CAUTION: Air for combustion must not be contami-
nated by halogen compounds, which include fluoride,
chloride, bromide, and iodide. These elements are found
in aerosol sprays, detergents, bleaches, cleaning solvents,
salts, air fresheners, and other household products.
All fuel-burning equipment must be supplied with air for combus-
tion of the 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 the furnace
cabinet and the return-air duct to prevent pulling air from the
burner area and draft safeguard opening.
CAUTION: The operation of exhaust fans, kitchen ven-
tilation fans, clothes dryers, 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.
The requirements for combustion and ventilation air depend upon
whether the furnace is located in an unconfined or confined space.
A. Unconfined Space
An unconfined space must have at least 50 cubic ft for each 1000
Btuh of input for all the appliances (such as furnaces, clothes
dryer, water heaters, etc.) in the space.
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 sized the same as for a confined
space. A minimum opening with a total of at least 1 sq in. per 5000
Btuh of total input rating for all equipment must be provided.
Return air must not be taken from the room unless an equal or
greater amount of air is supplied to the room.
B. Confined Space
A confined space has volume less than 50 cu ft per 1000 Btuh of
the total input rating for all appliances installed in that space. A
confined space MUST have 2 permanent openings, 1 within 12 in.
of the ceiling and the other within 12 in. of the floor. (See Fig. 2.)
NOTE: In determining the free area of an opening, the blocking
effect of the louvers, grilles, and screens must be considered. If the
free area of a louver or grille design is unknown, it may be
assumed that wood louvers have a 20 percent free area, and metal
louvers or grilles have a 60 percent free area. Screens, when used,
must not be smaller than 1/4-in. mesh. Louvers and grilles must be
constructed so they cannot be closed.
The size of the openings depends upon whether the air comes from
inside or outside of the structure.
1. All air from inside the structure:
a. Each opening MUST have at least 1 sq in. of free area
per 1000 Btuh of the total input for all equipment within
the confined space, but not less than 100 sq in. per
opening. (See Fig. 2.)
b. If the building is constructed unusually tight, a perma-
nent opening directly communicating with the outdoors
should be provided. This opening shall have a minimum
free area of 1 sq in. per 5000 Btuh of total input rating
for all equipment in the enclosure.
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 the furnace, all air for
combustion must come from outdoors.
2. All air from outside the structure:
a. If combustion air is taken from outdoors through vertical
ducts, the openings and ducts MUST have at least 1 sq
in. of free area per 4000 Btuh of the total input for all
equipment within the confined space. (See Fig. 3.)
b. If combustion air is taken from the outdoors through
horizontal ducts, the openings and ducts MUST have at
least 1 sq in. of free area per 2000 Btuh of the total input
for all equipment within the confined space.
For Example:
395CAV FURNACE
INPUT BTUH MINIMUM SQ FT WITH
7-1/2 FT CEILING
44,000 293
66,000 440
88,000 587
110,000 733
132,000 880
154,000 1026
Fig. 2—Air For Combustion and Ventilation (Inside Air)
A89012
SUPPLY
AIR
6″ MIN
(FRONT) †
RETURN AIR
VENT THROUGH ROOF
1 SQ IN.
PER 1000
BTUH* IN DOOR
OR WALL
12″ MAX
1 SQ IN.
PER 1000
BTUH* IN DOOR
OR WALL
12″ MAX
INTERIOR
HEATED
SPACE
* Minimum opening size is 100 square in. with
minimum dimensions of 3-In.
† Minimum of 3-In. when type-B1 vent is used.
UNCONFINED
SPACE
CONFINED
SPACE
For Example:
395CAV FURNACE
INPUT BTUH FREE AREA PER OPENING
(SQ IN.)
44,000 100
66,000 100
88,000 100
110,000 110
132,000 132
154,000 154
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