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12409-2-0403
WATER VAPOR: A BY-PRODUCT OF UNVENTED ROOM HEATERS
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, includ-
ing ample combustion air and circulation air.
2. If high humidity is experienced, a dehumidifier 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
Exhaust fans, fireplaces, clothes dryers and fuel burning appli-
ances draw air from the house to operate. You must provide
adequate fresh air for these appliances. This will insure proper
venting of vented fuel-burning appliances.
PROVISIONS FOR ADEQUATE COMBUSTION & VENTILATION AIR
This heater shall not be installed in a confined space unless pro-
visions are provided for adequate combustion and ventilation air.
The National Fuel Gas Code defines a confined 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 in-
stalled in that space and an unconfined space as a space whose
volume is not less than 50 cubic feet per 1,000 Btu per hour (4.8
m
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 unconfined 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.
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
or less with openings gasketed or sealed, and
b. Weatherstripping 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
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.
Warning: If the area in which the heater may be operated is
smaller than that defined as an unconfined 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, Section 5.3. or applicable local
codes.
Example of Large Room with 1/2 Wall divider.
Figure 1
The following formula can be used to determine the maximum
heater rating per the definition of unconfined space:
Btu/Hr =
(L
1
+ L
2
)FT x (W)FT x (H)FT
x 1000
50
If the area in which the heater may be operated is smaller than
that defined as an unconfined space, provide adequate combustion
and ventilation air by one of the methods described in the National
Fuel Gas Code, ANSI Z223.1, Section 5.3.
Adhere to all codes, or in their absence, the latest edition of THE
NATIONAL FUEL GAS CODE ANSI Z223.1 or NFPA54 which
can be obtained from:
American National Standards Institute National Fire Protection Association, Inc.
11 West 42nd St. Batterymarch Park
New York, NY 10018 Quincy, MA 02269