Gas Furnace Heat Exchanger: Definition, Replacement Cost, and Common Problems

A furnace heat exchanger is a sealed, steel component connected to the furnace’s combustion chamber. Heat from furnace combustion is transferred through the heat exchanger to heat the air flowing over it and through your ductwork.

The average cost of heat exchanger replacement is $2,150.

The most common heat exchanger problems are cracking or separating from the excess heat and corroding from acidic moisture which results from combustion.

The only proper solution to these problems is to replace the heat exchanger or the entire furnace because leaked gasses contain deadly carbon monoxide.

What is a Furnace Heat Exchanger?

Your furnace’s heat exchanger is a steel part located in the center of the furnace. This vital part is usually designed with sealed steel tubes that travel back and forth to create more surface area. Many are produced with stainless steel or coated with a material like Teflon to better resist corrosion.

Intensely hot combustion gasses leave your home by traveling through the heat exchanger to the flue that runs through the outside wall of your home or the roof. At the same time, the furnace blower fan pulls cool air into the furnace where it passes over the heat exchanger. Heat is transferred out of the exchanger to heat the air before the warm air is pushed through the ductwork into the rooms of your home.

In an upflow furnace, the heat exchanger is located directly above the burners. In a downflow furnace, it is below the burners. And in a horizontal furnace, the heat exchanger is placed right next to the burners on the warm air supply side.

High-efficiency furnaces, also called condensing furnaces, have a secondary heat exchanger. The purpose of a second heat exchanger is to transfer additional heat from combustion gasses as they cool and water vapor condenses into a liquid. The liquid is acidic, so secondary heat exchanger parts are typically made from stainless steel to resist corrosion.

How Much Does It Cost to Replace a Furnace Heat Exchanger?

The average cost to replace a gas furnace heat exchanger is $2,150.

The heat exchanger replacement cost depends primarily on whether the heat exchanger is under warranty. If it is under warranty, then you will be charged only for labor, not for the part. The labor cost to replace a heat exchanger under warranty is $550 to $2,500.

The cost to replace a heat exchanger that is not under warranty is $2,200 to $4,000.

Most heat exchangers remain under warranty until the furnace is replaced. This is because warranties on heat exchangers range from 20 years to lifetime.

Heat exchanger replacement cost factors are:

  • Whether the heat exchanger is under warranty. If it is under warranty, you pay only for labor to replace it. No furnace warranties cover labor. If it is not under warranty, then you pay for both the part cost and labor.
  • Furnace size. The larger the furnace, the larger the heat exchanger, typically. Larger parts cost more.
  • Furnace brand. Parts for Lennox, Trane, Carrier and the brands they own tend to cost more than parts for other brands like Goodman and third-party brands.
  • Heat exchanger design. The more complex the design, the higher the cost.
  • Heat exchanger material. Stainless steel heat exchangers cost 25% to 40% more than galvanized steel heat exchangers.
  • Whether one or two heat exchangers are replaced. When a furnace has a secondary heat exchanger and both exchangers are replaced, cost is $350 to $800 higher.
  • Access to the heat exchanger. Some furnaces are designed to make access to internal components easier. This lowers the labor cost, because the design reduces the time it takes to replace the heat exchanger.

Should I replace other parts together with the heat exchanger?

Yes. If you replace the heat exchanger, also replace the ignitor, flame sensor, high limit switch and pressure switch. Our HVAC repair experts recommend replacing these parts as part of regular maintenance. They are affordable, and replacing them reduces the risk of later mechanical failure.

What are the Common Problems of a Gas Furnace Heat Exchanger? 

Cracking, corroding and separating are the most common problems gas furnace heat exchangers have. Each one causes carbon monoxide to leak from the unit into the air in your home. As a result, these common heat exchanger repair issues require immediate attention.

The single most common heat exchanger problem is cracking due to age. Intensely hot gasses pass through the heat exchanger whenever the furnace burners run, and the heat eventually causes the steel exchanger to crack and leak.

A second common problem is corrosion or rust. Water with high acidic content is a byproduct of furnace combustion. And if the water isn’t properly drained, it can get into the heat exchanger and cause corrosion through the metal which will lead to holes in the steel and leaks.

A final common heat exchanger repair problem is separating of the exchanger. This usually happens due to heat and the eventual warping of the steel. Older clamshell heat exchangers can come apart. Any type of exchanger can separate from the rest of the furnace. Both problems cause leaking.

