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START-UP – CHECKING AIRFLOW
The air distribution system has the greatest effect on
airfl ow. The duct system is totally controlled by the
contractor. For this reason, the contractor should use
only industry-recognized procedures. The correct air
quantity is critical to air conditioning systems. Proper
operation, effi ciency, compressor life, and humid-
ity control depend on the correct balance between
indoor load and outdoor unit capacity. Excessive
indoor airfl ow increases the possibility of high humid-
ity problems. Low indoor airfl ow reduces total capacity
and causes coil icing. Serious harm can be done to
the compressor by low airfl ow, such as that caused
by refrigerant fl ooding. Heat pump systems require a
specifi ed airfl ow. Each ton of cooling requires between
375 and 450 cubic feet of air per minute (CFM). See
the manufacturerʼs spec sheet for rated airfl ow for the
system being installed. Duct design and construction
should be carefully done. System performance can be
lowered dramatically through bad planning or work-
manship. Air supply diffusers must be selected and
located carefully. They must be sized and positioned
to deliver treated air along the perimeter of the space.
If they are too small for their intended airfl ow, they be-
come noisy. If they are not located properly, they cause
drafts. Return air grilles must be properly sized to
carry air back to the blower. If they are too small, they
also cause noise. The installers should balance the air
distribution system to ensure proper quiet airfl ow to all
rooms in the home. This ensures a comfortable living
space.
These simple mathematical formulas can be used to
determine the CFM in a residential or light commercial
system. Electric resistance heaters can use:
CFM = volts x amps x 3.413
SHC x temp rise
Gas furnaces can use:
CFM = Output Capacity in BTUH*
SHC x temp rise
*Refer to furnace data plate for furnace output capac-
ity. SHC = Sensible Heat Constant (see table below),
an air velocity meter or airfl ow hood can give a more
accurate reading of the system CFM. The measure-
ment for temperature rise should be performed at the
indoor coil inlet and near the outlet, but out of direct
line of sight of the heater element or heat exchanger.
For best results, measure air temperature at multiple
points and average the measurements to obtain coil
inlet and outlet temperatures.
Start-Up
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