Combustion Air Quality
The recommended source of combustion air is to use the outdoor air supply. However, the use of indoor air in most applications is acceptable except as follows:
1.If the furnace is installed in a confined space it is recommended that the necessary combustion air come from the outdoors by way of attic, crawl space, air duct, or direct opening.
2.If outdoor combustion air is used, there must be no exposure to the installations or substances listed in Item 3 below.
3.The following types of installation may re- quire Outdoor Air for combustion, due to chemical exposures:
•Commercial buildings
•Buildings with indoor pools
•Furnaces installed in laundry rooms
•Furnaces installed in hobby or craft rooms
•Furnaces installed near chemical storage areas
Exposure to the following substances in the combustion air supply may also require Outdoor Air for combustion:
•Permanent wave solutions
•Chlorinated waxes and cleaners
•Chlorine based swimming pool chemi- cals
•Water softening chemicals
•
•Carbon tetrachloride
•Halogen type refrigerants
•Cleaning solvents (such as perchloroethy- lene)
•Printing inks, paint removers, varnishes, etc.
•Hydrochloric acid
•Cements and glues
•Antistatic fabric softeners for clothes dry- ers
•Masonry acid washing materials
Air Requirements For One-Pipe Installation
When air for combustion is to be taken from around the furnace, a protective screen must be installed over the combustion air intake opening. This screen is provided with the furnace installa- tion instructions and functions to prevent debris
from entering the combustion system. It should be installed on the combustion air intake collar or inlet PVC. If furnace location is such that this opening might be unintentionally obstructed, a 3” PVC elbow must be installed on the collar, and the screen placed inside the inlet of the elbow. See Figure 10.
Installation In An Unconfined Space
!CAUTION:
“Tight” buildings (with weather strip- ping and caulk to reduce infiltration), may require special provisions for introduction of outside air to ensure satisfactory combustion and venting, even though the furnace is located in an unconfined space.
An unconfined space is an area including all rooms not separated by doors with a volume greater than 50 cubic feet per 1,000 Btuh of the combined input rates of all appliances which draw combustion air from that space. For ex- ample, a space including a water heater rated at 45,000 Btuh and a furnace rated at 75,000 Btuh requires a volume of 6,000 cubic feet [50 x (45 + 75) = 6,000] to be considered unconfined. If the space has an 8 foot ceiling, the floor area of the space must be 750 square feet (6,000
/8 = 750). In general, a furnace installed in an unconfined space will not require outside air for combustion.
!WARNING:
Furnaces installed with combustion air drawn from a heated space which includes exhaust fans, fireplaces, or other devices that may produce a nega- tive pressure should be considered confined space installations.
Installation In A Confined Space
A confined space is one which does not meet the unconfined space volume requirements, and typically involves installation in a small room. All such installations must have specific provisions for introduction of combustion and ventilation air. Codes require that two openings be provided for this - one with bottom edge within 12” of the fl oor and one with top edge within 12” of the ceiling.
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