When calculating the size unit for a
house, each room is calculated individually.
The rooms are then added up to
determine the total volume of air for the whole house.
It is a simple matter of measuring the
air to each room with a flow hood to determine if the air volume is OK.
If the airflow is out of balance,
adjusting or resizing is necessary.
Sometimes the cause of uneven
temperatures is oversized equipment.
This results in short run cycles which
does not let the blower run long enough for even temperatures.
A fairly new concept which finally has
the bugs worked out is the use of a special variable speed motor on the indoor blower.
The motor uses only about 1/8th the electricity at low speed and gives about 50% of
the normal air volume of high speed. When the thermostat calls for heat or cool the
fan runs on high, and when the thermostat is satisfied, the fan runs on low. This
results in constant air movement 24 hours a day, which keeps the house very even in
temperature. An added benefit to variable speed motors is the ability to maintain
If you have 5 supply registers supplying 200 CFM each, the total is 1000 CFM. With
this system you could close one of the registers and the 4 that remain open would now
supply 250 CFM each for a total of 1000 CFM! Also with a normal blower motor,
as your air filter becomes dirty the CFM of the system goes down because the air flow is
restricted. With a variable system, the motor speeds up automatically to maintain the full
CFM. Eventually if the filter is neglected too long the high velocity air will
create a whistle at the air filter prompting you to replace it. All this time the
A/C has not suffered from low air flow as it would with a conventional fan motor.