Why Do Heat Exchangers Fail? 7 Common Causes And Safety Tips

When you turn on your furnace on the first cold night of the season, you expect warm, safe air to flow through your home. At the heart of that process is the heat exchanger, a metal chamber where the flame heats the air that moves through your ductwork. When a heat exchanger fails, it is more than an inconvenience. In severe cases, it can allow carbon monoxide to enter your home.

Many homeowners around Rancho Cordova and Sacramento call us because their furnace has shut down or is making unusual noises. Our technicians always make time to inspect the heat exchanger during a diagnostic or tune-up. When you understand why heat exchangers fail and what warning signs to watch for, you can make better decisions about your furnace and your family’s safety.

This article explains what a heat exchanger does, seven common reasons they fail, what a cracked heat exchanger means for your home, and how to reduce the chance of a failure in the first place.

What Is A Furnace Heat Exchanger?

In a gas furnace, the heat exchanger is a sealed metal chamber or a series of metal tubes that separate the burner flame from the air that flows into your home. Combustion gases stay inside the heat exchanger and vent safely to the outside through the flue pipe. The blower moves room air across the outside of the heat exchanger so that air warms up without ever touching the flame or exhaust.

Most standard efficiency furnaces have a primary heat exchanger. Higher efficiency models may also have a secondary heat exchanger that squeezes extra heat out of the exhaust before it leaves the furnace. The metal in these components heats up every time your furnace runs, then cools down when the cycle ends. Over many seasons, this constant expansion and contraction can take a toll on the metal.

Because the heat exchanger is a sealed part that handles hot gases, the manufacturer designs it to meet strict safety standards. A crack, hole, or separation in that chamber means the furnace no longer meets those standards and should be taken out of service until the issue is addressed.

7 Common Reasons Heat Exchangers Fail

Heat exchangers rarely fail for a single reason. In most homes, several factors work together over many years before a crack or hole appears. Here are seven of the most common contributors our technicians see in the field.

1. Oversized Furnace

Many older homes in the Sacramento Valley have furnaces that are larger than they need to be. An oversized furnace brings the temperature up very quickly, then shuts off, only to start again soon after. This short cycling means the heat exchanger heats up and cools down more often than it should.

Every time that hot flame hits the cool metal firebox, a small amount of condensation forms inside the chamber. A properly sized furnace runs long enough to evaporate that moisture. An oversized furnace may shut off before the condensation has a chance to dry. Over the years, that extra moisture promotes rust on the inside of the heat exchanger. Enough rust will eventually eat through the metal and create holes.

If your furnace seems to start and stop constantly, or certain rooms get very warm very quickly while the rest of the house is still cool, oversizing could be part of the problem. A load calculation and proper sizing are critical when it is time to upgrade your system.

2. Not Enough Return Air

Your furnace can only heat the air it receives through the return duct system. Many systems installed before about 2015 were built with return ducts that are too small for the equipment. The furnace is designed to see a specific amount of air passing over the heat exchanger with a specific gas input. If the return duct cannot bring in that amount of air, the metal runs hotter than it should.

Hotter metal expands more during each cycle, then contracts again when the furnace shuts off. Over time, this extra expansion and contraction stresses the metal and makes it more likely to crack or separate. The furnace may also trip on high limit, causing additional short cycling.

Signs of poor return air include loud return grills, whistling sounds at the filter slot, or rooms that never quite feel even in temperature. Have you ever pulled out a filter and heard the ductwork spring back into shape? That is a sign the system is struggling to get enough air.

3. Dirty Or Restrictive Air Filters

Even with a properly sized return, a dirty or overly restrictive filter can choke airflow across the heat exchanger. As the filter loads up with dust, pet hair, skin cells, and other particles, it becomes harder for air to pass through. The result is similar to having a small return duct. The heat exchanger runs hotter, and the metal experiences more stress with each cycle.

Filters now come in many sizes and efficiency ratings, including high MERV filters that capture very fine particles. In the right application, these can be helpful, but if the system is not designed for that level of restriction, the filter can cause more harm than good.

