NOTICE
IMPORTANT: If the attic has an exhaust fan (power
vent), it may create a negative pressure suffi
ciently large
enough to prevent the attic from being an eff ective 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 thermo-
stat, which may allow some operation during the heating
season. Th
e 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 Confi ned Space, Outdoor Air
Ducted Horizontally
Similar to Case 3, outdoor air for combustion and ventilation may
be drawn through horizontal ducting. Th e free area for each opening
is calculated on the basis of a minimum of 1 square inch per 2000
BTU/Hr input.
Th
e following table shows minimum free areas and round pipe sizes
when drawing combustion air horizontally from the outdoors for
the furnace only. If other fuel burning appliances are present, their
combustion air and ventilation air requirements must be added to
those of the furnace.
TABLE 5 - HORIZONTAL AIR SUPPLY (CASE 4)
Model
Free Area
Ea. Opening
Round Pipe Size
60
30 in.²
7 in.
80
40 in.²
8 in.
100
50 in.²
8 in.
120
60 in.²
9 in.
NOTICE
IMPORTANT: If grilles are used on the outside wall, they
must be sized properly. Most sidewall grilles have only 50%
free area. In the case of a unit with 100,000 Btuh input,
which requires a pair of 8” round pipes to obtain suffi
cient
combustion and ventilation air, the duct could be an equiv-
alent rectangular duct; 8” x 7” for example. Based on 50%
free area for the inlet grilles, the actual grille size would
have to be 14” x 8” or its equivalent. A transition may be
used to reduce to the smaller duct size if necessary.
NOTICE
IMPORTANT: Th
e outdoor grilles must be installed in a
location where they will not be obstructed in any manner.
CASE 5 - Furnace Located In A Confi ned space, Outdoor Air
Ducted with Single Opening (US Only)
One permanent opening, commencing within 12 in. (300 mm) of
the top of the enclosure, shall be provided. Th
e appliance shall have
clearances of at least 1 in. (25 mm) from the sides and back and 6 in.
(150 mm) from the front of the appliance. Th
e opening shall directly
communicate with the outdoors or shall communicate through a
vertical or horizontal duct to the outdoors or spaces that freely com-
municate with the outdoors and shall have a minimum free area of
the following:
1.
1 in
2
/3000 BTU/hr (700 mm
2
per kW) of the total input rating
of all appliances located in the enclosure, and
2.
Not less than the sum of the areas of all vent connectors in that
space”
Connection To Furnace Non-Direct Vent
When using indoor air or the non-direct vent confi guration, the
combustion air inlet to the furnace must be protected from block-
age. Use a double elbow arrangement if supplying combustion air
through the top, or a downward pointing single elbow if supplying
combustion air through the side.
(
Figure 8
)
11 - DETERMINING COMBUSTION AIR
Figure 7 - Outdoor Air For Combustion, Horizontal (Case 4)
Figure 8- Non-Direct Combustion Air Inlet
17
