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Page 4
furnace
opening
confined
space
Figure 1
Outdoor Air
Outlet and inlet air can be brought into the confined space via openings
into a ventilated attic and ventilated crawl space.
furnace
outlet air
inlet air
attic ventilation louvers
crawl space ventilation louvers
Figure 2
Confined Space
Outdoor Air Using Vertical or Horizontal Ducts
If combustion air is taken from outdoors through vertical ducts, the
openings and ducts must have a minimum free area of one square inch
(6.5cm
2
) per 4,000 Btuh of total appliance input. In installations
drawing combustion air from a ventilated attic both air ducts must
extend above the attic insulation.
furnace
outlet air
inlet air ends one foot above floor
vertical
furnace
inlet air duct
outlet air duct
horizontal
Figure 3
If combustion air is taken from outdoors through horizontal ducts the
openings and ducts must have a minimum free area of one square inch
(6.5cm
2
) per 2,000 Btuh of total appliance input.
Ventilation and Combustion Air
WARNING: Danger of property damage, bodily injury or death,
this furnace and any other fuel burning appliance must be
provided with enough fresh air for proper combustion and
ventilation of flue gases. Most homes will require that outside air
be supplied. Do not draw air from a corrosive environment such
as a workshop or laundry room.
The requirements for providing air for combustion and ventilation are
listed in the National Fuel Gas Codes NFPA 54/ANSI Z223.1 (in
Canada - CAN/CGA B149).
Note: Air requirements for operation of exhaust fans, kitchen ventilation
systems, clothes dryers, fireplace and any other fuel burning or ventilating
equipment used in the space must be considered in determining
combustion air requirements.
Ventilation Air Openings and Ducts
In determining the free area needed consideration must be given to the
blocking effect of louvers, grills or screens protecting openings.
— If a screen is used to cover openings it must not be smaller than 1/4"
mesh.
— Use the free area of a louver or grill to determine the size opening
required to provide the free area specified. If the free area is not
known assume a 20% free area for wood and a 60% free area for a
metal louver or grill.
— Ducts must have the same cross sectional area as the free area of the
openings to which they connect.
— The minimum dimension of air ducts must not be less than 3 inches.
Installation in an Unconfined Space
An unconfined space is an area including all rooms not separated by
doors with a volume greater than 50 cubic feet per 1,000 Btuh of the
combined total input rates of all appliances which draw combustion air
from that space. For example, a space including a water heater rated at
40,000 Btuh input and a furnace rated at 40,000 Btuh requires a volume
of 4,000 cubic feet (50 x (40 + 40) = 4,000) to be considered as
unconfined. If the space has an 8 ft. ceiling, the floor area of the space
must be 500 sq. ft. In general, particularly in older homes, a furnace
installed in an unconfined space will not require outside air for
combustion. However in a "tight" newly constructed home, outside air
may be necessary to insure adequate combustion.
Installation in a Confined Space
A confined space is an area with volume less than 50 cubic feet per 1,000
Btuh of the combined input rates of all appliances drawing combustion
air from that space. Small areas such as equipment rooms are confined
spaces. Furnaces installed in a confined space which supply heated air
to areas outside the space must draw return air from outside the space
through tightly sealed return air ducts. A confined space must have 2
openings into the space for combustion air. One opening must be within
12 inches of the ceiling and the other must be within 12 inches of the
floor. The required sizing of these openings is determined by whether
inside or outside air is used to support combustion, the method by which
the air is brought to the space (vertical or horizontal duct) and by the total
input rate of all appliances in the space. See Figure 1.
All Air From Inside — Confined Space
If combustion air is taken from the heated space the 2 openings must
each have a free area of at least one square inch per 1,000 Btuh of total
input of all appliances in the confined space but not less than 100 square
inches (645cm
2
) of free area.
For example: for a 40,000 Btuh furnace only in the confined space each
opening must be 100 square inches (645cm
2
) each of free area.
WARNING: Combustion air must not be drawn from a heated
space which includes exhaust fans, fireplaces or other devices that
may produce a negative pressure in the space.