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.
The operation of exhaust fans, kitchen ventilation 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.
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 avoid pulling air from the burner
area and draft safeguard opening.
The requirements for combustion and ventilation air depend upon
whether the furnace is located in an unconfined or confined space.
UNCONFINED SPACE — An unconfined space must have at
least 50 cu 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.
CONFINED SPACE — A confined space has volume less than 50
cu ft per 1000 Btuh of the total input ratings of 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, assume 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 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 permanent
opening directly communicating with the outdoors should
be provided. This opening should 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. (See Fig. 3.)
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.
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.
c. When ducts are used, they must be of the same cross-
sectional area as the free area of the openings to which they
connect. The minimum dimension of rectangular ducts
must not be less than 3 in. (See Fig. 3.)
For Example:
58RAV FURNACE
INPUT BTUH
MINIMUM SQ FT
WITH
7-1/2 FT CEILING
46,000
307
69,000
460
92,000
613
115,000
767
135,000
920
Fig. 2—Air for Combustion and Ventilation
(Inside Air)
A93387
RETURN
AIR
6
″
MIN
(FRONT)
†
SUPPLY AIR
VENT THROUGH ROOF
(CATEGORY I)
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 B-1 vent is used.
UNCONFINED
SPACE
For Example:
58RAV FURNACE
INPUT BTUH
FREE AREA PER
OPENING (SQ IN.)
46,000
100
69,000
100
92,000
100
115,000
115
135,000
138
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