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Problems with Gas Appliances and Older Chimneys

Modern Gas FireplaceApproximately 70% of the hearth appliances purchased today are fueled by natural gas or propane, according to the Hearth, Patio & Barbecue Association. Gas fireplaces are popular for many reasons, including that the fuel is cost-effective and the appliances are wonderfully convenient, offer authentic-looking logs, and provide a wider variety of options for installation than appliances that run on other types of fuel. There is, however, one very important issue related to safety that is often misunderstood by homeowners and it is that special precaution needs to be used when a gas appliance is vented through a traditional masonry chimney.

The Chimney Safety Institute of America (CSIA), an authority on fireplace safety, cautions homeowners that gas appliances can cause serious problems in your chimney and that those issues can be dangerous.


The high-efficiency appliances of today cannot vent in the same way that traditional fireplaces do. For instance, wood-burning fireplaces create a substantial amount of heat in the chimney, causing the draft to operate as it was designed to. The heated air naturally rises, as it should, and exits the home. With gas fireplaces, there is less heat in the chimney. For a masonry chimney to properly vent a gas fireplace, the connector pipe and the flue must be the appropriate size, as recommended by the manufacturer of the appliance. Without careful attention to proper installation, the flue and connector pipes are often too large, hindering overall operation and safety.

For gas to be a clean-burning fuel, conditions must be right. There must be air for combustion. Various elements could be in the air, and those elements often create acids, when combined with moisture. If the air contains chlorides that are picked up from the use of household cleaning products and if that polluted air mixes with moisture, the result is acidic condensation.

During combustion, water vapor is always produced. Two cubic feet of water vapor is produced by burning one cubic foot of gas. When the venting system is correctly matched with the heating appliance, the water vapor is transported out of the system quickly enough and at a warm enough temperature that no condensation is formed inside the flue. When heat is lacking, the moisture turns to acidic condensation that will deteriorate the inside of the flue.

Visible Symptoms

The expert eye of a professional chimney technician is sometimes required to identify problems with the venting of a gas appliance. Oftentimes, however, there is visible evidence that the heating system and the venting system are not properly matched. Any of the following could be evidence of the mismatch between a gas fireplace and a masonry chimney:

  • Wallpaper begins peelingFlue Installation for Gas Appliances
  • Damp patches appear on exterior or interior walls
  • Paint becomes blistered
  • The ceiling is stained
  • On the outside of the masonry chimney, there is efflorescence, which is white staining
  • Bricks begin crumbling
  • Mortar joints erode


Professional Installation and Inspections

Safety should always be a top consideration in the use of fuel-burning appliances, including gas fireplaces. The best way to ensure that the connector pipes of a gas appliance are properly matched to a flue is to have your gas fireplace professionally installed. In addition, have your gas heating systems inspected annually.

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