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MAKEUP AIR FOR Combustion Equipment

Boilers, furnaces, and water heaters need adequate fresh air to function properly. Here's how to prevent problems.

Try breathing through a soda straw. It takes effort, and you'll probably get dizzy soon. A fuel-burning boiler, furnace, or water heater can also end up gasping for air when installed in a very tight house or a confined space. The appliance won't get dizzy, but it can't burn its fuel properly -- resulting in reduced efficiency, and possibly exposing occupants to harmful gases such as carbon monoxide.

A fuel-burning appliance needs a lot of air to burn properly. For example, a 100,000 Btu/h boiler or furnace needs 1,250 cubic feet of air per hour for proper combustion. With the exception of vent-free gas fireplaces and kitchen ranges, all gas- and oil-fired appliances are vented, using either fan-assisted venting or traditional atmospheric venting via a chimney.

Thanks to the natural buoyancy of hot flue gas -- which rises rapidly up the vent pipe, just as a hot air balloon rises in the open air -- vent pipes and chimneys suck flue gas out of appliances and suck in additional air to replace it. Here's the rub: You must replace the same amount of air that rises up the vent, and the makeup air must flow to the burner as easily as the flue gas was vented.

Causes of Poor Draft

If a vent pipe has poor draft, the burner may perform poorly. There are several possible causes of poor draft:

An undersized, restricted, or blocked vent.

Try to locate the appliances as close to the chimney as you can. Long horizontal vent runs, called "lateral piping," decrease the capacity of the vent (see Figure 1). The maximum length of lateral piping is listed on the vent capacity tables in the National Fuel Gas Code, as well as the tables provided by the vent pipe manufacturers.

A cold chimney.

A cold chimney or vent will cool the flue gas and reduce its buoyancy. In some cases, this "cold stacking" is enough to allow the moisture in the flue gas to condense. Outside chimneys (chimneys exposed to the weather on three sides) are vulnerable to cold stacking, especially when a setback thermostat shuts down a heating appliance for several hours at night. To address the problems of cold stacking, the 1996 National Fuel Gas Code included new regulations covering the use of outside chimneys. In most cases, the new regulations require that an outside chimney have a metal liner with a surrounding air space, which allows a cold flue to come up to operating temperature as quickly as possible.

House depressurization.

Under certain conditions, the indoor air pressure in a house may fall so low that it interferes with normal venting of flue gas .

Downdrafts of wind.

Wind can interfere with normal chimney venting, especially if the chimney is lower than the roof ridge.

Lack of makeup air.

If the house is extremely tight, the heating appliance may be unable to suck in enough makeup air to allow complete combustion.


Figure 1.Avoid long horizontal runs of vent pipe, which can cool the flue gas and reduce the draft. This gas-fired water heater should have been located closer to the chimney.