air are different.

Combustion

Many heating systems burn a hydrocarbon fuel to produce heat. Gas systems burn natural gas (methane) or propane. Oil systems burn heating oil (#2 fuel oil), although some small oil-burning appliances burn kerosene. Solid fuel systems burn wood, wood pellets, coal, and sometimes materials such as corn cobs (biomass). -All combustion produces byproducts. The byproducts are usually gasses, but solids and liquids are also possible depending on the fuel being burned, the conditions of combustion, and the conditions in the chimney or vent. Home inspectors should have a basic understanding of combustion to help them identify and interpret possible defects observed during an inspection.

Typical Defects, Category I Vent Systems

Many of these defects can cause backdrafting which can allow combustion products to enter the house. Typical defects that home inspectors should report include: 1.Vent made with or insulated with asbestos: asbestos vents are common in some areas in houses built before the 1970s. The asbestos material is often called by the brand name Transite. The asbestos material can be a dull light gray or light brown with a rough appearance. Similar material can be found with oil vents and with chimneys. 2.Inadequate clearance to combustibles: a Type B vent should have at least 1 inch clearance; a single-wall pipe should have at least 6 inches clearance; combustible materials include insulation, electrical cables, and HVAC ducts not made from sheet metal; this is a fire hazard. 3.Inadequate distance to a vertical sidewalls or to gravity air intake openings: a Type B vent should terminate at least 8 feet from a vertical sidewall or at least 2 feet above any obstruction within 10 feet; a single-wall pipe should terminate at least 2 feet above any obstruction within 10 feet; these clearances include gravity air intake openings such as windows and eave ventilation openings; improper termination can cause backdrafting during certain wind conditions. 4.Inadequate distance to a mechanical air intake opening: all gas vents should terminate at least 10 feet horizontally or 3 feet vertically above a mechanical air intake opening; mechanical air intake openings include ventilation air openings serving the HVAC system, an energy recovery ventilation or a heat recovery ventilation system. 5.Disconnected or inadequately connected vent components: sections of vent pipes should be connected using at least three screws or the manufacturer's locking connection system; this includes the connection at a draft hood or flue collar; this is a fire hazard and an air quality (carbon monoxide) hazard. 6.Vent pipe sections or fittings turned the wrong way, or the bottom section inserted over the upper section: this can cause the vent to become disconnected and can allow escape of vent gasses. 7.Rusted, stained, or damaged vent components: this usually indicates a venting problem (such as condensation in the vent) or a water infiltration problem. 8.Rust, debris, staining seen around the draft hood or flue collar: this usually indicates a venting problem (such as condensation in the vent or backdrafting) or a water infiltration problem. 9.Type B vents do not terminate with a listed cap: field constructed caps are not acceptable. 10.Inadequate Type B vent height above roof: a Type B vent should extend far enough above the roof to avoid air currents that might blow combustion gasses back down the vent and into the house; the height depends on the roof slope and ranges from 1 to 4 feet for common roof slopes; the vent should extend far enough above the roof to avoid snow closure. 11.Vent extends too far above roof or along a sidewall: a vent should extend only far enough to satisfy the height above roof requirement; additional height could cause the combustion gasses to cool and backdraft into the house; this is more of an issue in cold climates. 12.Vent system too short or too tall: the vent system should extend at least 5 feet above the flue collar or draft hood, at least 12 feet above the bottom of a gas wall furnace, and should not extend more than 50 feet; vent systems outside these parameters should be designed by an engineer. 13.Vent system too small or too large for the connected appliance: verifying vent system capacity is out of scope of a home inspection; however, vent capacity is a typical defect especially for common vent systems and vent systems that use a chimney as the vent; vents that are too small may allow backdrafting; vents that are too large may allow the combustion gasses to cool and condense in the vent, this may allow backdrafting and condensation that can damage the vent. 14.Vent connector slopes down toward appliance: vent connectors should slope up to-ward the vent at least ½ inch per foot; this can cause backdrafting. 15.Improper vent system material: a Type B or Type L vent may be used as a vent and vent connector, where approved; single-wall galvanized steel pipe that is at least 28 gauge may be used as a vent and vent connector; use of other vent systems material is a fire hazard and a backdrafting hazard. 