While improper design and location is a major cause of poor
fireplace performance, tighter house construction and powerful
exhaust fans must share some of the blame. By installing vapor
barriers and using doors and windows that have sealing gaskets,
builders commonly reduce air leakage by more than 75% compared
with the standard construction of 20 years ago. And homes are
now commonly equipped with high-volume exhaust fans, such as
those in downdraft kitchen ranges, which can move air out of
the house at a rate of 600 cubic feet per minute (cfm) or more.
Because tightly sealed house walls will not allow this much air
back into the house through leakage, these powerful fans create
negative pressure that can cause a chimney to backdraft and
fill a house with smoke (Figure 4).
4. In tight houses, depressurization from cooktop vents,
dryer vents, and other exhaust fans can cause fireplaces to
backdraft and spill smoke into the room. Instead of ducting
combustion air to the fireplace, which does nothing to change
room pressure, add a makeup air system linked to the exhaust
One standard fix for smoky fireplaces has been to install a
supply of outdoor air in the belief that air starvation is the
root cause. While lack of combustion air may be a problem in
some cases, supplying outdoor air to the fireplace through a
duct is certainly not the cure. Two research studies, one
conducted in Canada on a series of factory-built fireplaces and
one done in the U.S. on a masonry fireplace, looked into the
behavior of outdoor combustion air supplies. In both studies,
the fireplaces were installed within chambers that could be
depressurized continuously after a fire was lit. As the fires
died down to charcoal, technicians monitored carbon monoxide
readings in the chamber to see when exhaust began to spill from
the fireplaces. Tests were done with and without combustion air
supplied from outside the depressurized chamber. No consistent
difference in spillage timing or amount could be found whether
or not outdoor air was supplied.
The reason is simple: Air flows to zones of lower pressure.
If a room is depressurized to the point where its low pressure
overwhelms the chimney draft, smoke will flow into the room.
Obviously, ducting makeup air to the fireplace doesn’t
work. In fact, building code authorities are currently removing
mandatory outdoor air requirements for fireplaces that were
added only a few years ago, just before research debunked the
Where a notorious air-guzzling downdraft kitchen range
causes excessive depressurization, many homeowners will simply
not use their range exhaust when the fireplace is burning. But
a better solution is to install a kitchen makeup air system
that is interlocked to the range exhaust switch. This kind of
makeup air system would force air into the house to compensate
for the kitchen range exhaust flow. This would prevent
depressurization, and solve the smoky fireplace problem.