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Unusually Tight Construction
The air that leaks around doors and windows may provide enough fresh air for combustion and ventilation.
However, in buildings of unusually tight construction, you must provide additional fresh air. Unusually
tight construction is defined as construction where:
a) Walls and ceilings exposed to the outside atmosphere have a continuous water vapor retarder with a
rating of one perm (6×10-11kg per pa-sec-m
2
) or less with openings gasket or sealed and
b) Weather stripping has been added on openable windows and on doors and
c) Caulking or sealants are applied to areas such as joints around window and door frames, between sole
plates and floors, between wall ceiling joints, between wall panels, at penetrations for plumbing,
electrical, and gas lines, and at other openings.
If your home meets all of the three criteria above, you must provide additional fresh air. See “Ventilation
Air From Outdoors”. If your home does not meet all of the three criteria above, proceed to “Determining
Fresh-Air Flow For Heater Location”.
6. DETERMINING FRESH-AIR FLOW FOR HEATER LOCATION
Using 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 × Width × Height = cu. ft. (volume of space)
Example: Space size 20 ft. (length) × 16 ft. (width)×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)
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 heater logs
BTU/hr
Other gas appliances* +
BTU/hr
Total =
BTU/hr
Example:
Gas water heater 30,000 BTU/hr
Vent-free heater + 26,000 BTU/hr
Total = 56,000 BTU/hr
*Do not include direct-vent gas appliances. Direct-vent draws combustion air from the outdoors and vents to the outdoors
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