16.Flexible Type B vent connector improperly installed: flexible Type B vent connectors should be installed per manufacturer's instructions; in general, they may not pass through partitions (e.g, walls, floors, ceilings), may not be installed in attics and crawl spaces, may not be bent more than 90°, may not be cut or modified, and may not be compressed more than 4 inches per foot. 17.Single-wall pipe used where not approved: single-wall pipe may not be used in attics, crawl spaces, and in garages in cold climates; single-wall pipe may not penetrate floors and ceilings, but may penetrate directly through the roof or through a side-wall if clearance to combustibles (at least 6 inches) is maintained; many gas appliance manufacturers do not recommend using single-wall pipe as a vent; unapproved use of single-wall pipe is a fire hazard and a backdrafting hazard. 18.Vent roof or sidewall penetration not properly flashed: a vent roof penetration should be flashed with a metal thimble or roof jack, and the thimble or roof jack should be protected by a storm collar; a sidewall penetration should be flashed with a metal thimble; lack of proper flashing is a water infiltration hazard. 19.Single appliance vent connector too long or vent system has too many elbows: a and May Have Too Many 90° Elbows. general rule states that further evaluation may be prudent if the vent connector length is equal to or greater than the vent system height or if the vent system contains elbows that total more than 180°; vents with these defects may allow the combustion gasses to cool and condense in the vent, this may allow backdrafting and condensation can damage the vent. 20.Common vent connector too long or has too many elbows: the length of a common vent connector (in feet) should not be longer than I½ times the common vent connector diameter; for example, a 4 inch diameter common vent connector should not be longer than 6 feet. If elbows are installed in a common vent connector, the elbows should total more than 180°. These situations are allowed, but the common vent connector size may need to be increased. An improperly installed common vent system may allow the combustion gasses to cool and condense in the vent or vent connector, this may allow backdrafting and condensation can damage the vent system. 21.Common vent has an offset (elbows): if a common vent contains elbows, the vent and vent connector size may need to be increased; an improperly installed common vent system may allow the combustion gasses to cool and condense in the vent, this may allow backdrafting and condensation that can damage the vent system. 22.Smaller diameter common vent connector connected below the larger diameter common vent connector: the smaller diameter vent connector should be connected above the larger diameter vent connector at a common vent because the common vent may be too large if the smaller appliances is the only appliance operating; heat from the larger appliance may help increase the vent temperature. Connecting the smaller vent connector higher in the space provides the maximum vertical rise for the smaller appliance vent. Greater vertical rise provides better natural draft. It usually does not matter which vent connector is installed above the other if the vent connectors are the same diameter. 23.Two or more appliances connected to a vent connector: this configuration is called a common vent manifold; this is allowed, but with restrictions; home inspectors may wish to consider recommending evaluation, especially if evidence of backdrafting is observed. 24.Common vent connectors enter the vent at the same level and at a 90° angle: vent connectors may enter the common vent at the same level, but they should enter at a 45° angle. 25.Common vented appliances located on different stories: this is allowed but determining if the vent system is properly installed usually requires additional calculations. 26.Unused chimney openings not sealed: this allows air to enter the chimney; this air can cool and dilute the combustion gasses, and can cause backdrafting and condensation in the chimney; sealing includes the chimney flue under the vent connector. 27.A chimney improperly used as the vent: refer to the discussion above about use of a masonry chimney as a vent. 28.Vent connector improperly inserted into a chimney: the vent connector should be inserted no closer to the opposite side of the chimney than the vent connector diameter, for example, a 3 inch diameter vent connector should be no closer than 3 inches to the opposite side of the chimney. 29.Vent connector loose or not sealed where inserted into a chimney: the vent connector should be secured and sealed where it enters the chimney; failure to do so could allow the connector to work loose, or could allow combustion gasses to enter the house.

