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3 Heat Exchanger Problems That Could Affect Your Furnace

by Amy Henry

Just as your air conditioner relies on its evaporator and condenser coils to transfer heat, your furnace relies on its heat exchanger to transfer heat from passing exhaust gases to your home's indoor air. While heat exchangers are usually reliable, they can fall prey to a number of problems that could ultimately get in the way of heating your home effectively.

The following not only talks about three of the most common problems you can face with your heat exchanger, but also how you and your HVAC technician can work to resolve these issues and keep your furnace operating as normal.


Heat exchangers are particularly vulnerable to corrosion due to the nature of their operation. The exhaust gases produced through the combustion process also contain water vapor. As the heat exchanger removes heat from the exhaust gases, it also causes small amounts of water vapor to condense into liquid form. Mixed with other corrosive byproducts, the acidic condensate can trigger corrosion within the copper or aluminum tubing, resulting in cracks, holes and the potential exhaust gas leakage that usually follows.

Secondary heat exchangers used on high-efficiency condensing furnaces are especially vulnerable to corrosion, given that these components are designed to extract even more heat from passing exhaust gases. Without proper neutralization and drainage, it's possible for your secondary heat exchanger to suffer significant damage from corrosion.

Venting problems can also lead to rust and corrosion buildup on the heat exchanger. Poor venting can also cause carbon monoxide (CO) levels to increase over time, which is why it's important to not only keep your CO detector in great shape, but also to address any venting problems you may have with your furnace.


Fouling is another common issue that could cause trouble for your heat exchanger and, by extension, your furnace. This phenomenon usually occurs when dirt, algae, lime scale or another type of solid material collects within the heat exchanger tubes. The buildup makes it difficult to move heat from the passing exhaust gases to the indoor air and continued buildup can even prevent those gases from passing through entirely.

Your HVAC technician may be able to clean the heat exchanger using a variety of tools and techniques, including the use of chemical descalers. In more extreme cases, it may be necessary for your technician to replace the entire heat exchanger.

In the meantime, you can prevent most instances of fouling by ensuring your furnace is able to burn its fuel as cleanly as possible. Keeping the furnace burners clean and properly aligned can help promote cleaner fuel burn. You'll also need to make sure your furnace receives the proper amount of outside air for combustion.


Overheating is another problem that not only affects your heat exchanger, but it can also affect the rest of your furnace, as well. An overheating heat exchanger can easily trip the temperature limit switch on your furnace, eventually leading to short cycling issues that place additional wear and tear on your furnace.

Most overheating issues usually stem from a lack of proper airflow. If there's not enough air blowing over the heat exchanger to carry away the constantly-building heat, the surface of the heat exchanger will continue to heat up and expand. Sooner or later, heat-induced stress cracks will form near welds, bends and other vulnerable areas along the exchanger.

Annual inspections and preventative maintenance both play a key role in preventing heat exchanger damage due to overheating. Although some smaller cracks can be sealed or patched shut if caught early enough, larger cracks and other defects caused by overheating usually require the entire heat exchanger to be replaced.