HVAC Unit 31 Gas Heat
What are the four factors used to determine efficiency of a furnace?
- Type of draft - Amount of excess air used in the combustion chamber - Delta-T- temperature difference of the air entering vs leaving the heat exchanger - Flue stack temperature
How do high efficiency furnaces differ from conventional furnaces?
- they have a secondary heat exchanger - they have a flue-gas condensate disposal system - a venting system that is either induced or forced power drafted. - they have sidewall venting - they have a sealed combustion chamber that prevents the heated conditioned air from being used as combustion air by using a direct vent to outside air.
How do you numerically write 1 microamp
.000001 amps
For proper venting into a confined room, what is the rule of thumb for grill size?
1 sq inch per 1000 Btu/h is required for a grill above and below the furnace. Thus a 100,000 Btu/h furnace would require a 10"x10" grill (100 sq inches) above and below.
What is the specific gravity of propane?
1.52 which means it is heavier than air and will sink if released to the air.
Some gas regulators can be adjusted. How are they adjusted?
By turning an adjusting screw clockwise, the pressure is increased. By turning the screw counterclockwise, the pressure is decreased.
T or F: both a water and digital manometer can measure positive and negative pressures.
False- only a digital manometer can measure negative pressure. A water manometer can only measure a positive pressure.
T or F: Natural gas is poisonous to breath and has a distinctive smell.
False. It is not poisonous and has no odor. In fact, an odorant is added to it to make leak detection easier.
How do you calculate the actual heat output of a gas furnace?
Find the input gas rating and multiple it by the efficiency rating. Thus a furnace with 80000 Btu/h and a 80% furnace effiency would be 80000 x .80 = 64000 Btu/h.
What is the fastest and surest way of proving there is a flame in an IP system?
Flame rectification.
What are the three elements for combustion?
Fuel Oxygen Heat
Describe a solenoid gas valve.
It is a NC valve with a plunger that opens the valve when the coil is energized.
What is a smart valve?
It is a gas valve and electronic control module in one package. It combines the features of an IP system and HSI system in one.
What is a manifold?
It is a pipe inside a gas furnace through which the gas flows to the burners. The burners are also mounted on the manifold.
What type of device is a limit switch?
It is a safety device.
What is another name for the rate at which a flame travels through the gas and air mixture?
It is also known as ignition velocity or flame velocity.
What is the technical term to verify that a standing pilot is lit?
It is called flame-proving.
What is excess air composed of?
It is composed of combustion air and dilution air. Combustion air is primary or secondary air. Dilution air is air after combustion and usually enters at the end of the heat exchanger. It is brought in by the draft hood of the furnace.
What is a flame rollout safety switch?
It is safety switch located outside the burners and opens when it detects flame outside the burners. When they open, the gas valve is closed and stops the gas. They must be manually reset by a service call.
Why is extra primary gas supplied in the mixing tube chamber of the burner?
It is supplied to make sure that all of the fuel is burned and that no methane is left behind from the fuel. This extra air is known as excess air.
What happens if there is flame impingement?
It is the cooling of the flame when it strikes the sides of the combustion chamber due to burner misalignment.
How much excess is usually added to the combustion process?
It is usually supplied at 50% over what would be needed if it were mixed thoroughly.
What does a gas valve do?
It stops, starts or modulates the flow of fuel gas.
How does a heat motor-controlled valve work?
It uses an electric heating element around an expanding rod to open and close the valve. When the element is energized, it heats up and causes the rod to elongate. This pushes the valve open and allows gas to pass. When the heat is de-energized, the rod contracts and closes the valve.
Which type of furnace is usually found in basements that have low headroom?
Low-boy
What is a 100% shutoff system?
When there is a failure in the flame-proving device system, both the pilot valve and the main gas valve are shut down and closed.
What is standard gap for electronic ignition systems?
1/8" gap.
How many Btu of heat energy do you get from 1 cubic foot of natural gas?
1050 Btu of heat energy.
How many Btu of heat energy do you get from 1 cubic foot of propane ?
2500 Btu of heat energy.
How many inches WC is in 1 psig?
27.7 " WC
How many Btu of heat energy do you get from 1 cubic foot of butane?
3200 Btu of heat energy.
How does a DSI system work?
A DSI is designed with a spark ignition direct to the main burner. No pilot is used. It uses flame rectification to verify that the furnace has fired. The system will go into safety lockout if the flame is not established within a set time (usually 4-11 seconds) due to large amount of fuel coming through the burners compared to an IP system.
Explain flame rectification.
