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