Step 2—Air For Combustion and Ventilation
Provisions for adequate combustion and ventilation air must be
provided in accordance with Section 5.3 of the NFGC, Air for
Combustion and Ventilation, or applicable provisions of the local
building codes.
Canadian installations must be installed in accordance with NSC-
NGPIC 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 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 blocked vent safeguard opening.
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.
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 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 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.
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, ventila-
tion, and dilution of flue gases using 1 of the following methods.
(See Fig. 3, 4, and Table 2.)
NOTE:
In determining free area of an opening, the blocking
effect of louvers, grilles, and screens must be considered. If free
area of 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 air comes from
outside of the structure or an unconfined space inside the structure.
1. All air from inside the structure requires 2 openings (for
structures not usually tight):
a. Each opening MUST have at least 1 sq in. of free area per
1000 Btuh of total input for all equipment within the
confined space, but not less than 100 sq in. per opening.
(See Fig. 3 and Table 2.) The minimum dimension of air
openings shall not be less than 3 in.
b. If building is constructed unusually tight, a permanent
opening directly communicating with the outdoors shall be
provided. See item 2 below.
c. If furnace is installed on a raised platform to provide a
return-air plenum, and return air is taken directly from
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
methods:
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. 4 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. 4 and Table
2.)
c. If combustion air is taken from outdoors through a single
opening or duct (horizontal or vertical) commencing within
12 in. of the top of the confined space, 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 space
and not less than the sum of the areas of all vent connectors
in the confined space. (See Fig. 4 and Table 2.) Equipment
clearances to the structure shall be at least 1 in. from the
sides and back and 6 in. from the front of the appliances.
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 ducts must not be less than 3 in. (See Fig.
4.)
AIR DUCTS
Step 1—General Requirements
The duct system should be designed and sized according to
accepted national standards such as those published by: Air
Conditioning Contractors Association (ACCA), Sheet Metal and
Air Conditioning Contractors National Association (SMACNA) or
American Society of Heating, Refrigerating and Air Conditioning
Engineers (ASHRAE). Or consult factory The Air Systems Design
Guidelines reference tables available from your local distributor.
The duct system should be sized to handle the required system
design airflow CFM at the design external static pressure.
When a furnace is installed so that the supply ducts carry air to
areas outside the space containing the furnace, the return air must
also be handled by a duct(s) sealed to the furnace casing and
terminating outside the space containing the furnace.
Secure ductwork with proper fasteners for type of ductwork used.
Seal supply- and return-duct connections to furnace with code
approved tape or duct sealer.
Example of unconfined spaces:
58PAV 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
5