A flame can conduct a small electrical current due to the ionized combustion gases. An AC current is fed to electrodes in the flames. Due to different sizes of the electrodes, current will flow better in one direction than the other. A small DC current (1-3 microamps) will flow as long as the flame is lit. This DC signal s what the furnace recognizes as proof of a flame. If for some reason, it received the AC signal back, it would prevent the main gas valve from opening.
If both a light gas and a heavy gas are at the same pressure, which will flow more through a given orifice?
A light gas will flow more through an orifice.
What is the differences between a prepurge, interpurge and postpurge?
A prepurge is to clear the heat exchanger of any unwanted flue gases, household fumes, or dust that may have accumulated. An interpurge allows the combustion blower to operate for a certain time period between ignition tries. It is generally used when the furnace does not light or does not sense a flame. A postpurge allows the combustion blower motor to operate for a certain time period after the heating cycle to remove any combustibles from the combustion chamber.
If you see yellow flames, what problem should you suspect?
A primary air restriction.
What is the difference between soft and hard lockout?
A soft lockout is a time period built into an ignition module that allows for a certain time to light or relight the pilot or main burner. If this time period is exceeded, it will go into a semi-shutdown but it will keep trying to relight the system. It will go into hard lockout after several soft lockouts have been attempted. If it goes into hard lockout, the module will need to be reset by interrupting the power to the module. This can be done by a service call.
Describe IP ignition.
A spark from an electronic module ignites the pilot which in turn ignites the main gas burners. The pilot only burns when the t-stat calls for heat. The pilot turns off to conserve fuel. It uses flame rectification to verify that the pilot has fired.
How does a thermocouple flame-prove a pilot flame.
A thermocouple produces a small voltage when heated by the pilot flame. The thermocouple is connected to a shutoff valve. As long as the electrical current energizes the coil in the valve, the gas can flow. If the flame goes out, no current will flow and the gas valve closes, shutting off the gas.
How is inches WC calculated with a water manometer?
A tube is formed into a U and filled halfway with water. Gas pressure is piped to one side of the tube and the difference in their heights is inches WC.
What is the fancy name for the reduction in burner diameter in the burner tube?
A venturi.
What is an orifice and a spud?
An orifice is a precisely sized hole in a gas fitting which gas flows from the manifold to the burners. A spud is another name for a gas fitting.
What does AFUE stand for
Annual Fuel Utilization Efficiency. The higher the number, the less amount of heat is being vented to the atmosphere.
How does a gas burner tube get the air it needs for combustion?
As the gas is metered to the burner, it pulls in air with it through the venturi. This is known as primary air. As the gas is ignited at the port, secondary air is drawn in around the burner ports to support combustion.
What can be done to prevent heated air from leaving a structure through the draft hood?
Automatic vent dampers can be placed in the vent to close when the furnace is off and opened when the furnace is started.
Why do multipoise furnaces have extra safety controls?
Because these furnaces can be placed in four different configurations, they must multiple safety controls designed into the furnace.
Which is better, blue flames with slightly orange tips or blue flames with yellow tips?
Blue flames with slightly orange tips. Yellow tips is an indicator of an air-starved flame that is emitting carbon monoxide fumes.
Describe a diaphragm gas valve.
By using gas pressure on one side of the diaphragm, it keeps the diaphragm pushed down and keeps the main valve closed. When the gas pressure is removed, the pressure from the below the diaphragm pushes it up and opens the main valve port.
When there is perfect combustion, what elements result from the combustion?
Carbon dioxide (CO2), water vapor (H2O) and heat.
If there is poor combustion, what is the dangerous by-product that always results?
Carbon monoxide (CO).
What are the efficiency ratings for conventional, mid-efficiency and high-efficiency furnaces?
Conventional - 78-80% Mid - 78 to 83% High - 87 to 97%
What is another name for a downflow furnace?
Counterflow
Does the dew point temperature increase or decrease as excess air decreases?
DPT increases as excess air decreases.
Name the four types of burners.
Drilled-port burner Slotted-port burner Ribbon burner Single-port burner
True or False: Modern furnaces allow for many adjustments in order to keep efficiencies at a maximum.
False - except for primary air adjustment, modern furnaces have very little adjustability.
T or F: a standing pilot is only lit when needed, it goes out to conserve gas when it is not lighting a burner.
False- a standing pilot burns continuously.
What is the difference between aerated and nonaerated pilot burners?
In an aerated pilot, the air is mixed with the gas before it enters the pilot burner. In a nonaerated pilot, only secondary air is used for combustion.
Where do you usually find horizontal furnaces?
