2. Weatherstripping has been added on operable windows and
doors, and/orCaulking or sealants are applied to areas such as
joints around window and door frames; between sole plates
and floors; between wall-ceiling joints; between wall panels;
at penetrations for plumbing, electrical, and fuel lines; and at
other openings.
If combustion and ventilation air must be supplied to an uncon-
fined space from outside, an opening with a FREE AREA of not
less than 1 sq in. per 1000 Btuh of total input of all appliances
within unconfined space (but not less than 100 sq in.) must be
provided. This opening must be located such that it can not be
blocked at any time.
Step 4—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.
When furnace is installed in a closet or enclosure, 2 ventilation
openings, with OPEN AREA as dimensioned in example below
are required for combustion air. The openings should be located
about 6 in. from top and bottom of enclosure at front of furnace.
(Refer to Table 3.)
NOTE: In calculating free area, consideration shall be given to
blocking effect of louvers, grilles, or screens protecting openings.
Screens used shall not be smaller than 1/4-in. mesh and shall be
readily accessible for cleaning. If free area through a louver or
grille is known, it shall be used in calculating size and free area
specified. If design and free area are not known, it may be assumed
that wood louvers have 20 percent free area and metal louvers and
grilles have 60 percent free area. Louvers shall be fixed in open
position or interlocked with furnace so they open automatically at
furnace start-up and remain open during furnace operation.
The size of the openings depends upon whether the air comes from
outside of the structure or an unconfined space inside the structure.
ALL AIR FROM INSIDE THE STRUCTURE
For a confined space, where air is taken from an interior space, 2
permanent openings of equal area are required. One opening must
be within 12 in. of ceiling and the other within 12 in. of floor. Each
opening must have a free area of at least 1 sq in. per 1000 Btuh of
total input rating but not less than 100 sq in. (Refer to Table 4.)
ALL AIR FROM OUTSIDE OF STRUCTURE
If outside air is supplied to a confined space, then the 2 openings
must be equal and located as above.
1. If combustion air is taken through a permanent opening
directly communicating with the outdoors, the opening shall
have a minimum free area of 1 sq in. per 4000 Btuh of total
input rating for all equipment in the enclosure.
2. 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. (Refer to Table 5.)
3. If combustion air is taken from 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. (Refer to Table 6.)
When ducts are used to supply air, they must be of the same cross
sectional area as free area of openings to which they connect.
The minimum dimension of rectangular air ducts must not be less
than 3 in.
DUCTWORK RECOMMENDATIONS
CARBON MONOXIDE POISONING HAZARD
Failure to follow this warning could result in personal injury
or death.
When supply ducts carry air circulated by furnace to areas
outside spaces containing furnace, return air MUST also be
handled by a duct sealed to furnace casing and terminating
outside space containing furnace.
FIRE HAZARD or UNIT MAY NOT OPERATE
Failure to follow this caution may result in property damage
or intermittent unit operation.
Return-air grilles and warm air registers MUST NOT be
obstructed.
The proper sizing of warm air ducts is necessary to ensure
satisfactory furnace operation. Duct work should be in accordance
Table 3—Combustion Air From Confined Space
58VMR FURNACE
INPUT BTUH
LENGTH
(IN.)
HEIGHT
(IN.)
70,000-105,000 16 8
119,000-154,000 20 10
Table 4—Combustion Air for Unconfined Space
58VMR FURNACE
INPUT BTUH
FREE AREA PER
OPENING
(SQ IN.)
70,000 100
91,000 100
105,000 105
119,000 119
140,000 140
154,000 154
Table 5—Combustion Air From Outdoors Through
Vertical Ducts
58VMR FURNACE
INPUT BTUH
FREE AREA PER
OPENING
(SQ IN.)
ROUND PIPE
(IN. DIAM)
70,000 17.5 5
91,000 22.8 6
105,000 26.3 6
119,000 29.8 6
140,000 35.0 6
154,000 38.5 6
Table 6—Combustion Air from Outdoors Through
Horizontal Ducts
58VMR FURNACE
INPUT BTUH
FREE AREA PER OPENING
(SQ IN.)
ROUND PIPE
(IN. DIAM)
70,000 35.0 7
91,000 45.5 8
105,000 52.5 9
119,000 59.5 9
140,000 70.0 10
154,000 77.0 10
5
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