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DETERMINING FRESH-AIR FLOW
FOR HEATER LOCATION
Determining if You Have a Conﬁned or
Use this work sheet to determine if you have a
conﬁned or unconﬁned space.
Space: Includes the room in which you will
install heater plus any adjoining rooms with door
less passageways or ventilation grills between
1. Determine the volume of the space (length x
width x height).
Length x Width x Height = ________cu. ft.
(volume of space)
Example: Space size 20 ft. (length) x 16 ft.
(width) x 8 ft. (ceiling height) = 2560 cu. ft.
(volume of space)
If additional ventilation to adjoining room is sup
plied with grills or openings, add the volume of
these rooms to the total volume of the space.
2. Multiply the space volume by 20 to determine
the maximum Btu/Hr the space can support.
(volume of space) x 20 = (maximum Btu/Hr
the space can support)
Example: 2560 cu. ft. (volume of space) x 20 =
51,200 (maximum Btu/Hr the space can support)
3. Add the Btu/Hr of all fuel burning appliances
in the space.
Vent-free heater ________
Gas water heater* ________
Gas furnace ________
Vented gas heater ________
Gas ﬁreplace logs ________
Other gas appliances* + _______
Total = _______
* Do not include direct-vent gas appliances.
Direct-vent draws combustion air from the
outdoors and vents to the outdoors.
Gas water heater ________
Vent-free heater + ________
Total = ________
4. Compare the maximum Btu/Hr the space can
support with the actual amount of Btu/Hr used.
_______Btu/Hr (maximum the space can support)
______ Btu/Hr (actual amount of Btu/Hr used)
AIR FOR COMBUSTION
Example: 51,200 Btu/Hr (maximum the
space can support)
70,000 Btu/Hr (actual amount of
The space in the above example is a conﬁned
space because the actual Btu/Hr used is more than
the maximum Btu/Hr the space can support. You
must provide additional fresh air. Your options
are as follows:
A. Rework worksheet, adding the space of an
adjoining room. If the extra space provides an
unconﬁned space, remove door to adjoining
room or add ventilation grills between rooms.
See Ventilation Air From Inside Building.
B. Vent room directly to the outdoors. See Ven
tilation Air From Outdoors, page 7.
C. Install a lower Btu/Hr heater, if lower Btu/Hr
size makes room unconﬁned.
If the actual Btu/Hr used is less than the maxi
mum Btu/Hr the space can support, the space is
an unconﬁned space. You will need no additional
fresh air ventilation.
WARNING: If the area in
which the heater may be oper
ated is smaller than that deﬁned
as an unconﬁned space or if the
building is of unusually tight
construction, provide adequate
combustion and ventilation air
by one of the methods described
in the National Fuel Gas Code,
ANSI Z223.1/NFPA 54, Section
5.3 or applicable local codes.
Ventilation Air From Inside Building
This fresh air would come from an adjoining un
conﬁned space. When ventilating to an adjoining
unconﬁned space, you must provide two perma-
nent openings: one within 12" of the ceiling and
one within 12" of the ﬂoor on the wall connecting
the two spaces (see options 1 and 2, Figure 2,
page 7). You can also remove door into adjoining
room (see option 3, Figure 2, page 7). Follow the
National Fuel Gas Code, ANSI Z223.1/NFPA 54,
Section 5.3, Air for Combustion and Ventilation for
required size of ventilation grills or ducts.