Hastings COUNTERFLO CF SERIES, Installation And Service Manual

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The following recommendations are not intended to supplant requirements of federal,
state or local codes having jurisdiction. All local authorities having jurisdiction should be
consulted before the installation is made. The heater should be installed in accordance
with the standard of the National Board of Fire Underwriters for the class and all wiring
connections must conform to the National Electric Code.
C. COMBUSTION AIR OPENINGS
When the heaters are installed in a separate heater room with fresh air and return air ducts
running direct to the base of the heater, or when the plant is under a high negative
pressure condition, provisions should be made for an outside source of combustion air. A
conservative rule of thumb for estimating the inlet area required is 90 square inches per
1,000,000 BTU/HR heater output. The following table lists the recommended free area of
vent opening in the outside wall for various heater sizes.
HEATER SIZE AREA OF COMBUSTION AIR CONNECTING DUCT
MODEL INTAKE OPENING – SQ.INCHES DIAMETER – INCHES
20 18 6
25 22 6
40 36 7
50 45 8
75 68 9
100 90 11
125 113 12
150 135 13
175 158 14
200 180 14
225 202 16
250 225 18
275 248 18
300 270 19
325 292 20
350 315 20
400 360 22
450 405 24
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