What to Look for When Inspecting a Home with a Combustion Furnace

Chalkboard with the words Furnaces 101

Learning about combustion furnaces helps you determine the safety level of a home.

Furnaces are a great invention. They heat our homes so well and so efficiently. But when something goes wrong with them, they can be dangerous. When you mix gas and air and fire, you have to do it right.

When you’re inspecting a home with a combustion furnace, what should you look for to make sure it’s operating safely?

How Does a Combustion Furnace Work?

First, you have to know how the combustion furnace works. Combustion simply means fire or burning. A combustion furnace burns gas to create heat to warm the home.

But fire can’t burn without oxygen. That’s why you can put out a fire with dirt or baking soda. A popular science experiment involves lighting a candle, placing a glass jar over it and then watching the flame use up the oxygen in the jar and then go out.

Some furnaces have an atmospheric combustion chamber while others are sealed combustion furnaces. An atmospheric combustion furnace usually has a grill through which it pulls air from the atmosphere, or the house. Once combustion takes place inside the furnace, the exhaust fumes are expelled from the house through a flue.

With a sealed combustion furnace, air is taken from the outside via a pipe. These are safer, inasmuch as the danger of carbon monoxide buildup inside the house is eliminated with this type of system.


Check the vents on a combustion furnace for backdraft.

Fresh Air is Necessary

Atmospheric combustion furnaces must have adequate air to function properly. Thus, if they are in a small basement room, either the doors must be left open, or louvered doors must be installed for adequate ventilation. Without this ventilation, the combustion process would be incomplete and negative air pressure would build up.

Further, it would create the potential for backdraft, which is when the air flows in the opposite direction through the furnace. This can cause combustion gases, or spillage, such as carbon monoxide, to be sucked back into the home.

The ASHI Reporter says that checking for backdraft is as simple as holding your hand near the furnace draft hood, though others use a lighter to check which way the air is flowing. With your hand, you should feel cool air flowing into the furnace. If instead, you feel warm or moist air coming out, there is a backdraft issue.

But give the furnace a minute or two to get the air flowing in the proper direction before you check. The flue will likely have residual cool air in it that will need to escape before the heating process is entirely underway.

Rust Is a Bad Sign on a Combustion Furnace

Also look for rust around the draft hood. Condensation is a byproduct of the combustion process when the gas is cooling. If gas is frequently spilling back into the home, the accompanying condensation can cause rusting on the hood.

Spillage can also result from a furnace that is too small for the chimney or from a blockage in the chimney. If you suspect the latter, you can attempt to look up the chimney while the furnace is off to see if you spot any objects in it.

If you find anything suspicious about the combustion furnace in the home you are inspecting, flag it and recommend to the homeowners that they get the furnace fully inspected by a reputable local company. Also, advise them to get a carbon monoxide detector for the home. These inexpensive devices can save the lives of the occupants in the event of a malfunction or breakdown.

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