My Furnace Is Leaking Gas: The Signs and Dangers of a Cracked Heat Exchanger

My neighbor recently asked me for advice. His kid, who’s 17, bought his first used car this summer, and the engine was rattling. I’m a bit of a car guy, so it didn’t take long under the hood to find a problem: the timing belt was close to snapping. I told the kid a real mechanic would have to replace it—that belt is a vital part of any engine. In fact, if it breaks while a car is running, it’ll destroy the engine and create a hazardous situation on the road. That’s just how machinery works, from car engines to furnaces—there’s always a vital part that, if it breaks, will spell disaster.

It’s a valuable lesson in my line of work as well. There’s no timing belt in an HVAC system, but there is a part that can crack, requiring you to spend a bunch of money getting the entire furnace replaced. It’s called the heat exchanger, and I see them go down constantly at this time of year. When they break, they’re liable to release carbon monoxide and other dangerous pollutants you don’t want in your air. So, I want to share some tips to keep yours healthy. Otherwise, next it’ll be you asking, “What’s that rattling?”

Heat Exchangers: The MVP of Your Furnace

If your furnace was an NBA team, the heat exchanger would be LeBron James, i.e the MVP. See, a heat exchanger is a set of coils or tubes that are looped inside your furnace. In the case of a forced air furnace, which is the common central heating system that uses ductwork and vents, the heat exchanger uses two different air streams: return air that comes from inside your home and hot exhaust produced by the combustion of the fuel.

These air streams are separate inside the heat exchanger. Coils or tubes snake back and forth between the two air flows, heated by the hot exhaust. The return air then passes over the now-hot tubing or metal, thus becoming hot itself, before being blown into the house, never coming into direct contact with the air from the combustion process.

Inside most furnaces, natural gas is burned to create combustion, also creating pollutants that can leak out through cracks and dangerously seep into the air in your home. These toxins include:

• Carbon dioxide
• Carbon monoxide
• Pure carbon, or soot
• Nitrogen oxides

All of these can make your family ill if inhaled in great enough quantities, and carbon monoxide can even prove lethal. Part of what the heat exchanger does, then, is to make sure that air for your house is not polluted by those byproducts. But, if a heat exchanger wears down and cracks, it can’t do its job, which can break your furnace beyond repair—and pollute the air in your home.

Why Heat Exchangers Dangerously Crack

Have you ever bent a paperclip and then tried to bend it back into shape? How many times can you do that before it breaks? Not many—the metal will snap pretty fast. Heat exchangers are also made of metal, albeit stronger than the metal of a paperclip, but they can bend and crack, too, if taxed hard enough.

The usual life expectancy of a furnace is between 15 and 18 years, and heat exchangers are designed to last at least that long. Unfortunately, there are things that speed up that lifespan, like:

• Overuse: In colder climates, furnaces may see as many as 800 hours of use in a given winter. Assuming that the average heating cycle is about 15 minutes, that means the heat exchanger heats up and cools down more than 3,000 times in a season.
• An improperly sized furnace: It’s important to make sure a furnace has been installed and calibrated correctly. If a furnace is oversized, it often short cycles because it heats a home too quickly. A short cycle is a quick burst of heat, rather than a sustained one, and, like overuse, mean that the heat exchanger is being cooled and warmed too quickly. A check-up from a skilled HVAC professional can spot this sort of issue and ensure that an oversized furnace uses a lower stage for its heat output.
• Poor airflow: The third major heat exchanger killer is a lack of airflow. This issue is primarily caused by a dirty air filter. If there’s too much dust and dirt clogging a filter, air simply can’t get through. That means heat can’t get out from your furnace to warm your home, and the furnace will overwork itself as the temperature on the thermostat remains cold, stressing the metal in the heat exchanger and potentially causing cracks.

Be Good to Your Heat Exchanger

Inside a heat exchanger is fire, soot, and carbon monoxide—all substances that can escape if it cracks. Don’t worry though, there are some simple steps you can take to avoid your heat exchanger failing before the lifespan of your furnace is up, including:

• Have regular furnace tune-ups. I’ve talked a lot about the importance of fall furnace maintenance, but this point can’t be overstated. A good furnace tune-up will involve checking your ductwork, as well as the performance and efficiency of the HVAC system, which will help you avoid the multiple scenarios that lead to a crack in the heat exchanger. It will also be evaluated for cracks so you can rest assured toxins aren’t polluting the air in your home.
• Replace your air filter regularly. This seems simple, but it’s not always easy to remember to do. If your furnace is being used regularly, you may want to have your air filter replaced monthly, or at least checked for clogs and obstructions—I recommend folks keep a printed record of when they last changed the filter next to their HVAC system.
• Verify your furnace size. As I mentioned, one of the major causes for cracked heat exchangers is that furnaces are oversized. A trained HVAC professional should be called in to ensure that your furnace has been installed correctly and sized right, making sure it isn’t overworking itself.

By the time you read this, I’m almost certain that my neighbor’s son will have had to get his timing belt replaced. Like I said, I only dabble in cars, but I imagine waiting until the belt started to act up in the engine didn’t make this repair any cheaper for him. So while it’s too late for him, I hope you learn from his mistake when it comes to your heat exchanger. Do the proper maintenance to ensure its running smoothly and not liable to crack unexpectedly, taking your furnace down with it.

Just remember our example of a paperclip. A strong one can be used to bind your papers together for months and months, but if you use it in ways it wasn’t intended, and start to bend it out and back into shape a bunch, you’re greatly increasing the chances it will crack. Don’t let that happen with your heat exchanger. Hire a trained HVAC professional to perform regular furnace tune-ups and verify that your furnace is functioning properly. It sure beats being caught unaware—or having dangerous toxins leak into your home and your family’s air.

The trained HVAC professionals at Bell Brothers can perform regular furnace maintenance, verify that your furnace is installed and sized correctly, and even regularly replace filters for you. Contact us today to make sure your heat exchanger is properly cared for.