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What Happens If Your Furnace's Heat Exchanger Fails?

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Natural gas, oil, and propane-fired furnaces are equipped with a heat exchanger, a set of long, looping tubes or coils that radiate heat from the combustion gases contained within. The heat is given off to the passing air generated by the blower fan, which eventually circulates throughout your home.

So what happens if this essential piece of equipment fails? The following takes a look at that scenario and what you can do to keep it from happening.

How Heat Exchangers Fail

The majority of heat exchangers fail due to metal fatigue, usually caused by the constant heating and cooling experienced by the heat exchanger throughout its working life. Metal fatigue can cause stress cracks to form near welds and areas where the metal is at its thinnest, allowing combustion gases to escape the heat exchanger. Metal fatigue can be brought on prematurely if the furnace overheats due to poor airflow, a faulty limit switch, or blower fan failure.

Heat exchangers can also fail due to rust. If corrosive condensation isn't properly removed from the heat exchanger, it can linger and cause rust to form within the chambers. Eventually, the rust will eat through the heat exchanger and allow combustion gases to escape.

The Consequences of Heat Exchanger Failure

A failed heat exchanger can pose a serious health and safety risk to everyone in your home. Escaping combustion gases contain carbon monoxide, a colorless and odorless gas that can cause a wide variety of hazardous and potentially fatal symptoms. Heat exchanger failure can also lead to a no-heat condition for your furnace.

Symptoms to Watch For

  • Little to no heat coming out of your furnace
  • Flame rollout, a condition where the burner flame protrudes from the furnace combustion chamber
  • Constant alarms from your carbon monoxide detectors
  • Visible signs of rust or cracks caused by metal fatigue

How to Prevent Heat Exchanger Failure

Preventive maintenance is the key to preventing heat exchanger failure. You should have your HVAC technician perform a comprehensive inspection of your furnace prior to every heating season. During the inspection, your HVAC technician should check for any and all signs of impending heat exchanger failure, including rust spots and stress cracks.

If and when heat exchanger failure occurs, your best and only option involves a complete replacement of the failed component. Attempts to patch your heat exchanger could result in a more serious failure, plus it can put you and others in your home at greater risk of CO poisoning. 

For more information, contact a professional in your area or visit a website like