When the furnace is installed in a residential garage, the
burners and ignition sources must be located at least 18 inches
above the floor. The furnace must be located or protected to
avoid damage by vehicles. When the furnace is installed in a
public garage, airplane hangar, or other building having a
hazardous atmosphere, the furnace must be installed in
accordance with the NFGC or NSCNGPIC. (See Fig. 6.)
Air for combustion must not be contaminated by halogen
compounds, which include fluoride, chloride, bromide, and
iodide. These elements are found in aerosol sprays, deter-
gents, bleaches, cleaning solvents, salts, air fresheners, and
other household products.
All fuel-burning equipment must be supplied with air for fuel
combustion. Sufficient air must be provided to avoid negative
pressure in the equipment room or space. A positive seal must be
made between the furnace cabinet and the return-air duct to
prevent pulling air from the burner area and from blocked vent
safeguard opening.
The operation of exhaust fans, kitchen ventilation fans,
clothes dryers, attic exhaust fans or fireplaces could create a
NEGATIVE PRESSURE CONDITION at the furnace.
Make-up air MUST be provided for the ventilation devices, in
addition to that required by the furnace. Refer to the Carbon
Monoxide Hazard warning in the venting section of htese
instructions to determine amount of make-up air required.
The requirements for combustion and ventilation air depend upon
whether the furnace is located in an unconfined or confined space.
Step 1—Unconfined Space
An unconfined space has a volume of at least 50 cu ft for each
1000 Btuh total input for all appliances (furnaces, clothes dryers,
water heaters, etc.) in the space.
For example:
If the unconfined space is constructed unusually tight, air for
combustion and ventilation must come from either the outdoors or
spaces freely communicating with the outdoors. Combustion and
ventilation openings must be equivalent to those used for a
confined space (defined below). Return air must not be taken from
the room unless an equal or greater amount of air is supplied to the
room.
Step 2—Confined Space
A confined space has a volume less than 50 cu ft per 1000 Btuh
of total input ratings of all appliances installed in that space. A
confined space must have provisions for supplying air for com-
bustion, ventilation, and dilution of flue gases using one of the
following methods in Table 2 and Fig. 7 and 8.
NOTE:
When determining the free area of an opening, the
blocking effect of louvers, grilles, and screens must be considered.
If the free area of the louver or grille design is unknown, assume
wood louvers have a 20 percent free area and metal louvers or
grilles have a 60 percent free area. Screens must not be smaller
than 1/4-in. mesh. Louvers and grilles must be constructed so they
cannot be closed.
The opening size depends upon whether air comes from outside of
the structure or an unconfined space inside the structure.
1. Air from inside the structure requires 2 openings (for struc-
tures not of unusually tight construction):
a. Each opening must have a minimum free area of not less
than 1 sq in per 1000 Btuh of total input rating for all gas
utilization equipment in the confined space, but not less
than 100 sq in. The minimum dimension of air openings
should be no smaller than 3 in. (See Table 2 and Fig. 7.)
b. If building construction is unusually tight, a permanent
opening directly communicating with the outdoors shall be
provided. (See next section).
c. If the furnace is installed on a raised platform to provide a
return-air plenum, and return air is taken directly from the
hallway or space adjacent to furnace, all air for combustion
must come from outdoors.
2. Air from outside the structure requires 1 of the following:
a. If combustion air is taken from outdoors through 2 vertical
ducts, the openings and ducts must have at least 1 sq in. of
free area per 4000 Btuh of total input for all equipment
within the confined space. (See Fig. 8 and Table 2.)
b. If combustion air is taken from outdoors through 2 hori-
zontal ducts, the openings and ducts must have at least 1 sq
in. of free area per 2000 Btuh of total input for all
equipment within the confined space. (See Fig. 8 and Table
2.)
c. If combustion air is taken from the outdoors through a
single opening or duct (horizontal or vertical) commencing
within 12 in. of the top of the confined space, the opening
and duct must have at least 1 sq in. of free area per 3000
Btuh of the total input for all equipment within the confined
Fig. 6—Installation in a Garage
A93044
18-IN. MINIMUM
TO BURNERS
FURNACE
INPUT (BTUH)
MINIMUM WITH
7–1/2 FT CEILING (SQ. FT.)
66,000
441
88,000
587
110,000
734
132,000
881
154,000
1028
7
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