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. They are at odds with the common sense of gas safety, and their safety should not be taken for granted.
If gas combustion is incomplete for any reason, soot and carbon monoxide are
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. These are all way beyond the inspection capabilities of a home inspector.
It is possible for well-designed backup systems to fail, no matter how carefully manufactured. Nothing in the realm of human invention is 100% safe. The risks inherent in unvented gas fireplaces may be small, but are not nonexistent.
Those who have unvented gas fireplaces in their homes should exercise caution in the following ways:
Install carbon monoxide detectors as directed by the manufacturer.
Never leave the gas logs burning while you sleep.
Keep the fireplace doors open when the fixture is in use.
Keep a window slightly open when the gas logs are burning.
Follow all the manufacturer’s installation instructions and safety precautions.
Problem such as these do not commonly occur, but again, it should not be assumed that unvented gas log systems are unconditionally safe.