If your central heating system or wall heater runs on propane or natural gas, and you smell rotten eggs or sulfur near it, the unit could be leaking. The source of a furnace gas leak isn't always clear, but if you suspect a leak, your course of action is: You should go outside immediately, then call the gas company. The U.S. Consumer Product Safety Commission reports that furnace leaks cause thousands of fires annually.

The Rotten Egg Smell

Both propane and natural gas are naturally odorless, so the gas company adds a chemical to give them them an unmistakeable odor. The pungent gas is methyl mercaptan -- which is produced by combining hydrogen sulfide gas -- infamous as the odorous gas in sewers -- with methanol. Detecting the rotten egg smell of hydrogen sulfide in water is unpleasant, but it's seldom dangerous. When gas heating or cooking systems are involved, however, it's a different story. If you can smell mercaptan, enough flammable gas is in the air to potentially explode.

Momentary, Localized Gas Odors

Each time a furnace or space heater switches on, a small amount of unburned gas flows through the pilot tube before the burner ignites, so a momentary whiff of sulfurous gas shouldn't alarm you. The odor typically lasts for less then a minute and is usually only detectable near the burner or the outdoor vent. Suspect a leak only if:

  • the odor persists.
  • the odor is present when the heater isn't on.
  • the odor is particularly strong.
  • you can smell the odor far from the furnace or room heater.

Possible Sources of Leaks

It isn't a good idea to look for sources of gas leaks yourself, but when gas company representatives arrive with proper testing equipment, they'll check several components of your heating system:

  • The furnace combustion chamber -- The furnace controls may be damaged, and the gas valve may be releasing gas even though the burner isn't igniting.
  • Other internal furnace components -- One or more of the gas delivery tubes inside the furnace may be cracked, or a connection may be leaking.
  • Gas pipes -- If your furnace is an older one, your gas piping system may be constructed from black iron. This rigid material is prone to cracks as it ages, and leaks are common from joints. Even if your gas pipes are made from corrugated stainless steel tubing, a modern alternative to black iron, they can still be compromised by an indirect lightning strike if they weren't properly grounded by being bonded to your home's electrical system.