Dec 28, 2016

Here’s “Why” Your Furnace Heat Exchanger Cracked

in Carbon Monoxide Testing, Central Heating and Air Conditioning, Furnace Repair
written by admin

In our blog, we’ve covered furnace failures, repairs, and replacements extensively. Now, let’s dive deeper into furnace heat exchanger design, how they work, and why they fail.

The Most Dangerous Furnace

*This 75k BTU furnace was installed in a crawlspace and vented to the roof using 3″ “B” vent. We resolved the issue by installing a high-efficiency furnace with PVC venting out the side of the home.

How a Furnace Heat Exchanger Works

A furnace’s heat exchanger burns a fuel/air mixture, generating intense heat. The blower motor (or fan) then blows air over the heat exchanger, absorbing the heat and warming the air. This warm air is distributed through your ductwork to heat your home.


Heat exchanger training for all makes and models of furnaces – 8/23/2016

Signs & Symptoms of Heat Exchanger Failure

All pictures below are real findings from our field service technicians.

This 10-year-old furnace had proper gas pressure, regular filter changes, and was well-maintained. Notice the wear on the left (2) burner inlets.

Above, you can see flame roll-out in the burner area, which tripped the safety switch. We discovered a 3″ crack in the top of the heat exchanger.

After years of neglect, improper installation, and drainage issues, the secondary heat exchanger clogged, melting the plastic flue pipe and ignition control. The residents experienced severe headaches and contacted the local gas company.

This cracked eyelet was found in a home for sale. The local gas company red-tagged the furnace due to excessive flame roll-out upon ignition.

This crack inside the heat exchanger cell was caused by a lack of airflow due to a neglected filter.

Common Forced Air Furnace Heat Exchanger Designs

Clamshell Heat Exchangers

Made of two concave halves joined by welding or folding, clamshell heat exchangers often fail at the narrow back of the chamber.

Serpentine Heat Exchangers

Used in 80%-90% efficient furnaces since the mid-80s, serpentine designs are formed by pressing one side through the other and crimping it back over the edge.

Tubular Heat Exchangers

Featuring U- or S-shaped chambers, tubular designs are newer and often use proprietary processes to strengthen metal at the curves.

What Causes a Furnace Heat Exchanger to Fail?

*Testing a natural draft furnace heat exchanger cell.

  • Oversized furnace BTU for the home (causes excessive corrosion and premature failure).
  • Insufficient return air (leads to metal stress, cracks, and open seams).
  • Restricted airflow from 1-inch pleated furnace filters.
  • Infrequent filter changes (restricts airflow across the heat exchanger).
  • Exposure to household chemicals like bleach, fabric softener, or hair spray.
  • Furnaces running during home construction or remodeling (exposure to out-gassing materials).
  • Water leaks from unmaintained humidifiers or air conditioning coils.
  • Manufacturing defects or poor design.

Thermal Fatigue – Crack Initiation

Repeated heating and cooling cause thermal expansion and contraction, leading to cracks at sharp corners, welds, or areas with restricted airflow.

Furnaces fail for many reasons: improper installation, lack of combustion air, insufficient return air, over-firing burners, improper venting, humidity, dirty filters, and age.

In conclusion, when investigating furnace failures, we often find installation issues like poor duct design, improper filter selection, neglected maintenance, or incorrect blower settings. These factors significantly shorten a heat exchanger’s lifespan.

Your safety depends on the expertise of your HVAC service provider. Annual maintenance is the best way to prevent premature failure and ensure your system’s reliability. Our maintenance program starts at $139.00/year. Call us today at 219-476-3776 to schedule your service!