All fireplaces, including those that burn natural gas instead of wood, have flues to vent their carbon monoxide and smoke out of a home safely. Flues, however, also represent breaches in the home's thermal insulation. You can close a gas fireplace flue, but only when the fireplace is off.
When a gas fireplace is on, or burning, the fumes it produces rise through the flue and out of the home through a flue, or chimney or wall vent. The pressure from the rising air keeps heat in the home, reducing the amount of fuel burned to keep it comfortable. It also prevents cold air from drafting downward, which can happen under certain wind conditions if the flue is open and no air rises through the flue. Closing the flue when the gas fireplace is not burning prevents the home's heat from escaping up the flue and prevents cold air from entering the home through the flue.
The procedure for closing a gas fireplace's flue is relatively straightforward. To be able to close the flue, it needs a damper, which is a moving plate that engages to close off the flow of air through the flue. Some dampers are near the top of a chimney and engage by pulling a chain. Others are closer to the fireplace and require turning a handle or lever.
Closing a gas fireplace's damper while the flames are still burning allows carbon monoxide from the gas into the home. In large enough quantities, carbon monoxide can cause illness, unconsciousness and even death. Once the gas flame is out, it should be safe to close the damper. If a gas fireplace's fixture leaks, the closed flue will prevent natural gas from escaping, but the leak poses a problem even if the flue is open because natural gas is only slightly lighter than air and won't rise quickly through the flue.
Over time, a gas fireplace's damper may fail to seal. That situation will prevent you from taking advantage of the benefits of closing the flue until you can replace or repair the damper. To close your flue with fewer gaps and leaks, or for an extended period of time, you can add weather stripping to the damper. It will cut off the flow of air and keep your chimney from serving as a source of downdrafts and escaping heat.