Symptoms are the same for all heat exchanger damage. You might experience some of them or all of them. Looking for heat exchanger damage symptoms is how to tell if the furnace heat exchanger is cracked or damaged in another way:

  • Poor heating: Your furnace is not heating the air properly because heat is escaping. You might also notice the furnace or the area immediately around it being very hot.
  • Water leaking: The furnace is leaking water while it is in heat mode. This indicates that condensation isn’t being drained but is leaking because the heat exchanger is cracked, separated or has a hole in it caused by corrosion.
  • Alarm warning: Your carbon monoxide detector’s alarm goes off signaling carbon monoxide escaping into the air because the heat exchanger is leaking.
  • Foul smell: Combustion gasses have a chemical smell or odor. If it is coming from the furnace or heat grates, it indicates heat exchanger damage. The smell is similar to formaldehyde.
  • Feeling ill: One or more people in your household might have a headache or feel sick for no known reason. This can be a sign of carbon monoxide poisoning from a CO leak.

How to fix the problem, regardless of the causes, is clear.

You can either replace the heat exchanger or replace the entire furnace. Heat exchangers cannot be repaired, welded or patched.

the furnace heat exchanger is cracked

The furnace heat exchanger is cracked

What are Symptoms of a Cracked Furnace Heat Exchanger?

The symptoms of a cracked furnace heat exchanger are:

  • Water condensed from the combustion gas vapor is leaking.
  • The furnace and air around it are very hot while other rooms are cool.
  • A foul chemical smell is in the air.
  • Your carbon monoxide detector is going off.
  • You or others are feeling nauseous or having a headache for an unknown reason.

How to Clean a Heat Exchanger?

Here is how to clean a furnace heat exchanger in 4 steps.

1). Turn off electricity and gas to the furnace.

2). Using a wire brush, brush dirt, debris, soot, rust and scale from the exterior of the heat exchanger. Do the same in each chamber or port entering the heat exchanger. Once the heavy debris is removed, go over the surfaces with a damp cloth and cleaner.

3). Use a shop vacuum and its tools to clean every part of the heat exchanger including inside the chambers.

4). Turn on the power and gas, and start up the furnace to check that it is operating as it should.

Should I Replace a Heat Exchanger or Buy a New Furnace? 

The best solution is to replace your furnace instead of repairing the heat exchanger. Here is a look at both options – repair your furnace vs replace your furnace when the heat exchanger is bad.

When you should replace your furnace due to a damaged heat exchanger:

  • It is old: If the furnace is more than 10 years old, replace it. Most furnaces last 15-20 years. It doesn’t make sense to pay $2,000 or more to replace the heat exchanger now only to face the cost of furnace replacement in a few years.
  • Repair cost is high: If the cost of furnace heat exchanger replacement is more than half the cost of furnace replacement, get a new furnace.
  • You want a better furnace: You might like to use this opportunity to upgrade to a furnace that is more efficient or that has features you prefer such as 2-stage or variable capacity heating.

When you should repair your furnace rather than replace it when the heat exchanger is bad:

  • The furnace is newer: Only consider replacing the heat exchanger rather than the entire furnace if the furnace is less than 10 years old.
  • A leak into the furnace caused the damage: An example of this is an AC evaporator coil or drain pan leak that allows water into the heat exchanger and causes corrosion damage.
  • The fix is affordable: If the cost of the heat exchanger replacement is less than $2,000, then replacing just the heat exchanger on a newer furnace makes sense.

Even then, you should ask your furnace contractor why the heat exchanger went bad. If the reason for heat exchanger failure is poor furnace design or that it is cheap, then the problem is likely to happen again. And the entire furnace should be replaced instead.

Which Part Should Be Replaced Together When Replacing a Furnace Heat Exchanger?

The hot surface ignitor, flame sensor, pressure switch and high limit switch should be replaced when the heat exchanger is replaced.

These parts wear out with age, and they are inexpensive. If any of the parts fail, your furnace won’t ignite or stay ignited, and you’ll end up with a cold house when you want heat. So, the right decision is to replace these parts when the heat exchanger is replaced as a preemptive measure against future furnace failure.

Written by

Rene has worked 10 years in the HVAC field and now is the Senior Comfort Specialist for PICKHVAC. He holds an HVAC associate degree and EPA & R-410A Certifications.

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