9
House depressurization can cause back drafting or improper
combustion of gas-fired appliances, thereby exposing building
occupants to gas combustion products that could include carbon
monoxide.
If this furnace is to be installed in the same space with other gas
appliances, such as a water heater, ensure there is an adequate
supply of combustion and ventilation air for the other appliances.
Refer to the latest edition of the National Fuel Gas Code NFPA
54/ANSI Z223.1 or CAN/CSA B149 Installation Codes or
applicable provisions of the local building codes for determining
the combustion air requirements for the appliances.
Most homes will require outside air to be supplied to the furnace
area by means of ventilation grilles or ducts connecting directly
to the outdoors or spaces open to the outdoors such as attics or
crawl spaces.
The following information on air for combustion and ventilation is
reproduced from the National Fuel Gas Code NFPA 54/ANSI
Z223.1 Section 5.3.
5.3.1 General:
a) The provisions of 5.3 apply to gas utilization equipment
installed in buildings and which require air for
combustion, ventilation and dilution of flue gases from
within the building. They do not apply to (1) direct vent
equipment which is constructed and installed so that all
air for combustion is obtained from the outside
atmosphere and all flue gases are discharged to the
outside atmosphere, or (2) enclosed furnaces which
incorporate an integral total enclosure and use only
outside air for combustion and dilution of flue gases.
b) Equipment shall be installed in a location in which the
facilities for ventilation permit satisfactory combustion of
gas, proper venting and the maintenance of ambient
temperature at safe limits under normal conditions of use.
Equipment shall be located so as not to interfere with
proper circulation of air. When normal infiltration does not
provide the necessary air, outside air shall be introduced.
c) In addition to air needed for combustion, process air shall
be provided as required for: cooling of equipment or
material, controlling dew point, heating, drying, oxidation
or dilution, safety exhaust, odor control and air for
compressors.
d) In addition to air needed for combustion, air shall be
supplied for ventilation, including all the air required for
comfort and proper working conditions for personnel.
e) While all forms of building construction cannot be
covered in detail, air for combustion, ventilation and
dilution of flue gases for gas utilization equipment vented
by natural draft normally may be obtained by application
of one of the methods covered in 5.3.3 and 5.3.4.
f)
Air requirements for the operation of exhaust fans,
kitchen ventilation systems, clothes dryers and fireplaces
shall be considered in determining the adequacy of a
space to provide combustion air requirements.
5.3.2 Equipment Located in Unconfined Spaces:
In unconfined spaces (see definition below) in buildings,
infiltration may be adequate to provide air for combustion
ventilation and dilution of flue gases. However, in buildings of
tight construction (for example, weather stripping, heavily
insulated, caulked, vapor barrier, etc.), additional air may need to
be provided using the methods described in 5.3.3-b or 5.3.4.
Space, Unconfined.
For purposes of this Code, a space whose volume is not less
than 50 cu ft per 1,000 Btu/h 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.
5.3.3 Equipment Located in Confined Spaces:
a) All Air from Inside the Building: The confined space shall
be provided with 2 permanent openings communicating
directly with an additional room(s) of sufficient volume so
that the combined volume of all spaces meets the criteria
for an unconfined space. The total input of all gas
utilization equipment installed in the combined space
shall be considered in making this determination. Each
opening shall have a minimum free area of 1 sq. in.
(6.5 cm
2
) per 1,000 Btu/h of the total input rating of all gas
utilization equipment in the confined space, but not less
than 100 sq. in (645.2 cm
2
). One opening shall be within
12" (30.5 cm) of the top and one within 12" (30.5 cm) of
the bottom of the enclosure.
Equipment Located in Confined Spaces: All Air from Inside
Building. See 5.3.3-a.
NOTE: Each opening must have a free area of not less than
1 sq. in. (6.5 cm
2
) per 1,000 Btu of the total input rating of all
equipment in the enclosure, but not less than 100 sq. in.
(645.2 cm
2
).
b) All Air from Outdoors: The confined space shall be
provided with 2 permanent openings, one commencing
within 12" (30.5 cm) of the top and one commencing
within 12" (30.5 cm) of the bottom of the enclosure. The
openings shall communicate directly, or by ducts, with
the outdoors or spaces (crawl or attic) that freely
communicate with the outdoors.
1. When directly communicating with the outdoors, each
opening shall have a minimum free area of 1 sq. in.
(6.5 cm
2
) per 4,000 Btu/h of total input rating of all
equipment in the enclosure.
A. Chimney or gas vent
B. Opening(s)
C. Water heater
D. Furnace
A
B
B
C
D
