What Exactly Does A Heat Exchanger Do?

Has your furnace dropped significantly in how much heat it produces? You can hope it is a simple fix, like a clogged air duct, but it could be much worse: a cracked heat exchanger. Homeowners often ask, “Can’t you repair a cracked heat exchanger without replacing it? Can I ignore the cracked heat exchanger? How does a heat exchanger work?” These are common questions homeowners ask when they learn they have a cracked heat exchanger and the technician is calling for an expensive replacement.

What Does a Heat Exchanger Do?

For complex pieces of machinery like heat exchangers, it can be challenging to explain how things work to produce the results we see and feel. So, how does a heat exchanger work exactly? For your HVAC system to cool your home, it takes your indoor air, removes heat from it, and places that heat outside. When it’s winter, and your furnace is turned on to heat the house, gas is turned on to warm up the heat exchanger, and the heat is then passed on to the air, which is then blown through the house. In a boiler, it works much the same way. The heat exchanger is warmed up, and then that heat is transferred to the water.
heat exchange diagram

What Does a Heat Exchanger Look Like?

Heat exchangers come in many different forms, depending on the application. An AC heat exchanger will look different than a boiler’s heat exchanger, which will also look different than an industrial heat exchanger. They all have a similar concept of using contact to transfer heat between something hot and something cool. Pipes or plates will be heated up, and air or liquid will flow around these heated metal objects and pick up some of that heat. Then, the liquid or air will be pumped to where it is needed. Below is an image of a gas boiler furnace.

How Does a Heat Exchanger Work?


This graphic shows how your heat exchanger works. When you turn your furnace on, burners send hot, combustible gases to your heat exchanger. These gases get transferred to the metal walls within the exchanger, which heats the metal walls. Cold air from your home flows over these heated metal walls. The walls cool down as the heat is transferred to the air. The air never comes into direct contact with the heated combustible gases. Separating the two ensures no dangerous gases are pumped through your vents. These combustible gases are then vented up through the top of your roof, or in more advanced and efficient systems, cycled through a second heat exchanger. With a second heat exchanger, furnaces can reach efficiencies of up to 98 percent. The warm air is then distributed through your home with your air handler and ductwork system.