In many parts of the country, the National Fuel Gas
Code (NFPA 54) applies. Other applicable codes may include
NFPA 31, Standard for the Installation of Oil-Burning
Equipment; NFPA 211, Standard for Chimneys, Fireplaces,
Vents, and Solid Fuel-Burning Appliances; and Section 607
of the Uniform Mechanical Code. In addition, be sure to
consult your local code and to adhere to the appliance
manufacturer's instructions. If there is a conflict between
your local code and the manufacturer's instructions, get the
conflict resolved before installing any equipment. The
National Fuel Gas Code Handbook can be ordered from NFPA
by calling 800/344-3555.
Special Problems With Tight Houses
Thanks to the use of vapor barriers, housewrap, sill seal,
weatherstripping, and caulk, we can now routinely build the
kind of warm, draft-free homes our grandparents could only
dream of. But if you build a very tight house without providing
adequate ventilation, it can come back to bite you.
Customers are increasingly concerned with indoor air quality.
A while ago, a homeowner showed me his new superinsulated
house, which included magnetic weatherstripping on the doors. I
mentioned that with a house that tight, perhaps he should stop
eating chili. Several months later, he called me for advice
concerning the mildew and lingering odor problems in his house.
He eventually decided to install a heat-recovery ventilator to
improve the indoor air quality. A heat-recovery ventilator will
improve ventilation levels, but it is not designed to provide
makeup air for the heating appliances.
All fuel-burning appliances require combustion air. Strictly
speaking, combustion air has three components:
=Stoichiometric air, which is the air required for the
chemical combustion process.
=Excess air, which is the "little bit of extra air" that
appliance manufacturers require to ensure that the amount of
air available for combustion is adequate.
=Dilution air, which is the air required to dilute the flue
gases enough to allow their passage through the vent.
The term "makeup air" is used to describe these three
components of combustion air, along with cooling air, which is
the air required to cool the room in which the appliance is
Appliances in an Unconfined Space
In an unconfined space, the makeup air for heating appliances
is provided by the large volume of air present in the space
where the appliances are located -- usually, a basement or
According to the National Fuel Gas Code, a space is
defined as unconfined if it has a volume greater than 50 cubic
feet per 1,000 Btu/h of the combined input of the fuel-burning
appliances. For example, if a house is equipped with a gas
furnace with an input rating of 70,000 Btu/h, plus a gas water
heater with an input rating of 40,000 Btu/h, the total input of
the appliances would be 110,000 Btu/h. The space where these
appliances are located would need to measure at least 5,500
cubic feet to be considered unconfined -- equivalent to a space
about 25x28 feet by 8 feet high.
Makeup air enters an unconfined space through uncontrolled
infiltration from the exterior -- for example, by finding its
way between the top of the concrete foundation and the sill
plate. With the advent of tight construction practices,
however, some basements that meet the code definition of an
unconfined space may not provide enough makeup air for
How Tight is Too Tight?
How can a builder know when a house becomes too tight for
uncontrolled infiltration to provide adequate makeup air to
appliances in an unconfined space? Unfortunately, existing
codes do not provide clear answers. NFPA 31 refers to the
possibility that a building can have "insufficient air because
of tight construction," (NFPA 31-1-9.3.2) without defining when
that point is reached. The National Fuel Gas Code
standards for makeup air assume that a building has at least
1/2 air change per hour.
If you are building houses with housewrap, sill seal,
weatherstripped doors, and caulked windows, it is probably
unwise to depend on uncontrolled infiltration to provide makeup