What Is a Heat Exchanger and How Does It Work

There’s probably a lot that you don’t know about the heating equipment in your O’Fallon, MO, home. From your furnace’s electric ignition switch to its venting system, there are many important components working together to provide an even and consistent stream of heat. Among these is your furnace’s heat exchanger. Much like its name implies, the heat exchanger is where heat transfer occurs. Positioned just below the combustion chamber and the furnace blower, it has a major role in determining just how efficient and safe your furnace is.

How a Heat Exchanger Works

Heat exchangers are L-shaped metal shells with multiple internal and external tubes. This shell-and-tube configuration minimizes thermal energy loss during heat transfer. When your furnace burns or combusts fuel, the hot exhaust gases from this process leave the combustion chamber and enter the heat exchanger, where their heat is absorbed. As filtered air is blown across the exchanger, it collects this heat before entering your ductwork and being distributed throughout your home.

Heat exchangers experience regular and significant fluctuations in temperature along with incredibly high levels of heat. Their all-metal design makes them excellent heat conductors. This also makes them resistant to temperature-related cracking.

Cracked Heat Exchangers and Why They’re So Dangerous

In oil-fired and gas-fired heating systems, heat exchangers and furnace venting keep harmful exhaust gases like carbon monoxide (CO) from entering buildings. CO is an odorless, colorless gas that causes nausea, headaches, dizziness, and shortness of breath, among many other symptoms. If CO exposure is prolonged or extensive, this gas can even lead to death. It’s also one of many harmful contaminants produced by the incomplete combustion of fuels like natural gas and propane.

Although heat exchangers perform the important job of moving heat from combusted materials to living spaces, they also act as barriers between residents and potentially deadly toxins. Ongoing heating and cooling can wear out these components. Over time, small cracks and other structural imperfections may develop that allow exhaust gases to escape.

Fortunately, furnace manufacturers install failsafe measures to limit the potential for CO exposure caused by cracked heat exchangers. For instance, if your furnace won’t turn on, it may be that you have an excessively dirty or damaged heat exchanger and the unit has shut down. When heating systems detect problems with this component, they immediately stop combusting fuel and moving air.

Some Furnaces Have Two Heat Exchangers

At one time, all residential furnaces had a single heat exchanger. Any residual heat that wasn’t absorbed in these chambers was routed out of homes along with CO and other exhaust gases. Now, high-efficiency furnaces have two: primary heat exchangers and secondary heat exchangers.

When exhaust gases exit primary heat exchangers, they move into smaller, secondary units for further heat extraction. This minimizes the amount of thermal energy that’s lost via furnace venting and allows homes to heat up in less time.

Heat Exchangers and AFUE Ratings

Furnace AFUE ratings reflect the percentage of fuel that’s converted into usable heat energy by way of heat exchangers and other components. For instance, a furnace with an AFUE rating of 82% converts 82% of the propane, natural gas, or heating oil that it burns into heat. The other 18% exits the venting system as exhaust gases.

Medium-efficiency furnaces have AFUE ratings between 80% and 83%. A medium-efficiency furnace only has a single primary heat exchanger. Comparatively, high-efficiency furnaces with secondary heat exchangers can collect between 10% and 15% more residual heat before routing exhaust gases out. Along with other design improvements, their secondary heat exchangers earn them AFUE ratings as high as 98.5%. As little as 1.5% of the fuel combusted results in energy waste.

Heat Exchanger Problems

Cracks in the metal shells or tubing of a heat exchanger are among the most dangerous problems that can develop. However, even heavy buildups of dirt and soot on heat exchangers can affect the safety and functionality of furnaces. Build-ups like these may be the result of poor furnace maintenance. Sometimes this is caused by tilted furnace burners, cracked exchanger sidewalls, or other structural damage. Only licensed HVAC contractors can determine whether these accumulations are symptoms of a larger issue or the issue itself.

One sure sign of problems are burner flames that burn yellow rather than blue. Burner flames are blue when they’re produced in tightly sealed and leak-free combustion chambers. If there are cracks or other air leaks in either combustion chambers or heat exchangers, burner flames will immediately glow yellow. This occurs due to changes in the composition of the gas they’re burning. Yellow flames indicate an improper ratio of oxygen for optimum fuel combustion. If both the heat exchanger and combustion chamber are completely intact, yellow or orange-tipped flames could be an indication of dust within or around these components.

How Long Do Heat Exchangers Last?

Furnace heat exchangers typically last about 15 years. This is on par with the expected lifespan of many furnace models. As such, replacing damaged heat exchangers isn’t something that needs to be done too often. It’s often cheaper, safer, and easier to replace entire heating systems instead.

It’s important to note that by the time fuel-combusting appliances have provided a decade or more of service, they’ve often lost at least half of their efficiency. For instance, if you purchased a furnace with an AFUE rating of 80% two decades ago, this unit could be converting just 40% of the fuel that it consumes into heat energy, or even less. With as much as 60% of the fuel that your furnace uses being wasted, having a new furnace installed could save you money both now and in the future.

How to Care for Your Furnace’s Heat Exchanger

Heat exchangers don’t require any special cleaning or other hands-on care from consumers. However, it is important to have this component inspected at least once each year to verify its integrity and identify factors that may be expediting wear. The dangers of cracked heat exchangers are but one of many reasons to schedule furnace inspection and tune-up services annually. Keeping furnaces clean and functioning like they should will minimize stress on these important components.

Performing regular air filter changes is a great way to protect your furnace’s heat exchanger. Airflow problems arising from dirty air filters place heat exchangers and many other furnace components under undue stress. This extra stress can lead to overheating, dramatic temperature fluctuations, excess condensation, and more. You should check your furnace’s filter monthly. In most homes, these components should be swapped out every 30 to 90 days.

Another important step in protecting your furnace’s heat exchanger is keeping your HVAC air vents open and clean. If there are thick buildups of lint-like material on these features, wipe them off with a soft, damp cloth. You can also unscrew HVAC air vent covers and vacuum behind them.

When maintaining your HVAC air vents, walk through your home and make sure that all of these features are open. Much like dirty filters and built-up debris at air vents, prolonged vent closures create stress within the heating system. If too many vents throughout your home are closed, increased pressure within your ducting could wear your furnace down and cause damage to its heat exchanger and other components.

We proudly serve residents of O’Fallon, MO, and the surrounding communities. We offer top-notch heating, air conditioning, and air quality services. We also provide cutting-edge HVAC products, geothermal installation, and whole-house humidity control. If you suspect that your furnace has heat exchanger problems, call Hoff Heating & AC today to schedule an appointment.