Energy Source

One way to classify heating systems. -Some heating systems use solid fuels such as coal, wood and waste products (biomass) such as corn cobs. Most coal-burning systems have been replaced with cleaner, more efficient systems. Wood-burning and biomass systems are common as supplemental heating, and may be the sole heat source in some houses. Liquid fuel systems use heating oil and sometimes kerosene. These systems are common in some markets, such as the Northeastern United States, and are uncommon in others. Gas fuel systems use natural gas or propane. These systems can be found in most markets and are the predominant system in some markets. Electric resistance heating systems use electricity, and are basically giant toasters that heat wires. Some electric systems are heat pumps. See the

Typical Defects, Vented Room Heaters

Refer to the gas wall furnaces and oil wall furnaces section.

Vented Room Heater Components

Room heaters are simple devices consisting of a cabinet, a burner and controls, a heat exchanger, and a vent. They may be controlled by a thermostat located on a wall, or they may be controlled by a thermostat in the unit.

Vented room Heater Installation

Room heaters should be installed according to manufacturer's instructions. These instructions may be different from the general guidelines discussed here. Instructions are rarely available for old units, but should be available for newer units. Home inspectors should consider recommending evaluation based on manufacturer's instructions for installations that are significantly different from the general guidelines discussed here. Installations that do not follow these guidelines are not necessarily deficient. -Room heaters get very hot while operating. Clearance to combustible materials is important. Occupant burns from touching these units are not uncommon, especially for children. Room heater side clearances vary between about 6 to 12 inches; rear clearances vary between about 4 to 48 inches, top clearances vary between about 4 to 48 inches; front clearance is usually about 60 inches. -An exception exists for room heaters regarding the gas shut off valve. The shut off valve may be located in another room or space if the valve is permanently labeled, readily accessible, and serves no other appliance. -The vent for most gas room heaters should be a Type B. Refer to the Category I gas vent systems section for more about installing these vents. Gas room heaters may use a chimney as the vent if approved by the manufacturer. The vent for a gas and oil direct vented wall furnace should be the vent supplied by the manufacturer.

Unvented Appliance Components

Several types of unvented appliances that use a fixed fuel source are common. Cabinet appliances consist of a cabinet, a burner, and controls. Some appliances have no temperature control and are either on or off. Some appliances have components that looks like logs. These logs may be installed in an open firebox that looks like a factory-built fireplace but has no vent, or they may be installed in a factory-built or a masonry fireplace that was originally designed to burn wood. Some appliances look like a vented decorative gas fireplace, but without a vent.

Heating System

The system should be capable of maintaining 68° F. in every habitable space. Remember that spaces such as bathrooms, laundry rooms, and closets are not habitable spaces, so a heat supply source is not required in these spaces.

Category I Gas Vent Systems

The vent system for a Category I gas appliance may consist of three components: •manufactured vent pipe or a chimney, •vent cap, and •vent connector. The vent is usually a chimney, a Type B vent, or occasionally a single-wall pipe. A vent cap or listed roof assembly is required where a Type B vent or a single-wall pipe terminates above the roof. A vent cap is not required if a chimney serves as the vent; however, a rain cap is recommended for all chimneys. A vent connector is necessary if the appliance is not located directly under the vent, which is usually the case. The vent connector runs between the appliance and the vent. The vent connector may be a Type B vent or a single-wall pipe (with restrictions). It is important to distinguish between the vent connector and the vent. These are different components and are governed by different rules. -Three types of manufactured vents and vent connectors may be used in a Category I vent system. A Type B vent may be used in most vent and vent connector applications. Use of single-wall pipes is limited because these pipes lose heat more rapidly than Type B vents, and do not conduct the combustion gasses out of the vent system as effectively. Use of single-wall pipes is mostly confined to vent connectors in warm climates. A Type L vent (for oil-burning appliances) may be used as a vent or a vent connector, but these are more expensive so using these in a gas vent system is unusual. -A Category I vent system may serve a single appliance or it may serve more than one gas appliance. A vent system that serves two or more gas appliances is called a common vent system.

Restrictions when using Category I Gas Appliance Vent

There are several restrictions when using a chimney as a Category I gas appliance vent. These restrictions account for the fact that combustion gasses from gas appliances are relatively cool compared to gasses from solid-fuel burning fireplaces and compared to gasses from oil-burning appliances. A chimney designed for hot gasses may not produce enough draft to expel the cooler gasses generated by gas appliances. The cooler gasses may condense in the chimney producing acidic liquid that can severely damage the chimney. The cooler gasses may fall back down the chimney causing backdrafting. -Restrictions on using a masonry chimney as a Category I gas appliance vent include the following. •The chimney must be enclosed within the house until it penetrates the roof. The chimney may not be exposed to the outside on even one side until the chimney penetrates the roof. •The chimney may not serve as a vent for one induced draft furnace. The chimney may serve as a common vent for multiple induced draft or draft hood-equipped gas appliances. The chimney may serve as a vent for a single draft hood-equipped appliance such as a water heater. •The chimney area should be at least as large as the vent connector area, and smaller than seven times the vent connector area when the chimney serves as the vent for one draft hood-equipped appliance.

Typical Defects, Floor Furnaces

Typical defects that home inspectors should report include: 1.Furnace has been abandoned: this should be reported. 2.Cabinet or internal components are deteriorated: many of these furnaces are old and are subject to rust and normal age related deterioration. 3.Debris in furnace: debris can include dirt, animal hair, and objects; debris can be a fire hazard. 4.Improper venting: the vent connector lateral may be too long because the nearest vent is a long distance from the furnace; draft hood, vent connector, or vent may be loose or deteriorated; single-wall vent connector (not allowed in a crawl space) is installed. 5.Improper cuts in floor framing: floor joist cuts should be framed like other floor system openings. 6.Inadequate combustion air: combustion air may be drawn from inside the house or from the crawl space; refer to the combustion air section for more information. 7.soot in and around furnace: including around vent termination. 8.Inadequate service clearances: there should be at least 18 inches clearance on the service side. 9.Inadequate clearances to combustible materials: clearances are required above and below the unit; clearances are required to the draft hood and to the vent.

