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Step 3—Hazardous Locations
Fig. 5—Installation in a Garage
When furnace is installed in a residential garage, it must be
installed so that burners and ignition sources are located a
minimum of 18 in. above floor. The furnace must be located
or protected to avoid physical damage by vehicles. When
furnace is installed in a public garage, airplane hangar, or
other building having a hazardous atmosphere, unit must be
installed in accordance with requirements of National Fire
Protection Association, Inc. (See Fig. 5.)
AIR FOR COMBUSTION AND VENTILATION
Provisions for adequate combustion and ventilation air must be
provided in accordance with Section 5.3, Air for Combustion and
Ventilation, of the NFGC or applicable provisions of the local
building codes.
Canadian installations must be in accordance with NSCNGPIC
and all authorities having jurisdiction.
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 combus-
tion of fuel. Sufficient air MUST be provided to ensure there will
not be a negative pressure in the equipment room or space. In
addition, a positive seal MUST be made between furnace cabinet
and return-air duct to prevent pulling air from burner area and draft
safeguard opening into the circulating air.
The operation of exhaust fans, kitchen ventilation fans,
clothes dryers, or fireplaces could create a NEGATIVE AIR
PRESSURE CONDITION at the furnace. Make-up air MUST
BE PROVIDED for these devices, in addition to that required
by the furnace.
Combustion and ventilation air requirements are determined by
whether the furnace is located in a CONFINED or UNCONFINED
space.
18-IN. MINIMUM
TO BURNERS
A93044
SUPPLY
INTERIOR
HEATED
SPACE
RETURN AIR
* Minimum opening size is 100 sq in. with
minimum dimensions of 3 in.
†
Minimum of 3 in. when type-B1 vent is used.
Fig. 6—Confined Space: Air for Combustion and
Ventilation from an Indoor Unconfined Space
Step 1—Unconfined Space
An unconfined space has volume of at least 50 cu ft for each 1000
Btuh of total input for all appliances (such as furnaces, clothes
dryers, water heaters, etc.) in the space.
For Example:
58UHV AND 58UXV FURNACES
HIGH-STAGE UNPUT BTUH
60,000
80,000
100,000
117,000
If the unconfined space is of unusually tight construction, air for
combustion and ventilation MUST come from either the outdoors
or spaces freely communicating with the outdoors. Combustion
and ventilation openings must be sized the same as for a confined
space as 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 is defined as a space whose volume is 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 combustion, ventilation, and dilution of flue
gases using 1 of the following methods. (See Fig. 6 or 7, and Table
2.)
5
VENT THROUGH ROOF
AIR
12" MAX
1 SQ IN.
PER 1000
BTUH * IN DOOR
OR WALL
UNCONFINED
SPACE
6 " MIN
(FRONT)
1 SQ IN.
PER 1000
BTUH * IN DOOR
OR WALL
12" MAX
MINIMUM SQ FT WITH
7-1/2 FT CEILING
400
534
667
780
†
A89012
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