Filter
Top Products
26670-0-0110Page 8
Water vapor is a by-product of gas combustion. An unvented room
heater produces approximately one (1) ounce (30ml) of water for
every 1,000 BTU's (.3KW's) of gas input per hour.
Unvented room heaters are recommended as supplemental heat (a
room) rather than a primary heat source (an entire house). In most
supplemental heat applications, the water vapor does not create a
problem. In most applications, the water vapor enhances the low
humidity atmosphere experienced during cold weather.
The following steps will help insure that water vapor does not
become a problem.
1. Be sure the heater is sized properly for the application, including
ample combustion air and circulation air.
2. If high humidity is experienced, a dehumidier may be used to
help lower the water vapor content of the air.
3. Do not use an unvented room heater as the primary heat
source.
This heater shall not be installed in a conned space or unusually
tight construction unless provisions are provided for adequate
combustion and ventilation air.
The National Fuel Gas Code, ANSI Z223.1 denes a conned space
as a space whose volume is less than 50 cubic feet per 1,000 Btu per
hour (4.8m
3
per kw) of the aggregate input rating of all appliances
installed in that space and an unconned space as a space whose
volume is not less than 50 cubic feet per 1,000 Btu per hour (4.8m
3
per kw) of the aggregate input rating of all appliances installed in
that space. Rooms communicating directly with the space in which
the appliances are installed, through openings not furnished with
doors, are considered a part of the unconned space.
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.
where:
a. Walls and ceilings exposed to the outside atmosphere have
a continuous water vapor retarder with a rating of one perm
or less with openings gasketed or sealed, and
b. Weather-stripping has been added on openable windows and
doors, and
c. Caulking or sealants are applied to areas such as joints
around window and door frames, between sole plates and
oors, 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,”
page 9.
Use this worksheet to determine if you have a conned or unconned
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 16 ft. (length) x 10 ft. (width) x 8 ft. (ceiling
height) = 1,280 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: 1,280 cu. ft. (volume of space) ÷ 50 cu. ft. = 25.6 or
25,600 (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 replace logs BTU/Hr
Other gas appliances* + BTU/Hr
Total = BTU/Hr
Example: Vented gas heater 20,000 BTU/Hr
Vent-free heater + 18,000 BTU/Hr
Total = 38,000 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:
25,600 BTU/Hr (maximum the space can support)
38,000 BTU/Hr (actual amount of BTU/Hr used)
Warning: If the area in which the heater may be operated is smaller
than that dened as an unconned space or if the building is of
unusually tight construction, provide adequate combustion and
ventilation air by one of the methods described in the National
Fuel Gas Code, ANSI Z223.1/NFPA 54, Air for Combustion and
Ventilation, or applicable local codes.
WATER VAPOR: A BY-PRODUCT OF UNVENTED ROOM HEATERS
PROVISIONS FOR ADEQUATE COMBUSTION & VENTILATION AIR