This article was crafted with the help of Bob Underwood from Estes Services

When it comes to gas appliance and furnace safety, homeowners may assume that the flammability of gas is the main hazard to guard against. However, Bob Underwood, technical training manager at Estes Services in Atlanta, says that the more important safety concern with regard to gas furnaces is actually the accidental venting of carbon monoxide into the home. “There are high-temperature limiting devices on furnaces to protect against fire,” he points out, “so the primary problem is that carbon monoxide could leak into the home.”

In a properly functioning gas furnace system, the flue transfers carbon monoxide (and other gases produced by combustion) safely out of the house; a damaged or blocked flue can send them into the home instead. A gas furnace’s heat exchanger is another potential source of leaked carbon monoxide. The heat exchanger contains the gas combustion chamber. As air is circulated through the HVAC system, it passes over the heat exchanger and absorbs thermal energy before it’s circulated throughout the house. Over time, the heat exchanger can crack or separate, allowing carbon monoxide to enter the house. Bob not only knows why cracks in heat exchangers sometimes develop, but he also knows what homeowners should do when this problem occurs as well as how regular maintenance can help prevent it in the first place.

The Causes of Cracks in Heat Exchangers

Heat exchangers are subjected to extreme temperatures, and as they heat up and cool down over the course of a heating cycle, their metal expands and contracts; this stress alone makes them susceptible to cracking. However, there are additional factors that can worsen the chance of cracking. “Anything that pushes those high temperatures even higher will accelerate the deterioration of the heat exchanger,” says Bob. Overly high gas pressure is one cause of overheating. Too little airflow through the system is another, and it is often the result of an oversized furnace combined with comparatively undersized ductwork.

On the other hand, too much airflow, which causes lower-than-normal temperatures, can be just as damaging. “In a furnace, vent gases need to be kept at certain temperatures to keep the moisture in the gas from condensing in the wrong places,” Bob explains. “The moisture in burned gas is slightly acidic—about the same level as a tomato—and this acid will corrode a heat exchanger over time.”

How the Pros Identify a Cracked Heat Exchanger

At its outset, a cracked heat exchanger is a problem that will go unnoticed by most homeowners. However, professional HVAC technicians always check heat exchangers for damage during their pre-season maintenance inspection. The first step is simply to look where most homeowners can’t. “Our technicians do visual inspections using a telescoping camera,” says Bob. “However, it’s difficult to see the entire heat exchanger, so we use other methods as well.” According to Bob, in the brief period of time after a furnace is turned on—when the burner is operating and the blower is about to be activated—technicians often check the burner flame. “If a crack has developed, air from the house will be forced into the combustion chamber when the blower turns on, causing some visible disturbance of the flame,” he says.  

Is a Cracked Heat Exchanger Dangerous?

Bob’s answer to whether or not a cracked heat exchanger can be dangerous is unambiguous. “Any verifiable crack requires replacement of the heat exchanger or furnace. It’s only going to get worse, not better—the only question is when.” He adds that all homeowners with gas appliances or a fireplace should have a carbon monoxide detector in their home for just this possibility.

Some situations are more dangerous than others, according to Bob. “The more severe the crack, the swifter the response should be,” he warns. “Also, older furnaces, called natural-draft furnaces, can leak larger quantities of gas because the inside of the heat exchanger is under slightly higher pressure than the airstream into the house.” However, under any circumstances, Bob says, the problem needs to be resolved.

Enlisting Professional Help

For homeowners with newer-model furnaces, there may be a more favorable outcome in the event that a crack develops in their heat exchanger. “A heat exchanger can be replaced without changing the entire furnace,” says Bob. In fact, he adds, most heat exchangers have a longer warranty than most other parts of the furnace—often 10 to 25 years. However, the warranty rarely includes labor costs, which can add up. “Replacing a heat exchanger is a very labor-intensive process because the furnace has to be completely disassembled and reassembled,” he notes. Homeowners with older furnaces may find it more cost-effective to upgrade to a more efficient model, with added benefits that include lower operating costs.

Finally, Bob wants homeowners to know that they have some influence over how fast their heat exchanger wears out, and maintenance is the key. During yearly checks, a well-trained technician can resolve potential contributors to cracked heat exchangers with a few important tasks, including checking the gas pressure and improving airflow issues by setting proper fan speeds, cleaning dirty coils, and changing filters.

With each new model, modern gas furnaces are becoming safer and more efficient. However, regular maintenance is crucial to continued safety and efficiency. Bob likens it to the service you should provide your car. “Keep it cleaned and tuned up,” he says. “The more you keep your system clean and running efficiently, the better the performance you’ll receive from it.” Ultimately, as with many residential HVAC issues, preventative care is every homeowner’s wisest strategy.

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This spotlight article was crafted with the help of Estes Services, an Air Conditioning & Heating Best Pick in Atlanta. While we strive to provide relevant information to all homeowners, some of the material we publish may not pertain to every area. Please contact your local Best Pick companies for any further area-specific advice.