Heat Exchangers In Gas Furnaces: How To Know When They Fail

Gas furnaces are among the most fuel-efficient means of heating homes in the United States, and they are low-maintenance devices that offer long service lives. However, as with any mechanical object, parts do break and malfunctions occur with age. One common problem with gas furnaces is wear and tear on the heat exchanger. Here is what you need to know about heat exchangers and how to know if yours might need servicing or replacement:

What is a heat exchanger?

A heat exchanger is aptly named because its purpose is to pass heat from the flames inside the furnace to the air that passes through the furnace and on to the inside of your home. Residential heat exchangers are mechanically simple devices; they consist of a metal tube that makes an "S" shaped circuit through the inside of the furnace.

At the bottom of the tube, the burners heat the surrounding air which passes into the heat exchanger tube. As this hot air, which also consists of combustion gases, snakes its way through the exchanger tube, returning air from the inside of the home is blown into the interior of the furnace. This cooler interior air passes by the heated tubes and absorbs heat; by the time the interior air flow leaves the furnace, it is much warmer than when it entered. The combustion air passes out of heat exchanger tubes through a vent stack into the outside air.

What are the signs of a failing heat exchanger?

Heat exchangers are exposed to high temperature extremes for hours at a time, so it's not surprising that problems will eventually develop as the furnace ages. Another cause of heat exchanger breakdown is rusting due to exposure to high humidity and moisture levels. Common problems include cracking, pitting and separation along seams. All of these will allow interior air to enter the heat exchanger tubes, and this can cause several symptoms:

  • Wavering burner flames – when a small amount of the interior air enters a perforated heat exchanger, it can move the burner flames around much the same as a candle flame "flickers" in a breeze. This is often the first sign of a leak in the heat exchanger, but there is usually little or no effect on the output of the furnace.

  • Flame rollout – as a crack or hole in the heat exchanger grows larger, the incoming air can "push" the burner flame out of its normal position. This behavior is known as a flame rollout, and it can be severe enough to actually scorch the furnace outside of the heat exchanger. Rollout is a dangerous condition that can cause a fire or destroy your unit.

  • Temperature rise in exiting air – furnaces are designed to operate at a specified temperature range; if the temperature of the exiting interior air is beyond the upper limit, then a possible cause is a crack in the heat exchanger. Other problems, such as a malfunctioning blower, can also cause temperatures to rise, so don't assume that an exchanger is cracked on this basis alone.

What can be done about a damaged heat exchanger?

If you suspect that a heat exchanger is damaged, then the first step is to get a qualified heating professional to examine the unit. They can conduct diagnostic tests on the exchanger, and examine it for cracks and holes. If they believe that the problem is severe, they may make a recommendation that your furnace be shut-off until a replacement can be made. It is possible that a leaky heat exchanger can allow deadly carbon monoxide fumes to enter your home, though this is a rare occurrence in most situations.

Your options for repair usually are replacement of the exchanger or the entire furnace. Depending on the age of your furnace, your professional may encourage you to purchase a new unit if the current unit is advanced in years.

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