Ventilation and Combustion Air
WARNING: Danger of property damage, bodily injury or death, this furnace and any otherfuel 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 (645cm2) of free area.
For example: for a 40,000 Btuh furnace only in the confined space each opening must be 100 square inches (645cm2) 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.
Page 4
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.
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 (6.5cm2) 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.
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 (6.5cm2) per 2,000 Btuh of total appliance input.
