Duct, Duct, Goose
the pressure in a duct system is measured in
''WC
the design static pressure for the return duct system is what
.05
the design static pressure for a residential duct system on the supply is what
.10
in a given duct the air flow rate is increased from 600 cfm to 800 cfm. what is the increase in pressure loss per 100 feet if the original loss was .08'' of WC
.14'' WC
the air velocity of a 16'' duct is 800 feet per minute, what is the cubic feet per minute
1100 cfm
what is an ac systems capacity in tons, if the supply air is moving through the 12x24 main supply duct at a velocity of 400 cfm
2 tons, 800 cfm
temperature drop across an ac coil is approximately ________ degrees
20 Degrees
air velocity for air filter should be around_________fpm
250 to 350
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?
30
in a typical ac system,_________ cfm of air is moved through the evaporator for each 12000 btu/hour cooling capacity of the system
400
designed velocity in feet per minute in the main trunk duct is
700 to 1800 FPM
what is the system cfm if a 240 volt heater has 20 amps of current and the temperature rise across the heater is 20 degrees
758 CFM
if the velocity of air through an 8X20 duct is 700 fpm, how many cfm of air is being carried by the duct
778 cfm
by design, which of the following air ducts will allow 250 cfm to flow with a velocity of 500 ft/min?
9''x8''
describe the 3 major factors which determine the number, location, and type of supply outlets
CFM, room size, usage
24 gage metal is Less thick than 28 Gage
False
Because the blower in the air handler (furnace) draws the air down the return air duct system, the return air duct is the same size as the supply duct system.
False
air filters work better with more air velocity across the filter
False
a duct system is pressurized by what 3 pressures
Static, Velocity, Total
ideally the sum of all the air volumes feeding each area of a conditioned space should equal the tonnage of the system multiplied by 400
TRUE
To put trunk duct or extended plenum sections together, the device used is called a drive clip.
True
on an extended plenum duct system, supply air ducts should be no longer than 24 feet
True
return air duct systems are designed with less resistance than the supply air ducting
True
the free area of most registers and grills is 80% of the total face area
True
the static pressure in a duct is the total pressure less the velocity pressure
True
velocity pressure acts in the direction of air flow only
True
high sidewall registers work best in
WARM climates for COOLING
each fitting and transition used in a duct system has
a friction loss equal to a predetermined length of same size straight duct
friction losses in a duct system are referred to as
all of the above
cfm can be calculated by multiplying the ______by the _______
area in square feet/velocity in feet per min
one reason for using a reducing plenum system is to keep
both A and C
transitions are used to change
both b and c
tech A says that if the air filter on a AC system is totally blocked with dirt, the amp draw of the evaporator fan motor will go down. tech B says that if you close down one supply register on a duct system, the volume of air feeding the other locations will increase slightly.
both correct
explain the proper way to install a flex duct to a supply grille
connect duct to collar, reinstall insulation, use ziptie with tool, install tape or mastic
Branch ducts should always use what component to balance the air to the individual areas?
dampers
if the resistance against a centrifugal blower is doubled, the cfm output decreases, and the amp draw of the motor
decreases slightly
the fitting at the end of a duct that connects the duct to the floor register is called an
end boot
name 3 item that remove air from a home
exhaust fans, combustion air, fire places
air velocity is measured in
feet per min
in a residential air condition system, the only air moving device that is used is a
forward curved centrifugal blower
what factors can result in noise problems in a duct system
high air velocity, poor air return location
list four factors that result in the decrease in the pressure of the airflow due to friction loss
high air velocity. larger air flow, long duct, changes in direction of air flow, sudden contraction or expansion
when is insulation required on air ducts
if the air temperature is 15 degrees different from the air in the duct
what can cause room air statification
inadequate air velocity or bad location
what instrument is used to measure duct system pressures?
inclined manometer
what is the difference between infiltration air and ventilation air
infiltration is uncontrolled air coming into home, ventilation is controlled air coming into home
the reason for using a reducing extending plenum system is to
maintain a more even pressure throughout the entire length of the trunk duct
what is used to seal small air leaks in the duct system
mastic duct sealer
within an air distribution system, the highest pressure level is found at the
output of the blower
describe how you would balance a residential duct system
place a thermostat in each room,
why is inches of water column used to measure duct pressures instead of psig?
psig is a larger scale and the small duct pressures would not register accurately, therefore, a much smaller scale (inches of water column) is used
a 24X37 ft single story house with a basement has the heating unit in one end of the basement. what type of supply duct system would be used in this application
reducing extended plenum system
what is a stack head used for
return air register box
list what steps you would use to design a duct system for a residential heating/cooling job
select type of duct, select supply and return locations, determine total cfm, size supply and return duct
the duct material that has the least friction loss is
sheet metal
90 degree elbows in rectangular ducts can be classified in 2 ways.
short and broad way. short way makes smaller duct dimensions/broad way makes longer duct dimension
measuring velocity pressure in a duct indicates the
speed at which the air is moving
which of the following is typically measured with an inclined water manometer
static pressure in a duct system
To connect a branch duct to the main duct, a ___________ fitting is used.
take off
name 3 methods for calculating air flow in cfm in a duct system
temperature rise method, manufactures blower performance data, multiply duct area and duct velocity
floor diffusers along the outer wall of the room are the best way to introduce treated air into a room. why?
the air is introduced at the point of the heating and cooling load which counteracts the load before is has a chance to affect the comfort conditions in the area
when a filter rack return grill is located in the living space why is it a good idea to make the return duct as large as possible?
the larger the duct, the slower the air speed, the noise level will be lower
what can you do to reduce air flow and duct noises
use flex duct
draw a picture of a pittsburg seam and describe what it is used for
used to close duct
a home on a concrete slab has a perimeter loop air distribution system. what is the important advantage of this system over an overhead system
warm air at the outer edge of the slab keeps the outer area of the slab warm
can a 6'' flex duct carry the came cfm as a 6'' metal duct?
yes, but blower static pressure would have to be increased
