AIR251

The pressure in a duct system is measured in

" WC

With a dry-bulb reading of 70°F and a wet-bulb reading of 61°F, the dew point temperature is __________°F.

55.5

Relative humidity in question 10 is __________%.

60

State three ways the body gives off heat

Conduction, Convection, Radiation, and perspiration evaporation.

Name five changes that are made to air to condition it

Cool, heat, dehumidify, humidify and clean.

Branch ducts should always use what component to balance the air to the individual areas?

balancing dampers

The ____ or fan provides the pressure difference to force the air into the duct system, through the heat transfer surfaces in the conditioning equipment, through the grilles and registers, and into the room.

blower

Multiple-zone systems often use a ................. or a ................ to alleviate the problem of excess air when only some zones are calling for conditioning.

bypass or a dump zone

What are the three most common types of compressors used in air conditioning?

centrifugal, screw, and reciprocating compressors

The generalized comfort chart can be used as a basis to ____

determine combinations of temperature and humidity to produce comfort in summer and winter

Of the heat produced in the human body, some of it is removed by perspiration through _________ ?

evaporation

Each foot of duct offers a known resistance to airflow which is called ____________________.

friction loss

How can the cost of heating a home in the winter be lowered, and still have comfortable conditions in the home?

if you raise the humidity in winter, the temperature will decrease. this is a great way to save in energy bill.

The two ways that fresh air enters a structure are

infiltration and ventilation.

Explain how a wet-bulb thermometer works.

it measures the humidity in air

The reason for using a reducing extended plenum system is to ____.

maintain a more even velocity pressure throughout the entire length of the trunk duct

Baseboard hot water heat is an example of ____________________ heat

natural-draft

Air is primarily made up of ____________________ and oxygen.

nitrogen

A homeowner plans to build a house out in Las Vegas, Nevada, in the middle of the desert. It would be in his/her best interest to lower the "solar load," to locate most of the windows on the ____.

north and south sides of the house

Lower room temperatures can be offset in winter by

raising the relative humidity.

Relative humidity is measured using a

sling pyschrometer

Measuring velocity pressure in a duct indicates the ____.

speed at which the air is moving

Which of the following is typically measured with a water manometer?

static pressure in a duct system

The name of the fitting where the branch line leaves the duct is the .............

take off.

What are the components that make up the forced-air system?

the blower (fan), the air supply system, the return air system, and the grilles and registers

Most compressors are cooled using the ________

the discharge gas leaving the compressor._

What is the major advantage a top air discharge condensing unit has over a side air discharge condensing unit?

the hot air is discharged from the top into the air

More than one evaporator coil circuit is used when ____.

there would be excessive refrigerant pressure drop in the coil circuit

The ________________ of a section of sheet metal is directly related to its weight.

thickness

How is the centrifugal blower turned?

turned by motor

The three pressures created by moving air in ductwork are

velocity pressure, static pressure, and total pressure.

An instrument used to measure the actual air velocity is the ____.

velometer

An airpressure measuring instrument ____.

water manometer

_______ is an air pressure measuring instrument.

water manometer

If a high efficiency condensing unit is manufactured with a one-speed condenser fan motor, what is likely to happen if the system is operating during mild weather?

what would happen is usually a low head pressure which will result in the starvation of the expansion valve and less capacity

There are two kinds of attic ventilation, power and _____________.

NATURAL

On a package air conditioning system, which refrigerant lines must be field installed?

No lines need to be installed.

Three types of expansion devices are used in residential air conditioning systems. They are TXV, capillary tube, and _________________________.

ORIFICE

Why are dampers recommended in all branch line ducts?

So the air volume can be balanced for correct flow to each run. Adjusting the air volume at supply register can result in excessive noise

The device used to put trunk duct or extended plenum sections together is called a drive clip. Also locally known as slip and drive systems.

T

The duct pressure equals the velocity pressure plus static pressure.

