When is it acceptable to use indoor air as combustion air for a furnace?
Thursday, June 21, 2018
The gas furnace in new homes in Florida is usually installed in a sealed closet or the garage, with outdoor combustion air supplied by ducts into the attic. But it is still allowed by the International Residential Code (IRC) and Florida Building Code Residential Edition (FBC) to utilize indoor air for furnace combustion air as long it meets these two requirements:
1) The home must have an ACH (Air Changes per Hour) of greater than 0.40. That means the air in the home would be completely changed out every 2-1/2 hours or less. Homes built in the 1990s or earlier definitely meet this standard and often have an ACH closer to 1.0. The ACH climbs as a house ages and cracks accumulate, along with tiny openings caused by minor movement of the structure and deteriorated caulking.
2) The minimum volume of air in the room where the furnace is located must be 50 cubic feet per 1,000 BTU/hr of input or more. That means a 75,000 BTU furnace would require 75 x 50 = 3,750 cu. ft. of air. A 20-foot by 24-foot room with an average 8-foot ceiling, for example, would be acceptable. An adjoining room that is not connected by a door and has a directly connected opening or openings that meets size requirements of the code can be used n the calculation. If the ACH is less than 0.40, you may still be able to use indoor air by applying one of the more complex equations at “G2407.5.2 - Known Air Infiltration Rate” of the codes.
An exception to the indoor combustion air allowance is that a furnace cannot be located in a bedroom, bathroom, or an adjoining closet.
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