For Example:
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.
V.
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, ventilation, and dilution of flue
gases using 1 of the following methods. (See Figs. 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 combus-
tion 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 Figs. 3, 4,
and Table 2.)
b. If combustion air is taken from outdoors through 2
horizontal 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
This section covers installation of the 376CAV Series H and J
furnace.
I.
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
Guidelinesreference 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.
Flexible connections should be used between ductwork and
furnace to prevent transmission of vibration. Ductwork passing
through unconditioned space should be insulated to enhance
system performance. When air conditioning is used, a vapor
barrier is recommended.
Maintain a 1-in. clearance from combustible materials to supply air
ductwork for a distance of 36 in. horizontally from the furnace. See
NFPA 90B or local code for further requirements.
II.
DUCTWORK ACOUSTICAL TREATMENT
Metal duct systems that do not have a 90 degree elbow and 10 ft
of main duct to the first branch take-off may require internal
acoustical lining. As an alternative, fibrous ductwork may be used
if constructed and installed in accordance with the latest edition of
SMACNA construction standard on fibrous glass ducts. Both
acoustical lining and fibrous ductwork shall comply with NFPA
90B as tested by UL Standard 181 for Class 1 Rigid air ducts.
III.
SUPPLY AIR CONNECTIONS
A.
Downflow Furnaces
Connect supply-air duct to supply-air opening on furnace. The
supply-air ducts attachment must ONLY be connected to furnace
supply/outlet or air conditioning coil casing (when used), when
installed on non-combustible material. When installed on combus-
tible material, supply-air duct attachment must ONLY be con-
nected to an accessory subbase or factory approved air condition-
ing coil casing. DO NOT cut main furnace casing to attach supply
side air duct, humidifier, or other accessories. All accessories
MUST be connected external to the furnace main casing.
B.
Horizontal Furnaces
Connect supply-air duct to supply-air opening on furnace. The
supply-air ducts attachment must ONLY be connected to furnace
supply/outlet or air conditioning coil casing (when used). DO NOT
cut main furnace casing to attach supply side air duct, humidifier,
or other accessories. All accessories MUST be connected external
to the furnace main casing.
376CAV 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
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