In crawl spaces, attics and suspended from floor joists in basements.
What is the difference between an induced-draft system and a forced-draft system?
Induced systems have a combustion blower motor located on the OUTLET of the heat exchanger. They pull or suck combustion gases through the heat exchanger due to a slight negative pressure in the heat exchanger. Forced systems have the combustion blower motor on the INLET of the heat exchanger. They push or blow combustion gases through the heat exchanger and cause a positive pressure in the heat exchanger.
What is another name for the single-port burner?
Inshot or upshot burner
Where does gas combustion take place?
Inside the gas burners.
How does a HSI system work?
Instead of a flame, a HSI uses a material that is highly resistant to electrical current. Using line voltage, the material can heat up to 1100°. The HSI igniter can light a pilot or the main furnace burner. The system must flame prove that it was lit or it will go into safety lockout.
What are the three categories of ignition systems?
Intermittent pilot (IP) Direct-spark ignition (DSI) Hot surface ignition (HSI)
Why is specific gravity of a gas important?
It affects the gas flow through the piping and through the orifice of the furnace.
Why is there a delay in turning on the fan switch?
It allows the heat exchanger a chance to heat up before the fan is turned on. This delay keeps cold air from being circulated through the duct system.
What gases is liquified petroleum made of?
It can be liquified propane, butane or a combination of both.
Why is soot bad for a furnace?
It can collect on the heat exchanger and acts like an insulator. This reduces efficiency of the furnace.
Why is butane not a popular fuel source?
It has a high boiling point of 31° and is difficult to keep the pressure high enough to move the gas to the main burner.
Why is a draft diverter important?
It helps reduce the chance of the pilot flame being blown out due to a downdraft from windy conditions.
On a electronic module, what wires are connected to the following terminals? MV PV MV/PV GND TH IGNITER/SENSOR
MV - main valve PV - pilot valve MV/PV - common terminal for main and pilot valves GND - ground for burner TH - power from thermostat IGNITER/SENSOR - high voltage igniter and flame rectification for local sensing rod
Name the four main components of a gas fired heating system.
Manifold and controls Burners Heat exchanger Venting system
Why is manifold pressures expressed in inches WC instead of psig?
Manifold pressures are very small, less than 1 psig. Inches WC is a much more precise measurement for less than 1 psig.
Why is a metal vent often preferred rather than a masonry chimney?
Metal vents reach operating temperatures quicker than masonry vents do. Masonry chimneys tend to cool the flue gases. Masonry must be lined with a glazed-type tile to prevent corrosive materials from the flue gas from destroying the mortar joints.
What is an advantage of a modulating gas furnace?
Modulating gas furnaces use a modulating gas valve instead of a staged gas valve. It can vary the heating input and warm airflow. Thus it is able to follow the heat loss of a structure and have longer run times and reduce cycling. This increases the efficiency of the furnace.
What are the three commonly used gas fuels?
Natural gas Manufactured gas Liquified petroleum or LP gas
What are the manifold pressures for natural gas and LP gas?
Natural gas 3 to 3.5 in. WC LP gas 9 to 11 in. WC
If one of the elements are not available, can there still be combustion?
No, if any of the elements are missing, no combustion is possible.
Can burners operate efficiently with any combination or amounts of air and gas?
No- the gas flow rate must be correct for the quantity of air supplied.
Do LP gas installations use gas valves with built-in regulators?
No- we just said that the gas regulators are located at the supply tank. Duh.
What is used to ignite the gas burners on most conventional gas furnaces.
Pilot flames.
How can you measure the flame current?
Place the microammeter in series with the wire coming from the burner ground terminal to the module.
What can result from flame impingement?
Poor combustion that produces carbon monoxide and soot.
Why are gas regulators necessary?
Pressures in supply lines can vary greatly. In order to keep pressures to the lines constant, the actual pressure in the lines are much higher than required at the manifold. A gas regulator drops the pressure to the proper inches WC needed and keeps the pressure constant at the outlet where the gas is fed to the gas valve.
What are three safety devices used to flame-prove a standing pilot.
Thermocouples and thermopiles Bimetallic strips Liquid-filled remote bulbs
Describe the two types of flame rectification systems.
Single-rod (or local) sensing consists of an igniter and a sensor all in one rod. There is only one wire running from the ignition module to the pilot assembly. Dual-rod uses a separate igniter and flame-sensing rod. There are two wires running from the pilot assembly to the ignition module.
What are the three classifications of gas valves?
Solenoid Diaphragm Heat Motor
Which metal is best at resisting corrosion in a heat exchanger?