T

The forced-air system is different from a natural-draft system, where the air passes naturally over the conditioning equipment.

T

The human body is comfortable when the heat produced in the body is equal to the heat lost to the surroundings.

T

The rate at which water will evaporate from the wick on the wet-bulb thermometer is determined by the moisture content of the surrounding air.

T

Turbulence in airflow causes friction drop

T

The __________ expansion device ordinarily must use a compressor with a high starting torque.

TXV

What advantage does a thermostatic expansion valve have over a capillary tube?

TXV's are more efficient than capillary tubes

Name the four comfort factors

Temperature, Humidity, Air movement, & Air cleanliness.

Infiltration is the term used when referring to the air entering a structure ____.

around doors and windows

If the fresh air requirements for the preceding house were to be 0.4 air changes per hour, how much fresh air must be taken in per minute?

210

If the air leaving the cooling coil were to be 55°F dry-bulb and 53°F wet-bulb, what would the total heat content of the air be?

22

What are the three common evaporator designs used on air conditioning equipment?

- A coil - slant coil -H coil

Describe the plenum system and explain why it is economical from a first-cost standpoint.

-The plenum system has an individual supply system that makes it well suited for a job in which the room outlets are all close to the unit .-This system is economical from a first-cost standpoint and can be installed easily by an installer with a minimum of training and experience.

One reason for using a reducing plenum system is to ____.

-save materials -keep the duct pressures constant

37.8 Types of Drive Assemblies

1. Centrifugal blowers a. Centrifugal blowers must be turned by a motor. b. Two drive mechanisms are used: 1. Belt drive. 2. Direct drive. c. 1800 rpm motors were used exclusively for years. d. Actually operated at 1750 rpm under load. e. Motor normally used a start capacitor. f. From a stop position, motor will reach1750 rpm in about 1 second. g. Later manufacturers started using smaller blowers with 3600-rpm motors. h. 3600-rpm motors actually operate at 3450 under load. i. Sleeve bearings and resilient (rubber) mountings are used to keep bearing noise out of the blower section. j. Belt-drive blowers have two bearings on the fan shaft and two bearings on the motor. k. Motor pulleys and fan motor pulleys can be adjusted to change fan speeds. 2. PSC blower motors: a. Recently, PSC (permanent split-capacitor) are being used. b. PSC motors start very slow and take several seconds to get up to speed. c. PCS motors are direct-drive blowers. 3. Shaded-pole blower motor: a. Shaded-pole motors are used on some direct-drive blowers. b. Shaded-pole motors are not as efficient as PSC motors. 4. Direct-drive blower motors: a. Direct-drive motors use no pulleys or belts. b. Direct-drive motors can be multispeed motors. c. 1500 rpm down to 800 rpm are the normal speeds. d. Speeds can be changed by changing motor wire leads

The average velocity of a 16-in. round duct is 800 fpm; the airflow in the duct is

1120 cfm.

What is an air conditioning system's capacity in tons, if the supply air is moving through the 12" x 24" main supply duct at a velocity of 400 ft/min? Show me your calculation for credit.

12x24=288/144= 2 tons

The atmosphere exerts a pressure of ____ at standard atmospheric conditions.

14.696 psi

Using the answers from questions 14, 15, and 16, what would be the total heat gain due to the fresh air for the house in question 13?

14609,7

A(n) ____ degree F temperature difference from inside the duct to outside the duct is considered the maximum difference allowed before insulation is necessary.

15

If a rectangular duct system (12 times 26 in.) has an average velocity of 700 fpm, what is the airflow?

1519cfm

Flexible round duct comes in sizes up to about ____________________ in. in diameter. Use numbers to answer (17).

24

If the equivalent length of a transition piece of ductwork is 10 feet and the main duct in series with it is 20 feet, what is the length used to estimate the total frictional loss?