If you notice your filter looks heavily caked with debris or seems bowed into the duct, it is past time to replace it. Most homeowners in our climate should plan to check filters every month during the heating season and replace them at least every one to three months, depending on the type and home conditions.

4. Household Chemicals In The Air

Everyday products in your home can create chemical vapors that eventually end up in the furnace. Hair spray, fabric softeners, bleach, cleaning sprays, and other products can pass through the return, past the filter, and land on the metal surfaces inside the furnace. Over many seasons, those chemicals contribute to corrosion on the heat exchanger.

This is especially common when the furnace is located in a laundry room or near storage shelves filled with chemicals. There may be no immediate symptom, but the metal is quietly breaking down.

Whenever possible, store harsh chemicals away from the furnace area and keep lids tightly closed. If you are doing a lot of cleaning or laundry while the furnace is running, good ventilation helps reduce the buildup of fumes in the air stream.

5. Off Gassing From New Construction Or Remodeling

If you have recently remodeled your home or built an addition, the materials used in that project can release vapors into the air for months. Fresh paint, new carpet, adhesives, cabinets, and flooring all give off what is known as off gassing. That new carpet smell or paint smell is a sign of chemicals evaporating into the air.

When the furnace runs during or after a remodel, those vapors travel through the system and across the heat exchanger. Just like household chemicals, they can combine with moisture and heat to slowly erode the metal. This is not usually a single-year problem, but it adds to the overall wear on the heat exchanger surface.

If your home is under construction during the winter, talk with your contractor about protecting the furnace. In some cases, it may be better to use temporary heat rather than running the finished equipment during the dustiest phases of the project.

6. Frozen Evaporator Coils Dripping Onto The Furnace

In a vertical furnace and evaporator coil combination, the air conditioning coil typically sits directly above the furnace. If that coil freezes because of a refrigerant issue or restricted airflow, it eventually has to thaw. When it does, large amounts of water can pour off the coil in a short period of time.

Under normal conditions, condensate drains away through a pan and drain line. During a serious freeze-up, water can overflow the pan or leak out of the cabinet. On some installations, that melt water drops right onto the metal heat exchanger below. Repeated wetting and drying of the metal encourages rust and corrosion, especially at seams and bends.

Have you ever noticed water around the base of your furnace after running the air conditioner? That is a sign the condensate system or coil needs attention, and the heat exchanger may be at risk as well.

7. Poor Heat Exchanger Design Or Manufacturing

Not all heat exchangers are built the same way. There are clamshell designs, serpentine styles, and tubular heat exchangers. Each style has its own typical failure points.

Clamshell heat exchangers tend to fail in the back of the chamber, in areas that are hard to see with a simple mirror. Technicians may use specialized inspection methods, such as a hydro scan or small cameras, to look for cracks.

Serpentine designs use eyelets or rings to hold sections together. Those eyelets can deform, pop off, or crack. If a manufacturer would never ship a furnace from the factory with missing or damaged eyelets, then a furnace that develops those problems in the field should be considered failed.

Many newer furnaces use tubular heat exchangers. These are often made of stainless steel and bent into shape at the factory. The tubes are stamped into the collector box and burner faceplate. Failures on tubular designs often occur where the tubes connect to these plates or on the first bend at the back of the furnace. Corrosion and moisture play a large role in these failures.

Even a well-designed heat exchanger will eventually wear out. In our climate, a properly installed and maintained furnace often lasts around 15 to 20 years. Systems that are oversized, poorly ducted, or neglected may see problems sooner.

The Danger Of A Cracked Heat Exchanger

The main concern with a cracked or rusted heat exchanger is the potential for combustion gases to mix with the air you breathe. Those gases can contain carbon monoxide, a colorless and odorless gas that can cause headaches, nausea, confusion, and in severe cases, death.

Modern furnaces have multiple safety switches that shut the system down if something is clearly wrong, but those switches cannot detect a small crack in a heat exchanger wall. That is why inspections and carbon monoxide detectors are so important.

If a technician confirms that your heat exchanger is cracked, the furnace should be taken out of service until it is repaired or replaced. It is never safe to ignore a confirmed heat exchanger failure or to keep resetting the furnace to get through another night.

