SYSTEM OPERATION
13
If this furnace is to be installed in the same space with other
gas appliances, such as a water heater, ensure there is an
adequate supply of combustion and ventilation air for the oth-
er appliances. Refer to the latest edition of the National Fuel
Gas Code NFPA 54/ANSI Z223.1 (Section 9.3), or CAN/CGA
B149 Installation Codes (Sections 7.2, 7.3, or 7.4), or applica-
ble provisions of the local building codes for determining the
combustion air requirements for the appliances.
Most homes will require outside air be supplied to the fur-
nace area by means of ventilation grilles or ducts connecting
directly to the outdoors or spaces open to the outdoors such
as attics or crawl spaces.
The following infor mation on air for combustion and ventila-
tion is repro duced from the
National Fuel Gas Code NFPA
54/ANSI Z223.1 Section 9.3.
9.3* Air for Combustion and Ventilation.
9.3.1 General.
9.3.1.1
Air for combustion, ventilation, and dilution of flue
gases for appliances installed in buildings shall be obtained
by application of one of the methods covered in 9.3.2 through
9.3.6. Where the requirements of 9.3.2 are not met, outdoor
air shall be introduced in accordance with methods covered
in 9.3.3 through 9.3.6.
Exception No. 1: This provision shall not apply to direct vent
appliances.
9.3.1.2
Appliances of other than natural draft design and
other than Category 1 vented appliances shall be provided
with combustion, ventilation, and dilution air in accordance
with the appliance manufacturer’s instructions.
9.3.1.3
Appliances shall be located so as not to interfere with
proper circulation of combustion, ventilation, and dilution air.
9.3.1.4
Where used, a draft hood or a barometric draft regu-
lator shall be installed in the same room or enclosure as the
appliance served so as to prevent any difference in pressure
between the hood or regulator and the combustion air supply.
9.3.1.5
Makeup air requirements for the operation of ex
-
haust fans, kitchen ventilation systems, clothes dryers, and
fireplaces shall be considered in determining the adequacy
of a space to provide combustion air requirements.
9.3.2 Indoor Combustion Air.
The required volume of in-
door air shall be determined in accordance with the method
in 9.3.2.1 or 9.3.2.2 except that where the air infiltration rate
is known to be less than 0.40 ACH, the method in 9.3.2.2
shall be used. The total required volume shall be the sum
of the required volume calculated for all appliances locat-
ed within the space. Rooms communicating directly with the
space in which the appliances are installed through open-
ings not furnished with doors, and through combustion air
openings sized and located in accordance with 9.3.2.3, are
considered a part of the required volume.
9.3.2.1* Standard Method.
The minimum required volume
shall be 50 ft
3
per 1,000/Btu/hour (4.8m
3
/kW).
9.3.2.2* Known Air Infiltration Rate Method.
Where the air
infiltration rate of a structure is known, the minimum required
volume shall be determined as follows:
(1) For appliances other than fan-assisted, calculate using
the following equation:
Required Volume
other
> 21 ft
3
I
other
ACH 1000 Btu/hr
(2) For fan-assisted appliances, calculate using the following
equation:
Required Volume
fan
> 15 ft
3
I
fan
ACH 1000 Btu/hr
where:
I
other
= all appliances other than fan-assisted input in Btu
per hour
I
fan
= fan-assisted appliances input in Btu per hour
ACH = air change per hour (percent of volume of space
exchanged per hour, expressed as a decimal)
(3) For purposes of this calculation, an infiltration rate great
-
er than 0.60 ACH shall not be used in the equations in
9.3.2.2(1) and 9.3.2.2(2).
9.3.2.3 Indoor Opening Size and Location.
Openings
used to connect indoor spaces shall be sized and located in
accordance with the following:
(1)*
Combining spaces on the same story.
Each opening shall
have a minimum free area of 1 in.
2
/1000Btu/hr (2200
mm
2
/kW) of the total input rating of all appliances in the
space but not less than 100 in.
2
(0.60m
2
). One opening
shall commence within 12 in. (300 mm) of the top, and
one opening shall commence within 12 in. (300 mm) of
the bottom, of the enclosure
[see Figure A.9.3.2.3(1)].
The minimum dimension of air openings shall be not less
than 3 in. (80 mm).
Furnace
Water
Heater
Opening
Chimney or Gas Vent
Opening
NOTE: Each opening must have
a free area of not less than one
square inch per 1000 BTU of
the total input rating of all equip-
ment in the enclosure, but not
less than 100 square inches.
Figure A.9.2.3.3.(1) All Combustion Air from Adjacent
Indoor Spaces through Indoor Combustion Air Openings.
