The Plugged Secondary Heat Exchanger – Destroyer #6

Furnace destroyer #6 – The Plugged or failed Secondary!

In our Rapidtech Heat Exchanger Program, we teach technicians all about heat exchangers and their safety. In this destroyer post we will cover the plugged secondary heat exchanger .

Furnaces and boilers must transfer the heat from the burned fuel to your home or to the space that is to be heated. This is done by heating up an object and then passing water or air through or across that object allowing the heat to transfer. Thus, the term HEAT EXCHANGER.

All forced air condensing gas furnaces use two heat exchangers. The main heat exchanger is the primary heat exchanger and the condensing heat exchanger is called the secondary.

When we talk about heat exchangers failing, often times technicians and homeowners only think of the cracked heat exchanger. This is not always the case.

Here is a list of different types of heat exchanger failures:

  1. The cracked heat exchanger
  2. The pitted, rotted, rusted out heat exchanger
  3. The heat exchanger with holes in it
  4. The sooted primary heat exchanger
  5. The plugged or rotted secondary heat exchanger

If a technician shares that he or she has found a failed secondary heat exchanger the proof is easy to find. Here is what will happen or what you may notice if you have a plugged secondary heat exchanger

  1. The carbon monoxide readings in the exhaust will be elevated and sometimes off the charts
  2. The furnace will not run properly
  3. There may be water leaking from the lower portion of the furnace
  4. You may smell sulfur or a rotten eggs smell at the exhaust or near the furnace
  5. The drain hoses and furnace drain trap may be black or a dark brown

Is this dangerous? The chances of getting carbon monoxide poisoning is slim. However, if your furnace has a plugged secondary, I guarantee it will not make it through a heating season. I highly suggest that you get it repaired or replaced as soon as possible.

What causes the plugged secondary?

In 26 years, I have seen a number of plugged secondary heat exchangers. Most that I have found were because the polypropylene coating started pulling away from the metal causing a restriction and eventually a full blown clog. I have seen propane furnaces where the furnace was not burning properly thus causing soot and eventually caused a clogged secondary. Then there was a time when I found things like bees getting into the pipes and making the secondary a home.

Causes of failed secondary heat exchangers

  1. Breakdown of polypropylene
  2. Over-fired furnace
  3. Undersized duct system
  4. Improper air fuel mixture
  5. Lack of maintenance
  6. Normal wear
  7. Life cycle of that furnace has finally come to an end

Homeowners advice.

If your furnace has a secondary heat exchanger that is pitted, rotted or plugged it’s time for repair or replacement of the furnace.

Technicians advice.

Check secondary heat exchangers on EVERY call. Perform a combustion analysis on every gas furnace repair and maintenance call. If the secondary heat exchanger is the finned style make sure you check it for cleanliness and be sure that it is not restricting air. If you are seeing carbon monoxide readings in the exhaust above 200 ppm, this is reason to investigate further.

If you would like to learn more contact us regarding our Rapidtech Heat Exchanger Certification Program.


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