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Appliances Installed in a Confined Space

Any space smaller than an "unconfined" space as defined in the National Fuel Gas Code is considered to be a confined space and must be provided with openings to admit makeup air. Usually, this requires the installation of a register or grille in the utility room wall (Figure 4), or the installation of one or more ducts leading to the exterior (Figure 5, below). The National Fuel Gas Code provides four options for providing these vent openings :

Figure 4.
Figure 4. Combustion makeup air can be provided to a confined space through an opening in the utility room wall. Air enters the confined space from the adjacent basement area, which must meet the definition of an unconfined space.


=Provide openings or grilles between the confined space and an adjacent unconfined space.

=Provide two ducts to a ventilated attic.

=Provide two ducts or openings to the exterior.

=Provide a single duct or opening to the exterior.

Figure 5a

Figure 5.

In this basement, which is too small to be considered an unconfined space, two ducts provide combustion makeup air from the exterior. One duct terminates near the ceiling, and the other terminates near the floor. The air intakes are protected at the exterior with rain hoods and screening.    


This last option is a new addition to the 1996 code, and although the allowance of only one opening rather than two may appear to contradict the requirements of the previous three options, it is perfectly acceptable. This option was included to address the concern that two openings to the exterior might cause frozen pipes in colder climates.

Motorized louvers.

If you are concerned that makeup air from the exterior could cause pipes to freeze, you might consider installing motorized louvers on the air intake (Figure 6). Any motorized louvers used must be the type that lock out the burner circuit until the louvers are in the full-open position. Contact your heating contractor or supply house for more information on motorized louvers.

Figure 6.

Motorized dampers are installed in very cold climates, where exterior makeup air might be cold enough to freeze plumbing pipes in a utility room or basement.      


Forced Combustion Air Systems

Another way to provide makeup air to appliances in a confined space is to install a forced combustion air system, also called a powered air intake system. This approach uses a fan to introduce ducted exterior air to a utility room (Figure 7). The fan is wired to be interlocked with the burner. An airflow switch in the intake air duct prevents operation of the fuel-burning appliance when the air duct is blocked, and a damper prevents off-cycle airflow.


Figure 7. A forced combustion air system uses a fan to introduce makeup air into a utility room. This permits the use of a much smaller duct than when the makeup air enters by gravity.

The advantage of a forced combustion air system is the ability to use a small duct (usually, a single 3-inch round duct for a residential system), instead of the two larger ducts that would usually be required when exterior makeup air is provided passively.

Forced combustion air systems are sold by two different manufacturers, Tjernland Products and Field Controls. Although forced combustion air systems are not addressed by existing codes, BOCA is now in the process of developing a proposed standard for their use.

Providing adequate makeup air for a fuel-burning appliance is essential not only for the proper operation of the equipment, but also for safety. Do the job right, and the building's appliances -- not to mention its occupants and you, the builder -- will be able to breathe easy.

Carl Saundersis director of training at Utica Boilers in Utica, N.Y.

Manufacturers of Combustion Air Fans

Field Controls

2630 Airport Road

Kinston, NC 28504


Circle #15

Tjernland Products, Inc.

1601 Ninth Street

White Bear Lake, MN 55110


Circle #15