Air requirements for operation of exhaust fans, kitchen ventilation
systems, clothes dryers, and fireplaces shall be considered in
determining the adequacy of a space to provide combustion-air
requirements.
In unconfined spaces in buildings of conventional frame, brick, or
stone construction, infiltration MAY be adequate to provide air for
combustion, ventilation, and dilution of flue gases. This determi-
nation must be made on an individual installation basis and must
take into consideration the overall volume of unconfined space, the
number of windows and ventilation openings, the number of doors
to the outside, internal doors which can close off unconfined space,
and overall tightness of building construction. Consideration must
also be given to the amount of storage items (furniture, boxes, etc.)
within the unconfined space which takes away form air volume.
Many new buildings and homes (and older ones that have been
weatherized) MUST BE considered as being of tight construction,
therefore, infiltration will not be sufficient to supply necessary air
for combustion and ventilation.
A building can be considered as being of tight construction when:
1. Walls and ceilings exposed to outside atmosphere have a
continuous water vapor retarder with a rating of 1 perm or
less with openings gasketed or sealed, and/or
2. Weatherstripping has been added on operable windows and
doors, and/or
3. Caulking 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.
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.
If outside air is supplied to a confined space, then the 2 openings
must be equal and located as above. The free area of each must be:
1. One sq in. per 4000 Btuh of total input rating when air is
directly communicated from outdoors.
2. One sq in. per 4000 Btuh of total input rating when air is
brought in through vertical ducts.
3. One sq in. per 2000 Btuh of total input rating when air is
transferred through horizontal ducts.
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.
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 design of louver or
grille is known, it shall be used in calculating size design 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.
WARNING:
Do not block combustion-air openings in
the furnace. Any blockage will result in improper com-
bustion which may result in a fire hazard and/or cause
bodily harm.
The lack of a proper amount of combustion air can lead to serious
furnace operational problems. Some of these problems are:
1. Excessive oil burner after drip and oil fumes.
2. Sooting.
3. Melted oil burner couplings and/or ignitor/relay control.
4. A condition where air band or air shutter settings must be
more open than normal to achieve proper combustion.
5. Lockouts on start-up.
II.
DUCT WORK RECOMMENDATIONS
The proper sizing of warm air ducts is necessary to ensure
satisfactory furnace operation. Duct work should be in accordance
with the latest editions of NFPA-90A (Installation of Air Condi-
tioning and Ventilating Systems) and NFPA-90B (Warm Air
Heating and Air Conditioning Systems) or Canadian equivalent.
The supply duct work should be attached to flanged opening
provided at discharge end of furnace. See Fig. 7 for dimensions of
this opening.
Knock-outs are provided on both sides and bottom of furnace to
facilitate locating the panel cut-out required to connect to return
duct work. Only 1 opening should be used. Upflow installations
may use any 1 of the openings, as appropriate. For horizontal
installations, we recommend the use of bottom opening (although
side openings may be used if necessary). Furnace cutouts must be
full size specified by corner markers. (See Fig. 7.) Undersized
cut-outs adversely affect airflow capacity of furnace.
NOTE:
THE BACK MUST NOT BE CUT OUT FOR RETURN
AIR.
When the provided external filter rack is used, return-air duct
should be attached as shown in Fig. 1.
The following recommendations should be followed when install-
ing duct work:
1. Install locking-type dampers in all branches of individual
ducts to balance out system. Dampers should be adjusted to
impose proper static at outlet of furnace.
2. A flexible duct connector of noncombustible material
should be installed at unit on both supply- and return-air
systems. In applications where extremely quiet operation is
necessary, the first 10 ft (if possible) of supply and return
ducts should be internally lined with acoustical material.
3. In cases where return-air grille is located close to fan inlet,
there should be at least one 90° air turn between fan inlet
and grille. Further reduction in sound level can be accom-
plished by installing acoustical air turning vanes or lining
duct as described in item 2 above.
4. When a single air grille is used, duct between grille and
furnace must be the same size as return opening in furnace.
CAUTION:
Return-air grilles and warm air registers
must not be obstructed.
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