Makeup Air 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).

Figure 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 fans. 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 idea. 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.