Typical Defects, wall Furnaces

Typical defects that home inspectors should report include: 1.Furnace has been abandoned: this should be reported. 2.Cabinet or internal components are deteriorated: many of these furnaces are old and are subject to rust and normal age related deterioration. 3.Debris in furnace: debris can include dirt, animal hair, and objects; debris can be a fire hazard. 4.Improper venting: wrong vent type is used; refer to the Category I gas vent systems section for more information. 5.Inadequate combustion air: except for direct vented units, combustion air is usually drawn from inside the house; refer to the combustion air section for more information. 6.soot in and around furnace: including at vent termination. 7.Inadequate clearances to combustible materials: clearances are required above, to the side, and sometime below the unit.

Typical Defects, Category I and Category IV Furnaces

Typical defects that home inspectors should report include: 1.Return duct connected directly to furnace/air handler cabinet without a plenum: this can cause uneven air flow, especially over the evaporator coil if one is present; some manufacturers allow this, so recommend evaluation using manufacturer's instructions. 2.Dirty fan (blower): this could reduce the effectiveness and service life of the fan. 3.Fan loose, shakes, or rattles when operating: this causes unnecessary noise and could reduce the service life of the fan. 4.Noisy fan: this could indicate a damaged or worn bearing and could indicate that the fan is near the end of its service life; in older furnaces with a belt-driven fan this could indicate a loose or worn drive belt. 5.Inoperative or bypassed fan cover interlock switch: this usually indicates that the switch is defective; this safety device should not be bypassed. 6.Bypassed safety controls: this will usually present as wires connected with wire nuts where wire nuts are not usually seen. 7.Internal parts located where damage is likely: wires and tubes inside the burner and fan compartments should not be located where they could be melted or where they could be scraped by moving parts; the 120 volt power cable should not run through the burner compartment; it should enter the cabinet and terminate in the box provided by the manufacturer; all wires and cables should enter the cabinet through a grommet or bushing. 8.Cabinet is damaged or rusted: rust indicates a water intrusion problem; the cause of the damage or rust should be determined and repaired. 9.Air leaking from cabinet: air leaks reduce system efficiency; the cabinet should be reasonably well sealed. 10.Ignition source not elevated in a garage: the ignition source (burners) should be elevated at least 18 inches above the floor for appliances located in a garage. 11.Appliances installed in a garage are not protected from vehicle impact: appliances such as furnaces and water heaters should be protected by framing or by a vehicle stop such as a bollard. 12.Cabinet does not have adequate service access: refer to service access inspection in the Cooling System chapter for the discussion of service access and clearances. 13.Cabinet does not have adequate clearance to combustibles: clearances to combustibles depends on manufacturer's instructions which are often listed on the furnace label.

Typical Defects, Combustion Air

Typical defects that home inspectors should report include: 1.blocked combustion air openings, especially by insulation in the attic and crawl space, 2.combustion air ducts made from unapproved materials, 3.one combustion air duct contains multiple openings, 4.fuel-burning appliances installed in an unapproved location, 5.appliance closets or compartments not sealed and insulated when combustion air is drawn from outside, 6.combustion air duct openings do not terminate within 12 inches from the floor or ceiling (gas appliances), 7.exhaust appliances compete with fuel-burning appliances for air.

Typical Defects, Gas Fireplaces and Oil Fireplaces (Vented)

Typical defects that home inspectors should report include: 1.inadequate combustion air, 2.inadequate clearance to combustible materials, 3.improper flame color or pattern, 4.soot in or around appliance, including at vent termination.

Typical Defects, Unvented Appliances

Typical defects that home inspectors should report include: 1.inadequate combustion air, 2.inadequate clearance to combustible materials, 3.improper flame color or pattern, 4.soot in or around appliance, including at vent termination.