30FT

13. A house has 3500 ft of floor space and a 9-ft ceiling. What is the cubic volume of the house?

31500

If the outside fresh-air conditions were to be 93°F dry-bulb and 74°F wet-bulb, what would the total heat content of the air be?

3746

The design boiling point for most evaporator coils in residential air conditioning is _______°F.

75

For a room to be comfortable, the following conditions are considered average: __________°F dry-bulb and __________% relative humidity.

75---------50%

When is insulation required on air ducts?

A 15°F temperature difference from inside the duct to outside the duct is considered the maximum difference allowed before insulation is necessary

. The equivalent feet of pressure drop for a group 3-Gbranch line takeoff is

A. 30 ft.

Warm air heats a room better if distributed

A. low in the room.

Zoning an air-conditioning or heating system allows for

Both A and B are characteristics of a zoned system.

A gas furnace has an output of 60,000 Btu/h. The return air temperature is 72°F, and the temperature out of the furnace is 130°F. Disregarding the heat the fan motor may add to the airstream, how much air is the furnace handling?

CFM.TD.1,08=9578

What advantage does a capillary tube have over a thermostatic expansion valve?

Capillary tubes the refrigerant pressures are equalized during the off-cycle unlike the TXV

Which of the following types of fan is used to move large amounts of air against a low static pressure drop?

D. Propeller

In order for an air-conditioning coil to remove moisture from the air, it must be below the _________ __________temperature of the air.

DEW POINT

Name two types of blower drives.

Direct & belt drive

When an "A" coil is used in a horizontal application, the coil is laid on its side and a special ____________________ is required.

Drain Pan

Adjusting the tension on the belt will alter the rate at which air is moving through the system and but does not affect the amperage draw of the motor.

F

Air for cooling distributes better from down next to the floor.

F

Canvas collars are generally made of canvas rather than other materials such as rubber.

F

Evaporative air conditioning will work in any climate.

F

Friction due to air rubbing the walls of the ductwork can be eliminated.

F

Leaks in all-aluminum coils are easier to repair than the standard copper tube-aluminum fin coil.

F

The all-aluminum coil has a higher heat transfer rate than the standard copper tube-aluminum fin coil.

F

The return duct portion of an air handling system never needs to be insulated.

F

Zoning an air-conditioning system can only be done when the system is initially designed and installed

F

Pressure in ductwork is expressed in which of the following?

Inches of water column

Name the two types of return air systems in residential installations.

Individual & central returns

Why is the airflow from a register not the same across the entire surface?

It depends on how the register is fastened to the ductwork

What are the two types of heat that are removed by the evaporator coil in the air-conditioning process?

LATEN HEAT & SENSIBLE HEAT

The small interconnecting line between the indoor and outdoor coil is the _________ line.

LIQUID_

What is a common instrument used to measure pressure in ductwork?

Manometer

The most popular refrigerant used in the past in residential air conditioning is _________.

R-134a

What refrigerant blend can be used in retrofit applications as a long-term replacement for R-22 in residential and light commercial air-conditioning applications?

R-134a

What are two higher-efficiency refrigerant blends that can be used as long-term replacements for R-22 in residential and light commercial air-conditioning applications?

R-410a and R-134a

What are the advantages of using refrigerated air conditioning rather than evaporative conditioning

Refrigerated air conditioning uses 4 components that are assembled in several ways to accomplish the same goal, refrigerated air to cool a space

The duct sizes on the air friction chart are expressed in which of the following?

Round

Air that contains all of the moisture that it can hold is known as _

SATURATION

Name four materials used to manufacture duct.

Sheet steel, aluminum, fiberglass duct board, and flexible duct.

A duct system is pressurized by what three pressures?

Static pressure, static pressure, and total pressure

What three pressures make up the duct system's pressure?

Static pressure, static pressure, and total pressure

What is the component that distributes air in the conditioned space?

Supply register

Duct systems can be plenum, extended plenum, reducing plenum, or perimeter loop.

T

Equipment will perform at its best efficiency when running at full load.

