Duct Work
The duct system should follow the design standards of Air Conditioning Contractors of
America (ACCA) or ASHRAE. The duct system should be sized for the maximum CFM capabilities
of the furnace being installed.
NOTICE
All trunk lines, take-offs, registers and grill free areas must be figured when determining the air
handling capacity of a duct system. By utilizing the Tables 3 through 5, one can obtain the necessary
duct system size. Use a supplier's catalog for proper sizing of outlet and return air registers to insure
that the register will meet the CFM requirements of the run to which it is connected. Do not exceed
the recommended flow rate. The pressure drop for each should not exceed 0.05 inch water column.
The
return air ducts should equal the warm air duct system in CFM capacities. Avoid locating a return
air duct in rooms that may contain undue odors. Use only a return air filter mounted to or integral to
the furnace. Do not add additonal filters unless the duct system is sized to allow for the additional
pressure drop.
An open return in a basement does not meet the requirements of return air.
Instruct the homeowner not to block any returns.
Always check the size of existing ducts, particularly if you are adding air conditioning. The pressure
drop through the cooling evaporator coil reduces available air flow. If the ducts are too small the
system may not work satisfactorily or be noisy on either heat or cooling.
If the furnace is used in connection with summer air conditioning, the furnace should be installed
parallel with or on the upstream side of the evaporator coil to avoid condensation in the furnace heat
exchanger. The evaporator coil must be installed at least 6” above the heat exchanger for proper air
flow. Distances less than 6” will result in decreased air flow. In all cases refer to the manufacturers
data for static pressure losses to ensure the total system static pressure does not exceed 0.5” WC. If
the cooling unit is installed with a parallel flow arrangement, dampers or other means used to control
flow of air should be provided to prevent chilled air from entering the furnace. If such a damper is
manually operated, it must be equipped with a means to prevent operation of either unit, unless the
damper is in the full heat or cool position.
NOTE: When a return register is located in the same room as the furnace, the register must be at least 20 feet away from
the furnace.
Determining Air Flow CFM
The temperature rise through the furnace should not exceed the rated temperature rise as listed
on the Rating Label (Typically75
0
F) and should be at least 55
0
F for comfort.
The sensible heat temperature change for cooling would be approximately 30
0
F. Actual temperature
change will be approximately 20
0
F due to the humidity in the air.
To calculate the sensible heat change or temperature rise the following formula applies
∆
T = (Btuh – Output)/(1.1 x CFM)
To calculate the air flow when you know the temperature rise the following formula applies
CFM = (Btuh – Output)/ (1.1 x
∆
T)
An estimate of air flow can be achieved by the following rules of thumb:
Heating:
1300 CFM per 100,000 Btuh output
Cooling:
400 CFM per ton of air conditioning
Determine the required air flow for the system based on both heating and cooling requirements. Use
the larger of either for duct design.
Table 3 lists the maximum recommended air velocities for ducts. Velocities greater than those as
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