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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 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.
This heater shall not be installed in a confined space or unusually
tight construction unless provisions 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
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. 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. See
“Ventilation Air From Outdoors,”
page
8.
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 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 fireplace 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 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/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
