SIZING THE DUCT WORK FOR A COMBINATION HEATING AND COOLING SYSTEM:
Two formulas must be used in determining the CFM requirements of a combustion heating and cooling system.
1.HEATING CFM:
HEAT OUTPUT OF FURNACE (BTUH)
1.1 X TR (TEMPERATURE RISE, °F) = HEATING(CFM)
EXAMPLES:
A. 110,000 BTUH OUTPUT
1.1X 85°F TR = 1176 CFM FOR HEATING B. 110,000 BTUH OUTPUT
1.1X 70°F TR = 1429 CFM FOR HEATING
2.COOLING CFM: 400 CFM X COOLING TONNAGE (12,000 BTUH PER TON)=AIRFLOW FOR COOLING(CFM)
EXAMPLES:
A.400 CFM X 3 TON (12,000 BTUH) = 1,200 CFM FOR COOLING 1TON
B.400 CFM X 2.5 TON (12,000 BTUH) = 1,000 CFM FOR COOLING 1 TON
IMPORTANT: SIZE THE DUCT SYSTEM FOR THE LARGER OF THE TWO AIRFLOW REQUIREMENTS.
E. Air Filter Mounted External to Furnace:
On highboy furnaces, it is necessary to cut the return air opening in the side, rear casing or base, depending upon the needs of the specific installation.
The filter rack provided with the furnace, refer to Fig. 10a & 10b, will serve as a template to scribe a mark for the return air opening on the casing. Place the filter rack on a side casing approximately one inch up from the bottom of the furnace and centered from side to side. Place the securing flange against the casing when locating the return air opening. For your convenience, (4) locator knockouts have been placed at the proper locations on both the left and right side casings.
PLEASE NOTE: While scribing the return air opening, the filter rack can be held in position by tape or similar temporary means.
Position the open end of the filter rack so as to provide access for filter replacement. Once the filter rack is positioned correctly, scribe a line along the inside of the securing flange on three of the sides. To scribe a line on the fourth side (the open end), use the
Remove the filter rack and cut the return air opening in the casing. Now the filter rack can be anchored to the furnace with screws or
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