Unvented Appliances Installation

Unvented appliances should be installed according to manufacturer's instructions. These instructions may be different from the general guidelines discussed here. Instructions are rarely available for old appliances, but should be available for newer appliances. Home inspectors should consider recommending evaluation based on manufacturer's instructions for installations that are significantly different from the general guidelines discussed here. Installations that do not follow these guidelines are not necessarily deficient. -Unvented appliances get very hot while operating. Clearances to combustible materials are important. Side clearances for cabinet appliances vary between about 6 to 12 inches; top clearance is often about 36 inches; bottom clearance to carpet is often about 2 inches; front clearance is often about 36 inches. -Clearances for unvented log-type appliances installed in open fireboxes vary significantly based on the manufacturer and on the individual unit. Side clearance to walls is often about 16 inches. -Clearance to combustible side trim is often about 6 inches. Top clearance to the ceiling is often about 42 inches. A heat-resistant material, such as slate or marble, should be installed for about 12 inches vertically above the firebox opening. Top clearance to a mantel depends on the depth of the mantel. A 6 inch deep mantel, for example, may need between about 14 - 20 inches between the top of the firebox opening and the bottom of the mantel. Hearth extension depth is usually about 14 inches. -Unvented appliances should not be used in a room unless the room has adequate volume. Room volume includes other spaces that do not have doors between the room and other spaces. Permanent openings between rooms and other spaces may be used if allowed by manufacturer's instructions. The room should have at least 1 cubic foot of volume for every 20 Btu/h input of the unit. Unvented appliances may not be used in bedrooms and bathrooms, and may not be used in other areas if prohibited by manufacturer's instructions. -There are two exceptions to the rule about not installing gas appliances that draw combustion air from inside the house in bedrooms and bathrooms. One wall-mounted unvented appliance may be installed in a bathroom if the appliance Btu/h input is 6,000 or less, and if the bathroom is large enough to provide combustion air. One wall-mounted unvented appliance may be installed in a bedroom if the appliance Btu/h input is 10,000 or less, and if the bedroom is large enough to provide combustion air. -An exception exists for unvented appliances regarding the gas shut off valve. The shut off valve may be located in another room or space if the valve is permanently labeled, readily accessible, and serves no other appliance.

Unvented Appliances

Unvented gas appliances and oil appliances include room heaters, fireplace logs, and fireplaces. They are similar appliances that differ mostly in appearance. They share common characteristics. Most burn gas; a few burn oil. They are designed to burn fuel in a manner that does not produce harmful combustion gasses; however, they still produce combustion gasses that are expelled into the house. -The use of fuel-burning unvented appliances is controversial. Some discourage use of these appliances based on air quality concerns. All fuel-fired appliances emit carbon dioxide and water vapor as combustion byproducts. These byproducts remain in the house. -These appliances can produce carbon monoxide if improperly adjusted or if there is insufficient combustion air, although this risk is supposed to be mitigated by the mandatory presence of an oxygen-depletion safety system. -Some discourage use of these appliances based on fire concerns. Government estimates are that 25,000 fires each year and 300 deaths result from the use of these appliances. Some state and local governments do not permit new installation of these appliances. -Unvented appliances usually should not be used in houses built using recent energy codes. Recent energy codes restrict the amount of air that infiltrates into the house through the small cracks and joints between components. Lack of air while operating an unvented appliance could create air quality problems and could violate manufacturer's instructions. -Unvented appliances may be found in any house. Unvented logs and fireplaces are often decorative. Room heaters are often used as supplemental heating and for heating additions. Unvented appliances may not be used as the only heat source in a house or as the only heat source in a habitable space. The presence of unvented appliances, especially room heaters, could indicate a problem with the heating system or with heat distribution in rooms. The home inspector should consider recommending evaluation of the heating system if the unvented appliance appears to be installed for other than decorative purposes. -Unvented appliances may also be called space heaters and vent-free heaters. Unvented appliances may be supplied by a fixed fuel source or may be portable and have an internal fuel source. Portable appliances of any type are out of scope for a home inspection. Home inspectors should report the presence of portable unvented appliances and disclaim inspection. -Some unvented appliances distribute heat by radiation and convection; some use a fan to assist in heat distribution. Unvented appliances usually serve one room. These appliances may not be connected to ducts or registers to serve adjoining rooms.

Inspecting Vented Room Heaters

Use the same procedures as inspecting a wall furnace.

Special Gas Vent Systems

Vent systems for Category III and Category IV gas appliances use manufacturer specified materials and installation instructions. This makes inspecting these vent systems difficult because each manufacturer has different instructions and because some appliances can be installed in different vent and combustion air configurations.

Direct Vent Pipe Terminations

Vent terminations for Category III and Category IV vents can vary significantly among manufacturers in terms of how the vent terminates, and the distances between vent pipes and combustion air pipes. Some general rules include the following. •The vent and combustion air pipes should terminate in the same pressure zone. •Pipes terminating on the roof should terminate on the same roof section. •Sidewall terminating pipes should terminate on the same sidewall. •There are minimum and maximum vertical and horizontal separation distances between the vent pipes and combustion air pipes. •There are separation requirements when two or more direct vented appliances are terminated in the same pressure zone. •There are usually minimum and maximum pipe lengths.

Gas Fireplace Components and Oil Fireplace Components (Vented)

Vented gas fireplaces and oil fireplaces are simple devices consisting of a cabinet, a burner and controls, and a vent. Vented gas logs use the existing fireplace as the combustion chamber and the vent. These units have no thermostats and are either on or off.

