1. Furnace is Oversized – It’s normal for the cool metal firebox, or heat exchanger, and a hot flame to temporarily create a bit of condensation inside the tubing or chamber of them. An oversized furnace will not evaporate that condensation completely because oversized furnaces tend to short cycle. They turn on and off more frequently than they should because they’re so powerful they warm the house up very quickly. Too quickly! Too much rust over time will drill a hole in the tubing or chamber, and you’d have a failed heat exchanger.
2. Not Enough Return Air – The system wants to give you a certain amount of air to warm your home. The furnace will only give you what it gets from the return duct (the duct or plenum attached to the grill where you change your filter). Those of us in the industry know that a lot of systems installed before, let’s say, 2015, were installed with return ducts that were too small. The heat exchanger is designed to see a certain amount of air pass over it with the correct amount of gas input and heat the heat exchanger emits. Too small of a return duct will restrict the amount of air that can enter the furnace and cause the firebox to expand and contract too much, causing it to break sooner than it should.
3. Dirty Filters – Another way to slow down the proper amount of air flowing across the heat exchanger is to have a dirty filter. Filters come in several thicknesses. 1″ all the way to 5″. You can have any size and thickness for a filter. But if it becomes impacted with dirt, dust, skin, hair, and whatever else clings to it, the filter restricts airflow like the return duct we discussed in number two.
4.  Check out our blog on how the newer MERV 13 filters can cause problems for your heat exchanger and the rest of the system
5. Household Chemicals – Believe it or not, corrosion can form on the heat exchanger. Things like hair spray, fabric softeners, and bleach can enter the air stream, pass the filter, and attach to the heat exchanger for good. Over time, the chemicals bore a hole through the metal and cause it to fail.
6. Off-Gassing – Let’s say you have a home remodel or new construction. Running the furnace during the construction and even afterward will expose the heat exchanger to off-gassing. What is off-gassing? Like the smell of that fresh paint? Off-gassing. New carpet smell? Off-gassing. And many of those new products keep off-gassing long after that new smell wears off. It’s just more subtle. And just like household chemicals, it will contribute to the overall degradation of the furnace heat exchanger. Frozen Evaporators Coils – Ever had a refrigerant leak cause your air conditioning system to freeze up? Vertical furnace/evap coil combos leave the heat exchanger below the evap coil drain pan. When a frozen evap coil melts, it tends to melt rapidly enough that it overflows or straight drips right down onto the metal heat exchanger. And that causes corrosion over time.
7. Poor Manufacturing Design – There are clamshell, serpentine, and tubular heat exchanger designs.
   
   1. Clamshells tend to fail in the back of the chamber. But it’s tough to get an inspection mirror back there for inspection purposes. A process called hydro-scan is used to identify cracks in these types of heat exchangers. And it’s very effective if done right and if the professional is looking in the right spots. It takes a trained eye!
   2. Serpentine designs are pressed together with the middle areas held together with rings or eyelets. The eyelets on serpentine designs tend to deform, pop off, and crack. And if the manufacturer wouldn’t send a heat exchanger out into the field with cracked or missing eyelets, a technician who later finds them like that should fail your heat exchanger for not meeting the manufacturer’s specs.
   3. Tubular designs are a newer design and, in my opinion, superior to clamshell and serpentine designs. They are usually made with stainless steel and bent into form at the factory. The ends of those tubes are stamped into the collector box and faceplate of the burner compartment. At that connection is where some tubular designs fail. Corrosion plays a big part in tubular heat exchangers failing. Eventually, that moisture, condensation, chemicals, and gasses we discussed earlier can infiltrate the tubes and cause them to fail. And 90% of the time, those failures happen at the back of the tubes on the first bend.

Gas furnaces are very safe. Most people prefer the warmth emitted by gas furnaces. They’re cozy! Ensuring the furnace is installed correctly is almost 100% of the battle. Choosing a reputable contractor is important because they will read the manuals and abide by the building codes that mandate a proper installation. Homeowners need to make sure those filters are replaced regularly.

Hopefully, this helps you understand more on “Why do heat exchangers fail?”

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