Carrier 58SS/DH - Manual (Page 2)

within the space containingHlte fut'^ace. A positive seal
m u st 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 use the factory-supplied bottom closure, properly
sealed.
For complete combustion and ventilation, the furnace
requires approximately 20 cu ft of air for every 1000 Btuh
of gas consumed. Thus, for each 1000 Btuh of gas con­
sumed, a total of 20 cu ft of air must be supplied. For a
100,000-Btuh gas furnace, this equals 2000 cu ft of air per
hour (100 X 20) or 33 cu ft of air per minute (2000 60).
In the past, the infiltration of outside air assumed in heat
loss calculations (one air change per hour) was sufficient.
However, current construction methods using more insu­
lation and vapor barriers, tighter fitting and gasketed
doors and windows, or weatherstripping, and the
presence of fuel-burning fireplaces and household
exhaust fans require positive introduction of outside air.
Use of exhaust fans, kitchen ventilation systems,
clothes dryers and fireplaces may create conditions that
require special attention to avoid unsatisfactory furnace
operation. Exhaust fans and range hoods can handle
from 60 to 300 cfm (or more). Gas and electric clothes
dryers remove even more air. Use of multiple appliances
requires special mandatory provisions for positive intro­
duction of outside air. This makeup air requirement is
several times that required by the furnace and must be
replaced, in addition to the air required by the furnace.
A CAUTION
Combustion air must come from a source not con­
taminated by halogens, which include fluorides,
chlorides, bromides and iodides. These elements are
found in aerosols, detergents, bleaches, cleaning
solvents, salts, air fresheners and other household
products. Vapors from these products are highly
corrosive to gas-fired furnaces, even in extremely low
concentrations — as low as 1/2 part per million.
When such contaminants are present, outside air
m u st be ducted separately to the furnace room.
Requirements for combustion and ventilation air
depend upon whether furnace is located in a con fin ed or
u n con fin ed space. An unconfined space is defined as a
space where volume is not less than 50 cu ft per 1000 Btuh
of the total input rating of all appliances installed in that
space. Rooms directly open (no doors) to the space in
which the appliances are installed are considered a part
of the unconfined space. A confined space is defined as a
space with volume less than 50 cu ft per 1000 Btuh of the
total input ratings of all appliances in that space.
Furnaces in Confined Spaces (Typical Furnace
Closet)
ALL AIR FROM INSIDE BUILDINGS — The con­
fined space shall be provided with 2 permanent 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 one sq in. per 1000 Btuh
of the total input rating of all equipment in the enclo­
sure, but not less than 100 sq inches. The openings must
freely communicate with other interior areas served by
the furnace, of sufficient volume so that the combined
volume of all spaces meets the criteria for an unconfined
space. This can be accomplished through either per­
manent openings or louvered doors.
In addition, if ths building is of unusually tight con­
struction, there shall be one permanent opening commu­
nicating directly with outdoors. This opening shall have
a minimum free area of one sq in. per 5000 Btuh of total
input rating of all equipment in the enclosure. Ducts
shall be used to convey makeup air, and shall be of the
same cross-sectional area as the free area 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 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 the outside, as prescribed in
paragraph below.
ALL AIR FROM OUTDOORS — The confined space
shall be provided with 2 permanent openings, one com­
mencing within 12 in. of the top and one commencing
within 12 in. of the bottom of the enclosure. These open­
ings shall communicate directly, or by ducts, with out­
doors. See Fig. 2.
a. When communicating with the outdoors through
horizontal ducts, each opening shall have a minimum
free area of one sq in. per 2000 Btuh of the total rated
input of all equipment in the enclosure. If vertical
ducts or openings with louvers and grilles are used,
each opening or duct shall have a minimum free area
of one sq in. per 4000 Btuh of the total rated input of all
equipment in the enclosure.
b. When ducts are used, they shall be of the same cross-
sectional area as the free area of the openings to which
they connect. The minimum dimension of rectangular
air ducts shall not be less than 3 inches.
Furnaces in Unconfined Spaces (Basements,
Large Equipment Rooms, etc.)
— If the uncon­
fined space is within a building of unusually tight con­
struction, air for combustion, ventilation and dilution of
flue gases shall be obtained from outdoors or from spaces
freely communicating with the outdoors. A permanent
opening, or openings, having a total free area of not less
than one sq in. per 5000 Btuh of total input rating for all
equipment shall be provided. A duct may be used to con­
vey makeup air from the outdoors and shall be of the
same cross-sectional area as the free area of the openings
to which it connects. The duct may be connected to the
cold air return of the heating system
on ly if it con n ects
directly to ou tside air. The minimum dimension of
rectangular air ducts shall be not less than 3 inches.
A CAUTION
Return air m u st not be taken from the room, unless
an equal or greater amount of air is supplied to the
room. All duct connections to the furnace must be
airtight to avoid causing a negative pressure condi­
tion within the room.
Louvers and Grilles — In calculating the free area,
consideration shall be given to the blocking effect of
louvers, grilles, or screens protecting openings. Screens
used must not be smaller than 1 / 4-in. mesh. If the free
area through a design of louver or grille is known, it
should be used in calculating the size opening required to
provide the free area specified. If the design and free area
is not known, assume that wood louvers will have 20 to
25% free area and metal louvers and grilles will have
60 to 75% free area. Louvers and grilles that provide
combustion and dilution air must be constructed so they
cannot be closed.
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