Gas Fireplace Installation and Oil Fireplace Installation (Vented)

Vented gas fireplaces and oil fireplaces should be installed according to manufacturer's instructions. These instructions may be different from the general guidelines discussed here. -Instructions are rarely available for old appliances, but should be available for newer appliances. Home inspectors should consider recommending evaluation based on manufacturer's instructions for installations that are significantly different from the general guidelines discussed here. -Installations that do not follow these guidelines are not necessarily deficient. Gas fireplaces and oil fireplaces get very hot while operating. -Clearances to combustible materials are important. Side clearances to trim and sidewalls vary between about 3 to 6 inches; top clearances to mantels vary between about 8 to 16 inches; hearth extensions in front vary between 0 to 10 inches. -An exception exists for gas fireplaces regarding the gas shut off valve. The shut off valve may be located in another room or space if the valve is permanently labeled, readily accessible, and serves no other appliance. A Type B vent should be used with gas fireplaces listed for use with a Type B vent. Refer to the Category I gas vent systems section for more about installing these vents. Appliances listed for Type B venting may not use a chimney as the vent, but the chimney may be used as a chase for the Type B vent. The vent for a direct vented gas fireplace and a direct vented oil fireplace should be the vent supplied or approved by the fireplace manufacturer; these fireplaces may not use the chimney as a vent. Vented gas logs may use the existing chimney. -The chimney damper should be permanently opened when vented gas logs are installed in a factory-built chimney or a masonry chimney. This may be accomplished with a damper stop. This is a clamp installed on the damper that props the damper open. The damper stop, by itself, may not provide a large enough vent opening. A typical recommended opening area for a masonry fireplace damper varies between about 19 to 40 square inches. A typical recommended opening area for a factory-built fireplace damper varies between about 8 to 20 square inches. -Vented gas fireplace and vented oil fireplace vent terminations are sometimes surrounded by a decorative metal cover called a shroud. Shrouds should be tested for use with the specific appliance vent system, and should be approved by the appliance manufacturer. This includes all gas and oil appliance vents such as furnaces and water heaters. It is difficult for a home inspector to know if a shroud is approved for use with an appliance. Home inspectors may wish to consider recommending evaluation using manufacturer's instructions for shrouds that might inhibit exhausting of combustion gasses.

Wall Furnace Components

Wall furnaces are simple devices consisting of a cabinet, a burner and controls, a heat exchanger, and a vent. They are controlled by a thermostat located on a wall. Thermostat location is important with wall furnaces. Areas further away from the furnace will be cool if the thermostat is located too close to the furnace. Areas near the furnace are often too hot regardless of the thermostat location.

Wall Furnace Installation

Wall furnaces should be installed according to manufacturer's instructions. These instructions may be different from the general guidelines discussed here. Instructions are rarely available for old units, but should be available for newer units. Home inspectors should consider recommending evaluation based on manufacturer's instructions for installations that are significantly different from the general guidelines discussed here. Installations that do not follow these guidelines are not necessarily deficient. -Wall furnaces get very hot while operating. Clearance to combustible materials is important. Occupant burns from touching these units are not uncommon, especially for children. The wall furnace should be at least 12 inches from a door when open in any position, at least 4 to 6 inches from wall corners, and at least 12 to 18 inches from the ceiling or other vertical projection above the furnace. Many units recommend a metal plate or tile between the unit and carpet, and may recommend some physical distance between the unit and carpet. -The vent for most gas wall furnaces should be a Type BW while the vent is run inside a wall cavity. A Type BW vent is a Type B vent with an oval shape. Refer to the Category I gas vent systems section for more about installing these vents. Wall furnaces usually may not use a chimney as the vent. The vent for a gas or an oil direct vented wall furnace should be the one supplied by the manufacturer.

Wood Combustion

Wood is a more complex hydrocarbon compared with gas and oil. Burning wood produces many compounds including carbon dioxide, carbon monoxide, water vapor, carbon, and creosote. The amount of each compound depends on the completeness and the temperature of combustion, the temperature of the chimney, and the moisture content of the wood. -Creosote is a particularly dangerous combustion byproduct because it is flammable and can ignite in a chimney and cause a fire. It is most likely to form when burning wood with a high moisture content, and when burning with inadequate combustion air. -Creosote can present as a light brown or black flaky ash (Stage I, First degree), as a heavy tar-like ash (Stage II, Second degree), or as a hard and shiny coating (Stage III, Third degree). Stage I creosote is common and is easy to remove; it is the least flammable form. Stage II creosote is more flammable, and more difficult to remove. Stage III creosote is highly flammable and is quite difficult to remove; flue liner replacement may be necessary.

Chimney Connector

a component that conducts combustion products from a fuel-burning appliance to a chimney.

Vent Connector

a component that conducts combustion products from a fuel-burning appliance to a vent; vent connectors are single-wall or double-wall metal pipes.

