Vent-free gas log fireplaces are heating units that work without the use of a chimney. Homeowners appreciate their easy operation and that the units don't require stocking wood for burning. However, it's a fact that vent-free fireplaces don't make use of a chimney that has sparked debates in the energy and heating industries about their safety and effectiveness.
Burning natural gas creates moisture. If you have a vented gas log fireplace, that moisture is directed outside as it moves up the chimney, but it remains indoors if you have a vent-free unit. The Vent-Free Gas Products Alliance asserts that vent-free fireplaces "emit a very moderate level of water vapor." Furthermore, the alliance views the emission as a potential benefit since central heating systems tend to dry indoor air to the point where some homeowners operate humidifiers in winter.
Concerns have been raised about the potential for mold growth inside homes due to the moisture created by vent-free fireplaces. According to the Vent-Free Gas Products Alliance, an independent study concluded that vent-free gas products don't create enough water vapor to raise humidity levels high enough to support mold growth in most homes. An article by Arnie Katz with the nonprofit corporation Advanced Energy questions such claims. Katz asserts that if moisture from a vent-free unit makes contact with a cold surface, it condenses, releases heat and ultimately creates a "perfect vacation resort for mold."
Katz acknowledges in his article that vent-free fireplaces emit low levels of carbon monoxide inside homes. Even so, he adds that there is a growing amount of medical evidence that suggests even low levels of carbon monoxide can create "serious health consequences." Yet in its answers to vent-free safety questions, the Vent-Free Gas Products Alliance cites more research that indicates vent-free products meet various emissions requirements even when they're used for long periods.
Vent-free gas fireplaces are said to be energy-efficient because no heat escapes through a chimney as it does with vented models. Again, Katz disputes this claim because he notes that owner's manuals for vent-free fireplaces recommend opening a window when the fireplaces are in use. Yet doing so would affect energy efficiency as heat escapes through the window.
The Vent-Free Gas Products Alliance advises consumers to take special care in cleaning vent-free fireplaces. The alliance says it's important that users return the gas logs to the correct position over the burners as required by the manufacturer after cleaning a unit. Soot could form from natural gas combustion if the logs are out of place.