Healthy Home Basics - Heating and Cooling

64. Why don’t you like fireplaces or gas ranges?

Actually, we don’t like most fuel-burning appliances—fireplaces, wood stoves, gas ranges, oil furnaces, space heaters, water heaters, boilers, etc. Many people think the reason we don’t like them has to do with fuel leaks. While oil or gas leaks certainly can pollute a house, we’re much more concerned with poor chimney function. Whenever you burn something, you have combustion by-products with which to contend. And, it isn’t healthy to have any combustion by-products indoors. The worst offenders are unvented appliances because they have no chimney whatsoever, and they dump combustion by-products directly into the indoor air.

You might think that if a fuel-burning device is connected to a chimney, the situation would be better. Well, in many cases it is—but in many cases it isn’t. You see, chimneys don’t always function correctly. The combustion by-products (which are warm) rise up through a chimney because warm air rises. You can actually measure a slight upward pressure (called a draft) inside a chimney. Problems occur when there's a negative pressure (that wants to pull in) within the living space that’s greater than the draft pressure (that wants to push up and out). If that’s the case, air can come down the chimney, and the combustion by-products can’t go up. When this occurs, they spill into the living space.

If there's a complete reversal of flow in a chimney (downward), it’s called backdrafting , and all the combustion by-products enter the house. In many cases, the draft is only partially affected, and some combustion by-products go up the chimney while some enter the living space. This situation is called spillage.

Backdrafting and spillage are actually quite common. In some houses they’re regular occurrences. For example, we’ve all smelled wood smoke indoors—smoke that should’ve gone up the chimney. Experts estimate that 50-80% of chimneys have the potential to malfunction.

Sometimes chimneys don’t work because of poor design, lack of maintenance, or damage. But, an increasing problem has to do with very small air pressure differences. A typical draft pressure in a chimney is often less than 5 Pascals (Pa.). A Pa. is a tiny unit of air pressure. (There are over 200,000 Pa. of pressure in an automobile tire.) If any device is blowing air outdoors, it can cause a very slight negative pressure in the entire house. (Exhaust fans, clothes dryers, and central vacuums have all been implicated.) If that slight negative pressure is more than 5 Pa., you can have chimney problems. There are a number of factors that come into play (house tightness, temperature, wind condi...