SYSTEM OPERATION
11
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 other
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 applicable
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 furnace
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 ventilation
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 ap-
pliances.
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 man-
ufacturer’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 regulator 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 exhaust 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 indoor 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 located within the space. Rooms communicating
directly with the space in which the appliances are installed through
openings not furnished with doors, and through combustion air open-
ings 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 infil
-
tration 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:
21 ft
3
I
other
Required Volume
other
> ________ _________
ACH 1000 Btu/hr
(2) For fan-assisted appliances, calculate using the following equation:
15 ft
3
I
fan
Required Volume
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 ex-
changed per hour, expressed as a decimal)
(3) For purposes of this calculation, an infiltration rate greater 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.
(2)
Combining spaces in different stories.
The volumes of spaces in
different stories shall be considered as communicating spaces
where such spaces are connected by one or more openings in
doors or floors having a total minimum free area of 2 in.
2
/1000
Btu/hr (4400 mm
2
/kW) of total input rating of all appliances.
9.3.3 Outdoor Combustion Air.
Outdoor combustion air shall be
provided through opening(s) to the outdoors in accordance with
the methods in 9.3.3.1 or 9.3.3.2. The minimum dimension of air
openings shall not be less than 3 in. (80 mm).
