WARNING
DO NOT ALLOW GAS PIPING TO BE ROUTED THROUGH
JOIST SPACES THAT ARE USED FOR RETURN AIR PUR-
POSES. DO NOT USE JOIST SPACES FOR RETURN AIR
PURPOSES IF THE JOIST SPACE ALREADY CONTAINS
PLUMBING STACKS, CHIMNEY COMPONENTS, ETC.
UNLESS THE PORTION USED FOR RETURN AIR PURPOS-
ES CAN BE COMPLETELY ISOLATED FROM PORTIONS
WITH OTHER USAGES.
The outdoor grilles must be installed in a location
where they will not be obstructed in any manner.
If grilles are used on the outside wall, they must be
sized properly. Most sidewall grilles have only 50%
free area, so the grill size opening must be twice the
size of the free air opening requirement.
IMPORTANT
The choices are (a) use the direct vent option; (b) obtain
outdoor air from elsewhere; or (c) interlock the attic
exhaust fan with the furnace such that the two cannot
operate simultaneously.
CASE 4:
FURNACE LOCATED IN A CONFINED SPACE, OUTDOOR
AIR DUCTED HORIZONTALLY
Similar to CASE 3, outdoor air for combustion and ventila-
tion may be drawn through horizontal ducting.
Consult the appropriate installation code for the free area
for openings. If other fuel burning appliances are present,
their combustion air and ventilation air requirements must
be added to those of the furnace.
CASE 3:
FURNACE LOCATED IN A CONFINED SPACE, OUT-
DOOR
AIR FROM ATTIC OR CRAWL SPACE
In this circumstance, refer to the appropriate installa-
tion code for the free area of the combustion and
ventilation air openings. If other fuel burning appli-
ances are present, their combustion air and ventilation
air requirements must be added to those of the fur-
nace. If the attic has an exhaust fan (power vent), it
may create a negative pressure sufficiently large
enough to prevent the attic from being an effective
source of combustion and ventilation air.
Powered attic fans do not customarily run during the
heating season; however, some are controlled by a
humidistat as well as a thermostat, which may allow
some operation during the heating season.
If an exhaust fan, fireplace, clothes dryer or any simi-
lar device is present in the indoor area from which the
combustion and ventilation air will be drawn, negative
pressure could be a problem if natural infiltration
from the outdoors does not match the rate at which
air is exhausted.
IMPORTANT
If planning to use the inside air of an unconfined
space, remember to test for proper furnace operation
(as well as other fuel burning appliances located
within the unconfined space) with respect to adequate
combustion and ventilation air with fireplace dampers
open, clothes dryer running, bathroom exhaust fans
on, kitchen range hood on, etc.
NOTE
CASE 2:
FURNACE LOCATED IN A CONFINED SPACE
A confined space, (any space smaller than the minimums
discussed in CASE 1), must have two air openings one with-
in 12 inches of the ceiling and the other within 12 inches of
the floor. The air openings must be sized based on whether
the combustion and ventilation air is being taken from
indoors or outdoors, the method outdoor air (if used) is
introduced, and taking into account any other fuel burning
appliances in the confined space.
If sufficient indoor combustion and ventilation air is avail-
able for the furnace and all other fuel burning appliances,
even when clothes dryers, bathroom fans, range hoods, etc.
are running, size each opening according to the appropri-
ate installation codes.
If using grilles to cover the two openings, factor in the
free area of the grille. Typically, a grille will have a
free area approximately 50% of its nominal size.
Consequently, if the required opening is 10 inches x 10
inches , it will have to be doubled if using a sidewall
grille with 50% free area.
NOTE
CASE 1:
FURNACE LOCATED IN AN UNCONFINED SPACE
Unconfined space does not necessarily mean that combus-
tion and ventilation will not have to be introduced from
the outdoors, particularly in airtight homes. Refer to the
appropriate installation code requirements regarding the
minimum combustion air required for all fuel burning
appliances located within the unconfined area.
If the amount of combustion and ventilation air is insuffi-
cient to properly operate the furnace and other fuel burn-
ing appliances within the unconfined area, it will be neces-
sary to supply it from the outdoors based on the criteria
used when calculating the air supply for a confined space.
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