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FRESH AIR
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COMBUSTION
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VENTILATION
Continued
DETERMINING FRESH-AIR FLOW FOR HEATER LOCATION
Determining if You Have a Confined or Unconfined Space
Use this worksheet to determine if you have a confined or unconfined space.
Space: Includes the room in which you will install heater plus any adjoining rooms with
doorless passageways or ventilation grills between the rooms.
1. Determine the volume of the space (length x width x height).
Length x Width x Height = ___________________ cu. ft. (volume of space)
Example:
Space size 20 ft. (length) x 16 ft. (width) x 8 ft. (ceiling height) =
2560 cu. ft. (volume of space)
If additional ventilation to adjoining room is supplied with grills or openings, add the
volume of these rooms to the total volume of the space.
2. Divide the space volume by 50 cubic feet to determine the maximum Btu/Hr the space
can support.
____________ (volume of space) ÷ 50 cu. ft. = (Maximum Btu/Hr
the space can support)
Example:
2560 cu. ft. (volume of space) ÷ 50 cu. ft. = 51.2 or 51,200 (maximum
Btu/Hr the space can support)
3. Add the Btu/Hr of all fuel burning appliances in the space.
Vent-free heater ___________________ Btu/Hr
Gas water heater* ___________________ Btu/Hr
Gas furnace ___________________ Btu/Hr
Vented gas heater ___________________ Btu/Hr
Gas fireplace logs ___________________ Btu/Hr
Other gas appliances* + ___________________ Btu/Hr
Total = ___________________ Btu/Hr
Example:
Gas water heater 40,000 Btu/Hr
Vent-free heater + 18,000 Btu/Hr
Total = 58,000 Btu/Hr
* Do not include direct-vent gas appliances. Direct-vent draws combustion air from the
outdoors and vents to the outdoors.
4. Compare the maximum Btu/Hr the space can support with the actual amount of Btu/Hr
used.
_________________ Btu/Hr (maximum the space can support)
_________________ Btu/Hr (actual amount of Btu/Hr used)
Example:
51,200 Btu/Hr (maximum the space can support)
58,000 Btu/Hr (actual amount of Btu/Hr used)
The space in the above example is a confined space because the actual Btu/Hr used is more
than the maximum Btu/Hr the space can support. You must provide additional fresh air.
Your options are as follows:
A. Rework worksheet, adding the space of an adjoining room. If the extra space provides
an unconfined space, remove door to adjoining room or add ventilation grills between
rooms. See Ventilation Air From Inside Building, page 7.
B. Vent room directly to the outdoors. See Ventilation Air From Outdoors, page 8.
C. Install a lower Btu/Hr heater, if lower Btu/Hr size makes room unconfined.
If the actual Btu/Hr used is less than the maximum Btu/Hr the space can support, the space
is an unconfined space. You will need no additional fresh air ventilation.