Thermo Pride CLHS1-050P36N Installation And Operation Manual page 29

All installations and services must be performed by qualified service personnel.
5. The duct system shall be sized for the maximum CFM requirement of the installation whether it is for heating or
cooling. Two common rules for heating and cooling follow:
A. COOLING: 400 CFM (1200 BTU's) per ton of cooling is typically required.
B. HEATING: 14 CFM of heating per 1,000 BTU's of furnace output based on a nominal temperature rise.
EXAMPLE:
Heating output of a furnace is 100,000 BTU: 100 x 14 CFM = 1400 CFM
Air conditioning installed is 4 tons: 4 x 400 CFM = 1600 CFM
NOTE: The duct system must be sized for the larger CFM requirement for cooling. If only 3 tons of cooling were
installed: 3 x 400 CFM = 1200 CFM. The duct would then have to be sized for the 1400 CFM heating requirement.
Figure 11
All trunk lines, take-offs, registers and grill-free areas must be figured when determining the air handling capacity of
a duct system. One can obtain the necessary duct system size by utilizing the chart below. (For example, see Figure
11.) 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.
The main trunk lines, take offs, registers and grills of the supply return air duct system must have an adequate square
inch area to move the desired CFM in order to achieve proper movement.
Each of the system components (trunk lines, take offs, runs and register and grill-free areas) must be properly sized
and matched together to ensure the necessary air handling capacity of a duct system. A 12" x 8" duct with a 400
CFM capacity for example will not flow 400 CFM if the register(s) to which it connects only flows a total of 200
CFM.
The speed of the blower motor may have to be changed to obtain the proper 45°F to 75°F temperature rise for
heating when an air conditioning coil is installed. This depends on the static resistance of an individual duct system
and the size of the air conditioner.
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