Carrier 58 Series - Manual (Page 2)

The draft hood relief air supply must be in the
same atmospheric pressure zone as the combustion
air inlet supply to the furnace.
When a furnace is installed so that supply ducts
carry air to areas outside the space containing the
furnace, the return air must also be handled by
duct(s) sealed to the furnace casing and terminating
outside the space containing the furnace.
All fuel-burning equipment must be supplied with
air, which enters into the combustion process and is
then vented to the outdoors. Sufficient air must
enter the equipment room to replace that drawn up
the vent stack. Replacement air must be provided by
means of ducts from the outside to the equipment
location or conditioned space. Under all conditions,
enough air must be provided to ensure there will not
be a negative pressure condition within the equip­
ment room or space. A positive seal must be made
between the furnace base and platform or return-air
duct connections. Even a “slight” leak can create a
negative pressure condition in a confined closet or
basement, and affect combustion. If necessary, seal
the furnace base to floor opening with fiberglass or
other approved material. Furnaces with side-
connected returns should utilize an accessory
bottom closure, properly sealed.
Complete combustion requires approximately
10 cu ft of air for every 1000 Btu of gas fuel. In addi­
tion, approximately 10 more cu ft of air is required
for draft hood dilution, and 10 more for ventilation
of the furnace enclosure. Thus, for eaeh 1000 Btu of
gas consumed, a total of 30 cu ft of air must be sup­
plied. For a 100,000 Btuh gas furnace, this equals
3000 cu ft of air per hour (100 x 30) or 50 cu ft of air
per minute (3000
60). In the past, the infiltration
of outside air assumed in heat loss calculations (one
air change per hour) was assumed to be sufficient.
However, current eonstruction methods, utilizing
more insulation and vapor barriers, tighter fitting
and gasketed doors and windows or weatherstrip­
ping, and the presence of fuel-burning fireplaces and
mechanical exhaust fans from kitchens, laundries,
and baths require the positive introduction of
outside air.
CAUTION; Combustion air must come from a
source that is not contaminated by halogens,
which include fluorides, chlorides, bromides,
and iodides. These elements are found in aerosol
sprays, detergents, bleaches, cleaning solvents,
salts, air fresheners, and other household prod­
ucts. Vapors from these products are highly
corrosive to gas-fired furnaces, in even ex­
tremely low concentrations—as low as 1 / 2 part
per million. When such contaminants are pre­
sent, outside air mttft be ducted separately to
the furnace room.
Operation of exhaust fans, kitchen ventilation
systems, clothes dryers, or fireplaces may create
conditions requiring special attention to avoid un­
satisfactory operation of installed gas furnaces.
Commonly used exhaust fans and range hoods
handle anywhere from 60 to 300 cfm (or more) and
gas or electric clothes dryers may remove more air.
With multiple use of such appliances common in
both old and new homes and apartments, special
provisions for positive introduction of outside air
are mandatory. This air requirement is several times
that required by the furnace, and must be replaced,
in addition to the air required by the furnace.
The requirements for combustion and ventilation
air depend upon whether the furnace is located in a
confined or unconfined space. An unconfined space
is defined as a space with a volume not less than
50 cu ft/1000 Btuh of the total input rating of all
appliances installed in that space. Rooms communi­
cating directly with the space in which the appliances
are installed, thru openings not furnished with
doors, are considered a part of the unconfined space.
A confined space is defined as a space with a vol­
ume less than 50 cu ft/1000 Btuh of the total input
ratings of all appliances installed in that space.
Furnaces in Confined Spaces (Typical Furnace
Closet)
ALL AIR FROM INSIDE BUILDINGS:
The confined space shall be provided with 2 per­
manent openings, one beginning within 12 in. of the
top and one beginning within 12 in. of the bottom of
the enclosure. Each opening shall have a minimum
free area of 1 sq in. per 1000 Btuh of the total input
rating of all equipment in the enclosure, but not have
less than 100 sq inches. The openings must freely
communicate with interior areas served by the
furnace, either thru permanent openings or louvered
doors.
In addition, if building is of unusually tight con­
struction, provide one permanent opening directly
communicating with outdoors. This opening shall
have a minimum free area of 1 sq in. per 5000 Btuh
of total input rating of all equipment in the en­
closure. Ducts shall be used to convey makeup air
and shall be of the same cross-sectional area as the
free areas of the openings to which they connect.
This duct should be connected to the cold air return
of the heating system. The minimum dimension of
the rectangular air ducts should not be less than
3 inches. See Fig. 1.
If the furnace is instafled 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, as prescribed below.
ALL AIR FROM OUTDOORS:
The confined space shall be provided with 2 per­
manent openings, one commencing within 12 in.
of the top and one commencing within 12 in. of the
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