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 :
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
=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.
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
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
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
2630 Airport Road
Kinston, NC 28504
1601 Ninth Street
White Bear Lake, MN 55110