Damper (barometric)

a device used to control draft in an oil-burning appliance vent connector when using a masonry chimney as a vent; the damper is a round metal plate that is mounted on two hinges. It opens and closes based on the pressure in the vent connector.

Direct exhaust appliance

a fuel-burning appliance that obtains combustion air from inside the house and expels combustion products outside the house. These are usually high efficiency appliances. Some appliances can be configured in the field as a direct vent appliance or as a direct exhaust appliance. Also called a non-direct vented appliance.

Direct Vent Appliance

a fuel-burning appliance that obtains combustion air from outside the house and expels combustion products outside the house. These are usually high efficiency appliances.

Chimney

a generally vertical structure containing one or more flues that conducts combustion products from a fuel-burning appliance to a point outside the house; chimneys are constructed using masonry and metal pipes.

Hydronic Heating

a heating system that circulates hot liquid or steam through pipes; the pipes may be installed in the floor, ceiling, or wall, or may serve radiators or similar devices.

Draft (forced)

a method of expelling combustion gasses from gas-burning and from oil-burning appliances that uses a fan (blower) to force the gasses through the vent under pressure; also referred to as positive pressure draft. Forced draft fans are installed at the beginning of the vent system, usually inside the appliance. Forced draft vent systems must be sealed to prevent combustion gasses from escaping through the system.

Vent System

a passageway that conducts combustion products from a fuel-burning appliance to a point outside the house; the vent system begins at the appliance draft hood or flue collar and ends outside the house; the vent system consists of a vent or chimney and a vent connector, if one is used.

Damper (HVAC duct system)

a plate or louvers installed in a duct system that permits control of how much air flows in a duct; a damper may be controlled manually or it may be controlled by a motor; both are methods of installing a zoned HVAC system where one HVAC system is controlled by two or more thermostats.

Annual Fuel Utilization Efficiency (AFUE)

a ratio of heat generated by a heating system versus the energy used; the minimum AFUE for most gas furnaces is around 80, meaning that the furnace converts 80% of the fuel to heat and 20% is lost through the vent or by other means.

Manifold (Vent Connector)

a type of vent connector in which two or more vent connectors are joined together before being connected to the vent.

Draft (mechanical)

a vent system that uses an electrically powered fan to assist in expelling combustion gasses through a chimney or vent; mechanical draft may be forced draft (positive pressure) or induced draft (negative pressure).

Solar Heating Systems

are available, but are not common. Solar heating is sometimes used to supplement other heating systems. Most solar heating systems heat a fluid and pump the fluid through pipes. Some solar heating systems heat a mass such as concrete or stones and circulate the heat by radiation or through ducts. Most home inspectors should disclaim inspection of these systems and recommend inspection by a qualified contractor. Refer to the alternative energy section in the Electrical System chapter for more information.

Hydronic Distribution Systems

are common in some markets and are uncommon in others. Most newer hydronic systems pump a heated liquid through pipes that may be installed in the floor or that run through radiators or similar distribution devices. Some, mostly older, hydronic systems move steam through pipes that run through radiators. Hydronic distribution systems rely mostly on radiation and convection. Hydronic systems in floors produce an even temperature distribution. Hydronic systems that use radiators often suffer from uneven temperature distribution.

Heat Exchangers in Old Furnaces

are generally straight sections of cast iron that are welded together. Older furnaces often have a standing pilot light. Gas valves in older furnaces and less expensive furnaces allow gas to flow at one rate.

Cracks or Damages to Heat Exchanger

can allow the combustion gasses to enter the air flow that gets circulated in the house. This is why heat exchanger cracks are a major safety issue. Heat exchanger cracks can be caused by metal fatigue resulting from years of heating and cooling; metal expands when heated and reverts back to its original state when it cools. Heat exchanger cracks can also be caused by a faulty high limit switch that allows the heat exchanger to get too hot. Furnaces with a cracked or damaged heat exchanger usually need to be replaced.

Gas Furnace Components

gas furnace is made from the following components that are enclosed in a cabinet. The cabinet is usually made from galvanized steel or aluminum. The cabinet usually has two access covers. One covers the burner compartment. The other covers the fan (blower) compartment. Some covers remain in place through friction or tension devices. Some covers are sealed shut with screws. Some HVAC technicians also seal the covers with tape. Screws and tape make removing the covers difficult, and also makes replacing the covers difficult. -Return air enters the cabinet at the fan compartment and is moved using a fan (blower). In almost all furnaces a home inspector is likely to see, this fan looks like the exercise wheel in a rodent cage, so it is sometimes called a squirrel cage fan.

Heat Exchangers in Newer Furnaces

have many curves that increase the heat exchanger surface area. Newer furnaces usually have an igniter that glows hot to ignite the gas. Gas valves in newer and more expensive furnaces may allow gas to flow at two rates, or may modulate the gas flow at a variable rate.

