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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 ounce (30ml) of water for every
1,000 BTU's (.3KW's) of gas input per hour.
Unvented room heaters must be used 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.
PrOVISIONS FOr ADeQuATe cOMBuSTION & VeNTILATION AIr
This heater shall not be installed in a confined space unless provi-
sions are provided for adequate combustion and ventilation air.
A confined space is an area with volume less than 50 cubic feet per
1,000 Btuh of the combined input rates of all appliances drawing
combustion air from that space. Small areas such as equipment
rooms are confined spaces. Furnaces installed in a confined space
which supply heated air to areas outside the space must draw
return air from outside the space through tightly sealed return air
ducts. A confined space must have 2 openings into the space for
combustion air. One opening must be within 12 inches of the ceiling
and the other must be within 12 inches of the floor. The required
sizing of these openings is determined by whether inside or outside
air is used to support combustion, the method by which the air is
brought to the space (vertical or horizontal duct) and by the total
input rate of all appliances in the 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 un-
usually 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. 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
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.
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., International Fuel Gas Code,
or applicable local codes.
31722-1-1213
wArNING
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 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
(an entire house).
Example of Large Room with 1/2 Wall divider.
DIVIDER
FIREPLACE
H
W
The following formula can be used to determine the maximum
heater rating per the definition of unconfined space:
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:
A merican National Standards Institute N ational Fire Protection As-
sociation, Inc.
11 West 42nd St.
New York, NY 10018
L2
L1
Figure 1
Batterymarch Park
Quincy, MA 02269
Page 11
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