Featuring America's Home Inspector: Nationally Syndicated Columnist, Barry Stone

The House Detective by Barry Stone, Certified Home Inspector

Dear Barry: We read one of your articles on ventless gas fireplaces and are concerned about the safety of the one in our home. We have a code-approved unvented gas log fireplace. It has a built in carbon monoxide detector and other safety back-up devices, as listed in the owners manual. Are there any known problems with this type of setup? Bill

Dear Bill: Unvented gas log fireplaces are actually misnamed. They should be called interior-vented fireplaces. Most gas-burning fixtures, whether furnaces, water heaters, or fireplaces, vent combustion exhaust to the exterior. “Unvented” fireplaces vent exhaust to the interior of the building. Fortunately, there have been very few reported problems with these fixtures. But conceptually, they are at odds with the common sense of gas safety, and their inherent safety should not be taken for granted.

When gas is burned, the primary byproducts are carbon dioxide and water vapor. If gas combustion is incomplete for any reason, soot and carbon monoxide are also produced. Carbon monoxide, commonly recognized as an odorless, lethal gas, causes no harm when vented to the exterior of the dwelling. When vented into a home, it can cause debilitating sickness or death. For this reason, unvented gas log fireplaces are specially designed to maximize gas combustion. They are also equipped with carbon monoxide sensors, an oxygen depletion sensor, and a complex system of safety shutoff devices. As long as these safety features function perfectly, unvented gas fireplaces can operate without posing a threat to the health and safety of occupants.

The main concern with unvented gas fireplaces is not that they are likely to create a problem, but rather that no amount of well-intended safeguards can make any manmade device totally foolproof. It is possible for well-designed backup systems to fail, no matter how carefully manufactured, no matter how scrupulously engineered, no matter how well conceived. Tragic examples of this fact are two failed space shuttles, probably the most well built technological devices in history.

Nothing in the realm of human invention is 100% safe. The risks inherent in unvented gas fireplaces may be miniscule, but they cannot be deemed as nonexistent.

Those who have unvented gas fireplaces in their homes should exercise caution in the following ways:

  1. Install carbon monoxide detectors in various places throughout the home.
  2. Never leave the gas logs burning while you sleep.
  3. Keep the fireplace doors open when the fixture is in use.
  4. Be sure the gas log unit is not too big for the size of room in which it is installed.
  5. Keep a window slightly open when the gas logs are burning.
  6. Make sure that unvented gas logs are legal in your area.
  7. Follow the manufacturer’s installation instructions and safety precautions to the letter.

In addition to air quality safety, remember that water vapor is also a byproduct of gas combustion. During dry weather, this may not pose a problem. But if humidity is increased to high levels, condensation and mold could be the results.

Problem such as these do not commonly occur, but again, it should not be assumed that unvented gas log systems are unconditionally safe.