Visual System Observation

involves looking at the appliance cabinet and the components entering, leaving, and around the cabinet. Also involves opening access panels that are provided for homeowner inspection and maintenance, and that are not sealed in place.

Inspecting Heating Systems

involves two tasks. One is a visual observation of the system including the electric power and fuel supply components. The other is operating the system using normal operating controls (the thermostat).

Traditional Wall Furnace

is a vertical unit that, as the name suggests, is often inset between studs in a wall. -Gas wall furnaces are the most common. -Oil wall furnaces are uncommon, but are available. -Some wall furnaces project from the wall and some are configured as a console unit that is attached to the wall. Some wall furnaces distribute heat by radiation and convection; others use a fan to assist in heat distribution. Some wall furnaces are vented using a Type B or a Type BW vent (gas) and some are direct vented. Wall furnaces usually serve one room or adjacent rooms with no door between the rooms. Some models can be fitted with a register to serve an adjoining room with short ducts that extend out the side of the unit. Wall furnaces are usually found in older houses, but new units are available and are used as supplemental heating and for heating additions. They could be used as the sole heating source in small houses and seasonal houses.

Properly Burning Gas Flame

is blue, sometimes with a slight yellow tip.

Floor Furnaces

is installed, as the name suggests, under a floor. It usually hangs down into a crawl space. -Gas floor furnaces are the most common. -Oil floor furnaces are uncommon, and it may be difficult to find replacement parts and new units. -Floor furnaces distribute heat by radiation and convection from a grille that covers the furnace opening in the floor. They usually serve one room or adjacent rooms. They are usually found in older houses, but new gas-burning units are available and could be used as the sole heating source in small houses and seasonal houses.

Vented Room Heater

may be in a console cabinet that is usually installed on the floor, or it may be a wall-mounted unit. Some room heaters may be mounted in a window or through the wall. Most room heaters are gas-fired; oil-fired units are available but are uncommon. Some room heaters distribute heat by radiation and convection; others use a fan to assist in heat distribution. Some room heaters are vented using a Type B (gas) and some are direct vented. Room heaters usually serve one room or adjacent rooms with no door between the rooms. Room heaters may not be connected to ducts or registers to serve adjoining rooms. Room heaters may be found in any house, and are often used as supplemental heating and for heating additions to a house. They could be used as the sole heating source in small houses and seasonal houses.

Temperature

measure of how fast heat is transferred

Radiant Systems

move heat by heating a material and allowing the heat to flow by radiation, conduction, and convection. Electric resistance systems in the floor and baseboard systems are types of radiant systems. Fireplaces and wood stoves are another type of radiant system. Other types of radiant systems include gas floor furnaces and wall furnaces, vented and unvented gas room heaters, and hydronic heating systems. Radiant systems, except for systems installed in the floor, also suffer from uneven heated air distribution.

Radiation

occurs when molecules gain heat by absorbing electromagnetic energy in the infrared part of the spectrum. The heat you feel when you place your hand near the oven door is transferred, in part, directly by the infrared radiation from the oven.

Gas Supply

should enter the cabinet through solid pipe. Flexible gas connectors are not permitted to enter cabinets. The gas valve controls the supply of gas to the burners and to the pilot light, if any.

Vent (combustion)

the final vertical component in a vent system; this term most accurately describes manufactured products such as metal pipes.

Draft (natural)

the tendency of combustion gasses to rise in a chimney or vent due to the gasses being hotter and at a lower pressure than the surrounding gasses (also known as the stack effect); fireplaces and most gas-fired and oil-fired appliances rely on natural draft to expel combustion gasses.

Flue

this term most accurately describes a generally vertical passageway inside a chimney; a chimney has at least one flue and may have several flues.

Heat

transfer of energy between molecules

Gravity Distribution

uses convection and the stack effect to move heat from a furnace up through ducts. Gravity systems have no fan and filter. Gravity systems are relics of the late 1800s and early 1900s and most, but not all, have been replaced. Gravity systems are very inefficient and suffer from uneven heated air distribution and drafts.

Convection

when heat moves by circulation of a gas or liquid. -One aspect of convection is especially important in removing unwanted and dangerous combustion gasses from the house. Hot liquids and gasses are lighter, more buoyant, and at a lower pressure compared to the surrounding material. This allows the liquid or gas to rise relative to the surrounding material. This phenomenon is called the stack effect. Chimneys and natural draft vents rely on the stack effect to expel the combustion gasses. The combustion gasses can lose heat in chimneys and vents that are improperly installed. If the gasses lose enough heat, they can become too heavy to reach the top of the chimney or vent, and fall back down the chimney or vent into the house. This phenomenon is called backdrafting.

Conduction

when heat moves through solid matter