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CAUTION:
18-IN. MINIMUM
TO BURNERS
Fig. 16—Leveling Legs
Air for combustion must not be contaminated 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. Failure to follow this caution will result in minor unit damage.
A89014 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.
III.
CONFINED SPACE
A confined space has a volume of 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 which freely communicate with an unconfined space or the outdoors. (See Fig.
17 or 18.)
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 outside of the structure or an unconfined space inside the structure.
A.
All Air from Inside the Structure
1. 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. 17.) The minimum dimension of air openings shall not be less than 3 in.
For Example:
COMBUSTION AIR FROM UNCONFINED SPACE
345MAV Furnace
Input Btuh
Free Area per Opening
(Sq In.)
40,000
60,000
80,000
100,000
120,000
100
100
100
100
120
All fuel-burning equipment must be supplied with air for combustion 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 into the circulating air.
CAUTION: The operation of exhaust fans, kitchen ventilation fans, clothes dryers, fireplaces, or other appliances including attic and crawlspace exhaust fans 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. Failure to follow this caution will result in intermittent unit operation or performance satisfaction.
The requirements for combustion and ventilation air depend upon whether the furnace is located in a CONFINED or UNCONFINED space.
II.
UNCONFINED SPACE
An unconfined space must have at least 50 cu ft for each 1000
Btuh of total input for all the appliances (such as furnaces, clothes dryers, water heaters, etc.) in the space.
For Example:
MINIMUM FLOOR AREA FOR UNCONFINED SPACE
345MAV Furnace
Input Btuh
40,000
Minimum Sq Ft With
7-1/2 Ft Ceiling
267
60,000
80,000
100,000
120,000
400
533
667
800
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
—14—
2. If the building is constructed unusually tight, in addition to the 2 permanent openings that freely communicate with an unconfined space, a permanent opening directly communicating with the outdoors should be provided. This opening shall have a minimum free area of 1 sq in. per 4000 Btuh of total input rating for all equipment in the enclosure.
3. 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. 18.)
B.
All Air from Outside the Structure
1. 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. 18.)
For Example:
COMBUSTION AIR FROM OUTDOORS THROUGH
VERTICAL DUCTS
345MAV Furnace
Input Btuh
Free Area per Opening
(Sq In.)
Round Pipe
(In. Dia)
40,000
60,000
80,000
100,000
120,000
10.0
15.0
20.0
25.0
30.0
4
5
6
6
7
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