Stainless steel
How much oxygen is in the air we breathe?
The air contains 21% oxygen.
In a standing pilot automatic gas valve, there are two solenoids, how do they differ in their actions inside the valve?
The first solenoid pulls up on the valve and allows gas to flow past it. When the t-stat is is satisfied, the solenoid is de-energized, a spring closes the valve. The second solenoid is a servo-operated valve, which means that its position is controlled by the pressure of gas coming to a chamber beneath its diaphragm. A servo pressure regulator adds or removes pressure from this chamber. More pressure closes the valve. Less pressure opens the valve.
How does the system know if the pilot flame with a thermocouple goes out and what happens?
The pilot flame engulfs a thermocouple. The heat from the flame produces a small DC voltage. If the pilot flame is ever lost, the DC current will stop flowing to a power unit that is holding the safety shutoff open. When the power unit is de-energized, it will shut the valve and gas flow will stop.
If the power unit shuts off the gas flow, how is it started up again?
The red reset button on the valve must be pushed down in the pilot position before it can be lit again.
What is the specific gravity of natural gas and is it lighter than air?
The specific gravity is .60 and it is lighter than air so it rises.
Are vent limit / draft safeguard switches NC or NO and what do they do?
They are NC switches and monitor the draft hood temperature and open when there is any spillage of flue gas.
In modern furnaces, where are the gas regulators located?
They are built into the gas valve.
Where are flame rollout switches usually installed?
They are installed near the top of the burner box, where the flame is most likely to roll out.
Where are the burners located in relation to a conventional heat exchanger?
They are located at the bottom of the heat exchanger.
For LP gas regulators, where are they located?
They are located at the supply tank.
What are heat exchangers made out of?
They are made of sheet steel or aluminized steel to provide rapid transfer of heat from the combustion to the air.
What is a dip switch?
They are small, single-pole, double throw switches located on an IFC board that help in programming the board.
How do auxiliary limit switches work?
They are used for counterflow and horizontal furnaces. They are usually bimetallic and are mounted on the warm air blower housing to they can monitor room return air temperature and interrupt the burners when the temperature gets too hot.
What is so special about multipoise furnaces?
They can be installed in any position. They are very versatile and give installers the option of upflow, downflow, horizontal right or horizontal left discharge.
What does an integrated furnace controller or electronic ignition modules do?
They control the ignition and sequence of operation of most modern gas furnaces.
What are redundant gas valves?
They have two or three valve operators physically in series but wired in parallel which allows either the pilot or main operator to block gas from getting to the main burner.
What do automatic combination gas valves incorporate inside of it?
They have: A manual control the gas supply for the pilot the adjustment and safety shutoff for the pilot the pressure regulator the controls to operate the main gas valve dual shutoff seats for extra safety protection.
How do high-efficiency vent their flue gases?
They recirculate the flue gases through a special extra heat exchanger to keep more of the heat available for space heating. This causes condensation and corrosion so plastic pipe (that is not damaged by the corrosive materials) is used to vent the gases and drain the condensate.
Why is there a delay in shutting off the fan?
This allows the heat exchanger to cool off and dissipate the furnace heat at the end of the cycle.
Why do conventional furnaces vent their hot gas quickly?
To prevent cooling of the flue gas which produces condensation and other corrosive actions. This quick venting also cause these furnaces to lose some efficiency due to heat loss up the flue.
What do you call a mixture that has too much gas and a mixture that has too little gas? Will these mixtures burn?
Too much gas is known as too rich. Too little gas is known as too lean. Neither of these mixtures will burn.
What happens if there is too much or too little airflow across a heat exchanger?
Too much- the flue gas will become too cool and the products of combustion may condense and run down the flue pipe. Too little - the combustion chamber will overheat and stress can occur.
T or F: A standing pilot has a pilot burner that is lit all the time.
True
T or F: in modern, high efficiency furnaces, the burners are located at the top of the furnace in a sealed combustion chamber.
True
T or F: a pilot is technically just a burner with an orifice that gas passes.
True.
When is twinning used?
Twinning is the use of two furnaces side by side. It is used when a heating capacity greater than that of the largest capacity furnace manufactured is needed.
Name the four types of furnace flow patterns.
Upflow Low-boy Downflow Horizontal
What is different for a non-100% shutoff system?
When there is a failure in the flame-proving device system, themain gas valve is shut down and closed but the pilot valve continues to bleed gas. This is to prevent nuisance shutdowns caused by such things as windy days. The system will have a soft shutdown period of 5 minutes or more to dissipate fuels.
