Gas and oil furnaces burn fuel to provide heat for homes, and that presents a basic problem. Every combustion process produces noxious gases. How do you keep those gases from circulating throughout your home with the warm air?

Simple: Seal the combustion chamber. But that raises another question: How do you then move the warm air? That’s the job of the heat exchanger.

“Think of the heat exchanger as the heart of your heating system,” says Tim David, CEO of AirLucent. A heat exchanger could be as simple as a metal barrier separating the combustion chamber from the air that circulates into the house, but most are more complex.

“There are a few different styles based on the overall intention of where they will be used,” David says, “and they could be made of different materials [steel, cast iron or aluminum].”

A heat exchanger isn’t necessary in an electric furnace, which doesn’t burn anything. But every gas- and oil-burning furnace must have one.

About the Experts

• Tim David is the CEO of Airlucent, a leading HVAC, heating and air purification information website based in Hampton Cove, Alabama. He’s a retired HVAC tech with 25 years of experience with multiple certifications in air purification technologies.
• John Gabrielli specializes in furnace and heating systems, HVAC maintenance and repair. He owns Air Temp Solutions in New Castle, Delaware.

What Is the Heat Exchanger in a Furnace?

John Gabrielli, owner of Air Temp Solutions, describes a heat exchanger this way: “It’s a piece of your furnace that is responsible for transferring heat from the combustion of fuel to the air that is blown into your house. A heat exchanger is made up of metal tubes, which heat up when fuel is burned.”

Just as furnace designs differ, so do the designs of heat exchangers. They aren’t always made of metal tubes.

“There are a few different styles based on the overall intention of where they will be used, like tubular mazes, clamshell sandwiches and slithering serpents, each transferring heat in their own unique way,” says David.

How Does the Heat Exchanger in a Furnace Work?

According to Gabrielli, “Exhaust gases go through one side of the heat exchanger, while your home’s interior air goes through the other side. The heat transfers through the metal, warming up the air on the other side, which is then blown into your house.”

All fuel-burning furnaces have a primary heat exchanger. But high-efficiency gas furnaces also have a secondary one, which typically consists of an array of metal tubes. Hot air from the primary exchanger cools and condenses when it enters the tubes of the secondary one, releasing more heat.

Because it converts all exhaust gases into acidic water, which drips into a drainage system, this type of furnace doesn’t come with a conventional metal exhaust vent. But it does feature a corrosion-resistant PVC one to allow trace gases to escape.

How To Know if Your Furnace Heat Exchanger Is Bad

Because they’re exposed to temperature extremes, heat exchangers do go bad, says David. “Most of the issues I’ve dealt with over the years were due to cracking, rusting or corrosion,” he says.

This isn’t something you can ignore, David adds. A damaged heat exchanger can allow carbon monoxide to escape into the air circulating in your house. It’s also bad for the furnace, affecting its performance, raising your energy bill and even leading to a breakdown.

“If you have a carbon monoxide detector in the same room as your furnace, which I recommend everyone should, it can alert you to the presence of exhaust gases leaking into your home through the heat exchanger,” says Gabrielli.

Gabrielli and David suggest homeowners be on the alert for strange smells that won’t go away, and recommend periodic visual inspections of the furnace.

“If you have your furnace open, and you’re looking directly at the heat exchanger, some cracks might be big enough to spot,” says Gabrielli. David adds you may also notice soot buildup around the burners.

What Is the Life Expectancy of a Furnace Heat Exchanger?

Neither Gabrielli nor David addressed this question, so we turned to InspectaPedia, which lists composite opinions from several sources. The consensus? A steel heat exchanger lasts 15 to 20 years on average, while a cast iron one lasts longer — 40 to 50 years.

Some major manufacturers, including American Standard and Bryant, offer a lifetime warranty on all furnace parts. Others, including Carrier, Heil, Rheem and Janitrol, offer 20-year parts warranties on standard and high-efficiency models.

Do You Need To Call a Pro To Replace the Heat Exchanger in a Furnace?

Absolutely. This it isn’t a DIY job. It requires disassembly and reassembly of much of the furnace. It’s akin to doing complex repairs on a car engine.

The most important issue when replacing a heat exchanger is creating a hermetic seal that prevents carbon monoxide from escaping. A homeowner is unlikely to have the tools, gas monitoring equipment and general know-how to do this correctly.  Some jurisdictions may require the work be completed by a licensed technician.

The health consequences of a leak can be life-threatening, so it’s safer to leave the job to a pro.

How Much Does It Cost To Replace a Heat Exchanger in a Furnace?

It’s a major undertaking that can cost as much as $3,000 and take up to eight hours, depending on the furnace model.

A new furnace doesn’t cost much more than that, so it often makes more sense to replace the entire furnace. If the heat exchanger is cracked or corroded, other parts of the furnace are also probably damaged.