Signs Your Furnace Heat Exchanger Has Gone Bad

Signs Your Furnace Heat Exchanger Has Gone Bad

Heat exchangers allow heat to be transferred without mixing between different liquids and gasses, in this case, hot water and air. A water-to-air heat exchanger is a device that transfers heat from your outdoor wood boiler's water to the air in your home. This can be done by connecting your outdoor wood furnace to your forced air furnace, whether it is a gas, oil, or electric furnace. These exchangers are designed to transfer hot water heat through seamless copper tubes with high-density aluminum fins.

Forced Air Furnace


This device is absolutely the star and hero of any furnace including an outdoor furnace. It’s an essential component of your heating system. When it fails, it can cause damage to your furnace and prevents your home from getting any heat!



Why Do Heat Exchangers Fail?

The most common reason for a failed or cracked heat exchanger is normal wear and tear. The thin metal walls are constantly exposed to high temperatures when you use your furnace. The metal then expands and contracts and will cause stress cracks to grow due to repeated heating and cooling cycles.

The most common cause of a failed heat exchanger is NOT using a filter in the outdoor boiler water flow loop. These are highly specialized filters that can handle high-temperature water, at high flow rates. More than 95 percent of our customers have these filters because they remove sediment in the outdoor boiler…sediment is the “Secret Killer” of Outdoor Boilers. 

Those who do not have a filter usually call us after a few years to replace their heat exchangers, pumps, and other failed components. Learn more about outdoor boiler filters by clicking HERE. 

Three signs of a Failed Heat Exchanger

  1. Cracks. Visual verification is a clear indication that your heat exchanger is damaged. However, due to the location in your plenum, or bends in the metal and other components blocking your view, it is often difficult to discover these flaws without removing portions of the furnace. The most obvious sign of a crack is water leaking on the floor.
  2. Audible sounds. If the heat exchanger has a crack, you may hear hissing or dripping. Because it is constructed from metal, cracks will expand and shrink as it heats, which may cause audible rattling, popping, and/or banging noises.
  3. Slow or No Water Flow. If your heat exchanger is plugged up with sediment, the water flow will be restricted or stopped. If the outdoor boiler water is hot, but your indoor PEX tubing is not hot, and you are not burning much wood, you have a flow problem that could be caused by a faulty pump, or a plugged heat exchanger.

Preventing Damage to Your Wood Boiler Heat Exchanger

Heat exchangers do not last forever. But here are some things you can do to extend the life of a heat exchanger:

  • You should have a Water Filter. Not using a filter will cause damage to your heat exchanger which is more costly than even using a filter. Click Here to Learn More

Filter Kit Assembly

  • Flush the boiler regularly to remove rust. We recommend a complete flush every 4 years. Also, please do the “Periodic Flush” every six months - just open the drain valve (at the outdoor boiler) for five seconds, or until the water runs clear. This will remove any excess sediment in the bottom of your boiler.

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