T

Oversized duct systems cost more to fabricate and install and will also result in the slowing of the air as it flows through the duct.

T

The air leaving an evaporator has a dry-bulb temperature of 85 °F and a wet-bulb temperature of 65°F. The air entering the evaporator has a dry-bulb temperature of 95°F and a wet-bulb temperature of 75°F. How many grains of moisture per pound of dry air were removed by the evaporator?

The air leaving an evaporator: 99.01 gr/lb The air entering the evaporator: 53.26 gr/lb Δ grain = The air leaving an evaporator −The air entering the evaporator = 99.01 gr/lb− 53.26 gr/lb = 45.75 gr/lb

Describe a pitot tube and how it measures velocity pressure.

The pilot tube is constructed as two concentric tubes and is bent into a 90-degree angle. The open end of the bent tube is inserted into the duct section with the open end directed into the air stream. how it measures velocity? -The velocity pressure reading is obtained by subtracting the static pressure from the total pressure. This is accomplished at the manometer.

Name the two unknowns that are easiest to obtain for making plots on the psychometric chart.

WB & DB

The outside design temperatures for summer in Tucson, Arizona, for 2.5% of the time are _______°F dry-bulb and _______°F wet-bulb.

_71_______°F dry-bulb and ___66__

The large interconnecting line between the indoor and outdoor coil is the ________ line.

_COOL_

The four major components of an air-conditioning system are the ________, _________, ______, and __________.

_EVAPORATOR_________, __COMPRESSOR________, ____CONDENSER______, and __METERING DEVICE________.

The dew point temperature tells you how much_______ is in the air.

_HUMIDITY__

37.15 Duct Insulation

a. A 15°F temperature difference from the inside of the duct to the outside of the duct is considered the maximum difference allowed before insulation is necessary. b. Insulation is built into a fiberglass duct by the manufacturer. c. Metal duct can be insulated on the outside and on the inside. d. 2-in. thickness is the most common insulation. e. The insulation is joined by lapping it, stapling it, and taping it.

37.19 Measuring Air Movement for Balancing

a. Air balancing is accomplished by measuring the air leaving each register. b. Air quantity in a duct can be measured by using various instruments. c. Traversing patterns for different duct shapes 1. Measuring velocity of the duct in a cross section of the duct; an average of the readings taken is used for the calculation. 2. Once the average velocity of the air is determined in the duct, determine the cfm by using the formula: cfm 5 Area (in ft2) 3 Velocity (in fpm).

37.20 The Air Friction Chart

a. Air friction charts are used by system designers to size ductwork and duct systems. b. The charts give recommended duct sizes and velocities for optimum performance. c. Friction charts can be used to troubleshoot airflow problems. d. Pressure drops in duct fittings have equivalent lengths assigned to them. e. All lengths and equivalent lengths are added together to achieve the total.

37.1 Conditioning Equipment

a. Air has to be conditioned in most cases for us to be comfortable. b. One way of conditioning air uses a blower to move air over the conditioning equipment. c. Equipment includes: 1. Cooling coil. 2. Heating device. 3. Device to add humidity. 4. Device to clean air. d. Forced-air systems use the same room air over and over again. e. Fresh air enters the structure by infiltration around windows and doors. f. Fresh air can enter by ventilation from a fresh-air inlet connected to the outside. g. The forced-air system is different from a natural-draft system. 1. Air passes naturally over the conditioning equipment. 2. There is very little concern for the amount of air moving in a natural convection system.

