One reason for using a reducing plenum
system is to keep ____.
a. the velocity of the air constant
b. the noise level low
c. the duct pressures constant
d. both a and c
Answer : D
By design, which of the following air ducts will
allow 250 cfm to flow with a velocity of 500 ft/min?
a. 9" � 8"
b. 16" � 9"
c. 10" � 9"
d. none of the above
Answer : A
Technician "A" says that if the air filter on an
air-conditioning system is totally blocked with dirt, the amperage draw of the evaporator fan motor will go down. Technician "B" says that if you close down one supply register on a duct system, the volume
a. Technician "A" is correct. Technician "B" is correct.
b. Technician "A" is correct. Technician "B" is incorrect.
c. Technician "A" is incorrect. Technician "B" is correct.
d. Technician "A" is incorrect. Technician "B" is incorrect
Answer : A
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?
a. 10 feet
b. 30 feet
c. 60 feet
d. 20 feet
Answer : B
The pressure in a duct system is measured in:
a. psig
b. psia
c. " Hg
d. " WC
Answer : D
Measuring velocity pressure in a duct
indicates the ____.
a. volume of air through a duct
b. speed at which the air is moving
c. pressure against the sides of the duct
d. weight of the air moving through the duct
Answer : B
If the resistance against a centrifugal blower
is doubled, the cfm output decreases, and the
amperage draw of the motor ____.
a. decreases slightly
b. increases slightly
c. stays the same
d. overheats the motor
Answer : A
If the resistance against a propeller fan is
doubled, the cfm output decreases, but the amperage draw of the motor ____.
a. decreases
b. increases
c. stays the same
d. overheats the motor
Answer : B
Which of the following is typically measured
with an inclined water manometer?
a. static pressure in a duct system
b. pressure in a refrigeration system
c. manifold pressure in a gas fired furnace
d. nozzle pressure of an oil burner
Answer : A
The reason for using a reducing extended
plenum system is to ____,
a. save metal by using a smaller duct
b. reduce the operating cost of the system
c. provide easier means of fastening the take off fittings
d. maintain a more even pressure throughout the entire length of the trunk duct
Answer : D
The fitting at the end of a round duct that
connects the duct to the floor register is called a(n) ____.
a. boot
b. reducer
c. take off
d. stack head
Answer : A
When a canvas collar (vibration eliminator) is
properly installed between two duct sections, the metal surfaces of the duct-work should be rubbing against each other.
Answer : False
Ideally, the sum of all the air volumes feeding
each area of a conditioned space should equal the tonnage of the system multiplied by four hundred
Answer : True
The volume of air that an air conditioning
system can deliver can be increased by adjusting the belt tension on the blower.
Answer : False
To put trunk duct or extended plenum sections
together, the device used is called a drive clip.
Answer : True
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.
Answer : False
The static pressure in a duct is the total
pressure less the velocity pressure
Answer : True
The free area of most registers and grills is
65% of the total face area.
Answer : True
Over-sized duct systems cost more to
fabricate and install and will also result in the slowing of the air as it flows through the duct.
Answer : True
The main drawback of the extended plenum
system is that the velocity of the air is lower as the air reaches the last takeoffs on the extended plenum
Answer : True
In a typical air conditioning system, ___ cfm of air is moved through the
evaporator for each 12,000 Btu/h cooling capacity of the system.
Answer : 400 cfm
In a residential air conditioning system, the
only air moving device that is used is a
_____________.
Answer : Centrifugal Fan
To connect a branch duct to the main duct, a
___________ fitting is used.
Answer : Take Off
The________ of a section of sheet metal is
directly related to its weight.
Answer :Thickness
Branch ducts should always use what
component to balance the air to the individual areas?
Answer : Balancing Dampers
Which type of fan creates more pressure
from inlet to outlet and is more desirable for a commercial duct system?
Answer : Centrifugal Fan
A duct system is pressurized by what three
pressures?
Answer : Static, Velocity & Total Pressure.
Why is inches of water column used to
measure duct pressure instead of psig?
Answer : The Pressure
is too small to be measured by PSIG.
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?
Answer : 12x24=288/144= 2 tons
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?
Answer : The larger the duct the slower the speed of the air, the less
noise it will produce.
A 24 ft. x 37 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?
Answer : Reducing extended plenum
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?
Answer : The loop has a constant pressure around
the system & provides the same pressure to all outlets.
Floor diffusers along the outer wall of the
room are the best way to introduce treated air into a room. Why?
Answer : This is where the heat exchange occurs, placed next to the outside wall to accomplish
the load cancelling effect. cold air kept in & hot air kept out. depending on the season.
When is insulation required on air ducts?
Answer : A 15 degree temp. difference from the inside to the
out -side of the duct is the maximum difference before insulation is required.
If the velocity of air through an 8" x 20" duct
is 700 fpm, how many cfm of air is being carried by the duct?
Answer : 8x20=160/144=1.11x700=778