Signs Your Heat Exchanger Might Have A Problem

Only a trained technician can positively confirm a cracked heat exchanger, but there are symptoms that should prompt a call for service. Have you noticed any of these in your home?

• Soot or scorch marks around the burner compartment or vent pipe
• Flames that flicker, lift, or change color when the blower starts
• A strong metallic or burning smell when the furnace runs
• Water or rust around the base of the furnace cabinet
• Repeated furnace shutdowns that require you to reset power or the thermostat
• Carbon monoxide alarms going off inside the home

Any time a carbon monoxide alarm sounds, leave the home, get everyone to fresh air, and call for help. Once everyone is safe, the next step is to have a qualified technician evaluate the furnace and other fuel-burning appliances.

How Our Technicians Inspect Your Heat Exchanger

When our team is called to look at a furnace in Rancho Cordova, Sacramento, or anywhere in Northern California, a heat exchanger inspection is part of a thorough visit. For some furnace designs, that means using inspection mirrors and bright lights. For others, especially clamshell designs, we may use cameras or specialized testing methods that help reveal cracks at the back of the chamber.

Technicians also look for indirect signs of failure. Rust flakes, deformed metal, missing eyelets on serpentine exchangers, or unusual flame behavior can all point to a problem. In some cases, the manufacturer may require specific testing steps before they will honor a warranty claim on a heat exchanger.

As a homeowner, you should feel comfortable asking your technician what they found and how they reached their conclusion. A trustworthy company will be happy to explain their process, show you photos or video from inside the furnace when possible, and review your options in plain language.

Repair Or Replace When A Heat Exchanger Fails

Once a heat exchanger has cracked or rusted through, most homeowners are faced with a decision. Should you replace just the heat exchanger or the entire furnace?

Technically, some heat exchangers can be replaced as a part. On older units, however, that can be a labor-intensive repair. By the time a heat exchanger fails, other major components in the furnace may also be near the end of their useful life. In many cases, replacing the entire furnace is the safer and more cost-effective choice.

When we talk through this decision with homeowners, we look at:

• Age of the existing furnace
• Overall condition of the system and ductwork
• Cost of the heat exchanger part and labor
• Efficiency and comfort benefits of a new system
• Any manufacturer or utility rebates available at the time

If your system is relatively new and still under a manufacturer’s warranty, a heat exchanger replacement might be reasonable. For older equipment, especially those 15 years or more, most homeowners prefer to invest in a modern, efficient furnace rather than put a large amount of money into a unit that is already near the end of its life.

How To Prevent Heat Exchanger Failure

No furnace lasts forever, but there is a lot you can do to help your heat exchanger reach its full expected life.

1. Change filters regularly. Keep a schedule for filter checks, especially during the heating season. Replace filters before they become heavily loaded with debris.
2. Schedule annual maintenance. A professional tune-up includes checking temperature rise, verifying safe operation, and inspecting the heat exchanger and vent system.
3. Address airflow issues. If your system is noisy, some rooms are always uncomfortable, or the furnace cycles on and off frequently, have the ductwork and sizing evaluated.
4. Protect the furnace during remodeling. Limit dust, paint fumes, and off-gassing materials from circulating through the system when possible.
5. Keep chemicals away from the furnace. Store bleach, cleaners, and other harsh products in sealed containers away from the equipment.
6. Install and maintain carbon monoxide detectors. Place them on every level of your home and test them regularly.

Here in the Sacramento Valley, our coldest temperatures usually arrive in December and January, with overnight lows dipping into the 30s. It might not be the kind of winter you see in the Midwest, but when that first cold snap hits in late November, your furnace often goes from months of sitting idle to running hard for several hours. That is exactly when many heat exchanger problems show up.

Taking care of your system before the season starts gives you a better chance of smooth, safe operation all winter long.

Conclusion: Keep Your Home Safe And Warm

Understanding why heat exchangers fail helps you make informed choices about maintenance, repairs, and replacement. If you have concerns about your furnace, notice any of the warning signs described above, or just want peace of mind before the next cold snap, our team is ready to help.

Fox Family Heating and Air Conditioning serves Rancho Cordova, Sacramento, and Northern California. If you need help with your furnace or heat exchanger, give us a call!