37.14 Zoning

a. Allows a single forced-air system to maintain different temperatures in different portions of the structure. b. Utilizes thermostatically controlled zone dampers. c. Each zone has its own damper arrangement. d. Zone thermostats and dampers are wired to a central control panel. e. Zoning with a single-speed blower motor: 1. Problems can arise if a single-speed blower is used. 2. If too many zone dampers are closed, excessive velocity and noise can result. 3. Reduction in airflow can cause cooling coils to freeze. 4. Slightly oversizing the ducts and using a bypass duct help reduce the negative effects. f. Zoning with a multispeed compressor and a variable-speed blower: 1. Very desirable system configuration. 2. Computer-controlled panels. 3. Computers store information about each zone. 4. Advanced systems can provide heating and cooling at the same time to different zones. accessible website, in whole or in part. g. Adding zoning to an existing system: 1. Adding zoning is a popular option for existing systems. 2. Dampers can be added to existing air distribution systems. 3. Many new air handlers are manufactured with variable-speed blowers, making the transition to a zoned system easier.

37.7 Types of Fans and Blowers

a. Blowers, or fans as they are sometimes called, produce airflow and movement. b. Blowers are not positive-displacement air movers. c. Two main types of blowers: 1. Propeller fan 2. Forward curved centrifugal blower (referred to as squirrel cage blower or blower wheel) d. Propeller fan: 1. Used in exhaust fan and condenser fan applications 2. Will handle large volumes of air at low pressure differentials 3. Can be made of cast iron, aluminum, stamped steel 4. Set into a housing called a venturi 5. The venturi forces airflow in a straight line from one side of The fan to the other 6. Makes noise and is used where noise is not a factor e. Squirrel cage or centrifugal blower: 1. Desirable for ductwork 2. Builds more pressure from the inlet to the outlet 3. Moves more air against more pressure 4. Has a forward curved blade and a cutoff to shear the air spinning around the blower wheel 5. Very quiet when properly applied 6. Can be used in very large high-pressure systems 7. High-pressure systems have pressures of1 in. WC or more 8. The current draw of the blower motor is proportional to the number pounds of air it moves 9. The blower motor rated FLA (full-load amperage) is proportional to the capacity of air the fan is designed to move 10. If the current draw on the blower motor is down, the airflow is down 11. If the airflow is increased, the current draw of the motor will increase

37.3 The Forced-Air System

a. Components that make up the forced-air system: 1. The blower. 2. Air supply system. 3. Return-air system. 4. Grilles and registers. b. Correctly chosen components will result in the following: 1. No air movement will be felt by the occupants. 2. No air noise will be noticed. 3. No temperature swings will be felt. 4. Occupants will not be aware if the system is on or off.

37.13 Balancing Dampers

a. Dampers are used to balance the air in various parts of the system. b. Dampers should be located as close as practical to the trunk line. c. Near the trunk is the place to balance airflow. d. Handles allow the dampers to be turned at an angle to the airstream to slow the air down.

37.9 The Supply Duct System

a. Distributes air to the terminal units, registers, or diffusers in the conditioned space. b. Vibration eliminators are recommended on all duct installations. c. Duct systems: 1. Plenum system 2. Extended plenum system 3. Reducing plenum system 4. Perimeter loop d. The plenum system: 1. Plenum systems have an individual supply system. 2. Plenums systems are suited for a job where the room outlets are all close to the unit. 3. Supply diffusers are normally located on the inside walls. 4. Diffusers are used for heating systems that have very warm or hot air as the heating source. 5. Work better on fossil-fuel systems. 6. Supply-air temperature on a heat pump without strip heat is rarely more than 100°F. 7. Fossil-fuel supply-air temperatures could easily reach 130°F. 8. Return-air system could be a single return located at the air handler. e. The extended plenum system: 1. Extended systems can be applied to a long structure. 2. This system takes the plenum closer to the farthest point. 3. The systems are called the trunk duct system. 4. The ducts can be round, square, or rectangular. 5. Ducts called branches complete the connection to the terminal units. f. The reducing plenum system: 1. Reduces the trunk duct size as branch ducts are added. 2. Has the advantage of saving material and keeping the same pressure from one end of the duct system to the other. g. The perimeter loop system: 1. The perimeter system is well suited for installation in a concrete floor in a colder climate. 2. The system can be run under the slab close to the outer walls with the outlets next to the wall. 3. Warm air is in the whole loop when the furnace fan is running. 4. The perimeter system keeps the slab at a more even temperature. 5. Loop has a constant pressure around the system. 6. The system provides the same pressure to all outlets.

