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Code Requirements

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.

Makeup Air

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 located.

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 crawlspace.

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 fuel-burning appliances.

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 air.