Bryant 310JAV Minimum Free Area of Combustion Air Opening, Unconfined Space, Confined Space, Btuh

Table 2–Minimum Free Area of Combustion Air Opening*

	AIR FROM INDOOR	OUTDOOR AIR THROUGH		OUTDOOR AIR THROUGH		OUTDOOR AIR THROUGH
FURNACE	UNCONFINED SPACE	VERTICAL DUCTS		HORIZONTAL DUCTS		SINGLE DUCT
INPUT	Free Area	Free Area of	Round Pipe	Free Area of	Round Pipe	Free Area of	Round Pipe
(BTUH)	of Opening	Opening and Duct		Opening and Duct		Opening and Duct
			(in. Dia)		(in. Dia)		(In. Dia)
	(Sq In.)	(Sq. In.)		(sq In.)		(Sq In.)
44,000	100	11	4	22	6	14.7	5
66,000	100	16.5	5	33	7	22.0	6
88,000	100	22.0	6	44.0	8	29.3	7
110,000	110	27.5	6	55	9	36.7	7
132,000	132	33	7	66	10	44.0	8
154,000	154	38.5	7	77	10	51.3	8

*Free area shall be equal to or greater than the sum of the areas of all vent connectors in the confined space. Opening area must be increased if other gas appliances in the space require combustion air.

Step 1—Unconfined Space

An unconfined space has a volume of at least 50 cu ft for each 1000 Btuh total input for all appliances (furnaces, clothes dryers, water heaters, etc.) in the space.

For example:

FURNACE	MINIMUM WITH
INPUT (BTUH)	7–1/2 FT CEILING (SQ. FT.)
44,000	294
66,000	441
88,000	587
110,000	734
132,000	881
154,000	1028

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 equivalent to those used for a confined space (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 has a volume 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 com- bustion, ventilation, and dilution of flue gases using one of the following methods in Table 2 and Fig. 7 and 8.

NOTE: When determining the free area of an opening, the blocking effect of louvers, grilles, and screens must be considered. If the free area of the louver or grille design is unknown, assume wood louvers have a 20 percent free area and metal louvers or grilles have a 60 percent free area. Screens must not be smaller than 1/4-in. mesh. Louvers and grilles must be constructed so they cannot be closed.

The opening size depends upon whether air comes from outside of the structure or an unconfined space inside the structure.

1.Air from inside the structure requires 2 openings (for struc- tures not of unusually tight construction):

a.Each opening must have a minimum free area of not less than 1 sq in per 1000 Btuh of total input rating for all gas utilization equipment in the confined space, but not less than 100 sq in. The minimum dimension of air openings should be no smaller than 3 in. (See Table 2 and Fig. 7 ).

b.If building construction is unusually tight, a permanent opening directly communicating with the outdoors shall be provided. (See next section).

AIR	VENT THROUGH ROOF
DUCTS

		12″	MAX
				IN.
		1 SQ
		PER 1000
	CONFINED SPACE	BTUH* IN DOOR
		OR WALL
INTERIOR		UNCONFINED
		SPACE
HEATED
SPACE
		6″ MIN
		(FRONT) †
		1 SQ IN.
		PER 1000
		BTUH* IN DOOR
		OR WALL
				12″ MAX

AIR DUCTS

*Minimum opening size is 100 sq in. with minimum dimensions of 3 in.

†Minimum of 3 in. when type-B1 vent is used.

A02038

Fig. 7—Confined Space: Air for Combustion and

Ventilation from an Unconfined Indoor Space

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 furnace, all air for combustion must come from outdoors.

2.Air from outside the structure requires 1 of the following:

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. 8 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. 8 and Table 2.)

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