37.5 System Pressures

a. Duct system is pressurized by three pressures: 1. Static pressure 2. Velocity pressure 3. Total pressure b. Static pressure is the pressure of the air pushing against the walls of the duct sections. c. The velocity of the air and the weight of air moving in the ductwork create velocity pressure. d. Static pressure plus velocity pressure equals total pressure.

37.16 Blending the Conditioned Air with Room Air

a. When possible, air should be directed onto the walls. b. The diffuser spreads the air to the desired air pattern. c. Warm air for heating distributes better from the floor. d. Cool air distributes better from the ceiling. e. Place diffusers next to the outside walls. f. How far the air will be blown from the diffuser into the room depends on the air pressure behind the diffuser and the style of the diffuser blades.

37.6 Air-Measuring Instruments for Duct Systems

a.Velometer—Measures actual air velocity (how fast the air is actually moving in the duct). b.Pitot tube—Used with special manometers for checking duct pressure. c. Some devices can measure temperature and relative humidity as well as velocity. d. Some devices can calculate the actual cfm if the duct size is entered into the instrument.

37.11 Duct Materials

a. Ductwork must meet local codes. b. For years, galvanized sheet metal was used exclusively. c. Other ductwork materials: 1. Aluminum 2. Fiberglass ductboard 3. Spiral metal duct 4. Flexible duct d. Galvanized-steel duct: 1. Gauge is the measurement of the thickness of galvanized steel duct. 2. Metal duct can be round, square, or rectangular. 3. Fasteners for metal duct: a. Drawbands b. Wedge-type connection c. Drive cleats d. S-fasteners (slips) e. Self-tapping sheet metal screws f. Fiberglass duct: 1. Styles: a. Flat sheet b. Round prefabricated cut 2. Duct is normally 1 to 1.5 in. thick with aluminum foil backing. 3. Advantage is that the insulation is already on the duct. 4. Fiberglass duct is easily transported and assembled in the field. 5. Special knives are used to make special cuts to turn ductboard into ductwork. 6. All duct seams should be stapled and taped. g. Spiral metal duct: 1. Used more on large systems. 2. Comes in rolls of flat, narrow metal. 3. Runs can be made at the job site. h. Flexible duct: 1. Round duct comes in sizes up to about24 in. in diameter. 2. Some have a reinforced aluminum foil backing. 3. Some come with vinyl or foil backing and insulation on it, in lengths up to25 ft. 4. Keep duct runs as short as possible. 5. Do not allow turns that may collapse the duct. 6. Has more friction loss inside it than metal duct 7. Flex duct should be stretched as tight as possible. i. Combination duct systems: 1. All square or rectangular duct. 2. Metal trunk lines with round branch ducts. 3. Metal trunk lines with round fiberglass branch ducts. 4. Metal trunk lines with flexible branch ducts. 5. Ductboard trunk lines with round fiberglass branch ducts. 6. Ductboard trunk lines with round metal branch ducts. 7. Ductboard trunk lines with flexible branch ducts. 8. All round metal duct with round metal branch ducts. 9. All round metal trunk lines with flexible branch ducts

7.11 Duct Materials

a. Ductwork must meet local codes. b. For years, galvanized sheet metal was used exclusively. c. Other ductwork materials: 1. Aluminum 2. Fiberglass ductboard 3. Spiral metal duct 4. Flexible duct d. Galvanized-steel duct: 1. Gauge is the measurement of the thickness of galvanized steel duct. 2. Metal duct can be round, square, or rectangular. 3. Fasteners for metal duct: a. Drawbands b. Wedge-type connection c. Drive cleats d. S-fasteners (slips) e. Self-tapping sheet metal screws f. Fiberglass duct: 1. Styles: a. Flat sheet b. Round prefabricated cut 2. Duct is normally 1 to 1.5 in. thick with aluminum foil backing. 3. Advantage is that the insulation is already on the duct. 4. Fiberglass duct is easily transported and assembled in the field. 5. Special knives are used to make special cuts to turn ductboard into ductwork. 6. All duct seams should be stapled and taped. g. Spiral metal duct: 1. Used more on large systems. 2. Comes in rolls of flat, narrow metal. 3. Runs can be made at the job site. h. Flexible duct: 1. Round duct comes in sizes up to about24 in. in diameter. 2. Some have a reinforced aluminum foil backing. 3. Some come with vinyl or foil backing and insulation on it, in lengths up to25 ft. 4. Keep duct runs as short as possible. 5. Do not allow turns that may collapse the duct. 6. Has more friction loss inside it than metal duct 7. Flex duct should be stretched as tight as possible. i. Combination duct systems: 1. All square or rectangular duct. 2. Metal trunk lines with round branch ducts. 3. Metal trunk lines with round fiberglass branch ducts. 4. Metal trunk lines with flexible branch ducts. 5. Ductboard trunk lines with round fiberglass branch ducts. 6. Ductboard trunk lines with round metal branch ducts. 7. Ductboard trunk lines with flexible branch ducts. 8. All round metal duct with round metal branch ducts. 9. All round metal trunk lines with flexible branch ducts

37.18 Sizing Duct for Moving Air

a. Friction loss in ductwork is due to the actual rubbing action of the air against the side of the duct and the turbulence of the air rubbing against itself while moving down the duct. b. The smoother the duct's interior surface, the less friction there is. c. The slower the air is moving, the less friction there will be. d. Each foot of duct offers a known resistance to airflow (friction loss).

37.17 The Return-Air Duct System

a. Individual return-air systems have a return air grille in each room that has a supply diffuser. b. Normally, return grilles are not located in restrooms or kitchens. c. An individual return-air system will give the most positive return air. d. The return-air duct is normally sized slightly larger than the supply duct. e. Central return systems are usually satisfactory for a one-level residence. f. Larger return-air grilles are used in a central return-air system. g. In a central return system, a path must be provided for the air to return to the central return. h. Providing a path for air return on a central air system can be created by: 1. putting return grilles in doors. 2. open doorways. 3. an undercut door. i. Return-air plenums should not be located on the furnace because of noise. j. The return-air grille should be around an elbow from the furnace. k. Insulating a return-air plenum is another way of deadening fan noise.

37.4 The Blower

a. Provides the pressure difference to force the air into the duct system, through the grilles and registers, and into the room. b. Typically, 400 cfm of air must be moved per minute per ton of air-conditioning. c. Pressure in the ductwork is measured in inches of water column (in. WC). d. Air pressure in the ductwork is measured with a manometer.

37.21 Practical Troubleshooting Techniques

a. Residential and commercial systems. b. Residential systems are not checked with instruments.

37.12 Duct Air Movement

a. Special attention should be given to the point where the branch duct leaves the main trunk duct. b. This is to ensure that you get the correct amount of air into the branch duct. c. Branch ducts are fastened to the main trunk by a takeoff fitting. d. The takeoff encourages the air moving down the duct to enter the takeoff to the branch duct. e. Air moving in the duct has inertia—meaning it wants to move in a straight line. f. Turns in the duct should be carefully designed. g. Using turning vanes will improve the airflow around corners.

37.10 Duct System Standards

a. States and localities should have some minimum standards for air-conditioning and heating systems. b. Standards are called codes. c. Codes are not standard across the nation.

37.2 Correct Air Quantity

a. The object of the forced-air system is to deliver the correct quantity of conditioned air to the occupied space. b. Different spaces require different air quantities. c. Same structure may have several different cooling requirements.