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Cracked Heat Exchanger: What Does It Mean?

So you scheduled regular furnace maintenance or a furnace service call because your gas furnace isn’t working right. The HVAC technician shakes his head and tells you that you have a cracked heat exchanger. What does that even mean?

What is a heat exchanger?

First, you need to know what a gas furnace heat exchanger is. These furnace components play a critical role in heating your home and keeping you safe. A heat exchanger transfers heat to provide space heating. It also keeps toxic combustion separate from the heated air.

A furnace with an AFUE of less than 90 has one primary heat exchanger. A high-efficiency furnace with an AFUE rating of 90 or higher has both a primary and secondary heat exchanger. These furnaces are sometimes called condensing furnaces.

What does a heat exchanger do, and how does it work?

As mentioned, this critical component has two jobs. Help heat your home, and keep toxic combustion gases contained until they can be vented out of your home.

Heating function

The heat exchanger is found deep inside your furnace. It is a series of metal tubes located between the combustion chamber and the blower. When your furnace burns fuel, the exhaust and combustion byproducts enter the heat exchanger.

The hot flue gas heats the metal heat exchanger. At the same time, air flows over the heat exchanger and warms up. The blower then sends this warm air into your ductwork. The heat exchanger expands and contracts as it heats and cools.

Safety function

The heat exchanger contains exhaust and combustion byproducts such as carbon monoxide, carbon dioxide, nitrogen oxides, water vapor, and more. Some of these gases can be quite dangerous. A cracked heat exchanger can leak combustion gases into your home.

Contact an HVAC professional

What are some cracked heat exchanger symptoms?

Signs of a cracked heat exchanger can also be the same as other furnace problems, so it’s essential that you call a licensed HVAC company to evaluate the situation and make the necessary repairs.

Some warning signs that you might have a cracked heat exchanger include:

Furnace short cycling (turns on and off again quickly)
Banging or rattling sounds coming from your furnace
A strong, metallic odor
Yellow or flickering flames instead of a steady blue flame
Soot buildup around the furnace or flue pipe
Carbon monoxide detector keeps tripping
Corrosion or rust on the component
Visible crack in the heat exchanger

A heat exchanger should last 10-15 years or more, with good maintenance. So, if your furnace is newer than that, it’s unlikely a crack in the component is the problem.

What causes a heat exchanger to crack?

There are 4 main reasons: years of wear and tear, overheating, oversized furnace, and rust/corrosion.

Wear and tear

As we mentioned, the heat exchanger expands and contracts each time your furnace comes on. Over the years, wear and tear may cause it to crack.

Overheating

A furnace that repeatedly overheats can lead to a cracked heat exchanger. Fortunately, you can help prevent your furnace from overheating by providing unrestricted airflow. Your furnace needs good airflow to function properly without overheating.

Make sure you change the furnace filter every 1-3 months, depending on the quality of the filter. Make sure registers and return air vents are not blocked by furniture or curtains. Your furnace will thank you.

Oversized furnace

A furnace that is too big for your home and ductwork system can also lead to a cracked heat exchanger. An oversized furnace may have short cycling, which causes the heat exchanger to expand and contract too rapidly. Additionally, the short cycling can prevent the condensation inside the heat exchanger from evaporating as it should.

Rust/corrosion

Older heat exchangers can essentially rust from the inside. That can happen when the condensation that forms inside the heat exchanger doesn’t evaporate properly. Excess condensation can cause rust or corrosion to form, weakening the component and making a crack more likely.

How Trane protects you

Trane heat exchangers are crimped, not welded, to prevent cracks from heat stress. In addition, primary and secondary heat exchangers are made of stainless steel to resist corrosion.

Is furnace heat exchanger repair possible?

We wouldn’t recommend trying to repair a cracked heat exchanger. Your best bet for safety and comfort is to replace it or even replace the entire furnace. Before making this decision, ask to see the cracked unit yourself.

What does it cost to replace a furnace heat exchanger?

Furnace heat exchanger replacement isn’t cheap. In fact, it’s one of the most expensive furnace repairs there is. Your heat exchanger replacement cost might range from $1,000-$3,000. Your specific cost will depend on the furnace model, type of heat exchanger, geographic location, local labor costs, and more.

Since it is at the heart of the furnace, the furnace is essentially disassembled and put back together again. This means that labor costs will be high for this particular furnace repair.

If your furnace is 15 years old and you’re facing more frequent and more costly furnace repair bills, you may want to consider furnace replacement as a more cost-effective alternative. Refer to our Repair or Replace Guide for some tips on making this decision.

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Help prevent problems with furnace maintenance

You can help prevent furnace problems by adopting good furnace maintenance habits. The biggest thing you can do yourself is to regularly replace the furnace filter. A clogged air filter can cause all sorts of problems with your heating system, including furnace overheating, a major cause of a cracked heat exchanger.

In addition, schedule a yearly furnace tune-up every fall before heating season starts. As part of this visit, the HVAC technician inspects the heat exchanger for corrosion or damage and performs heat exchanger maintenance, such as cleaning the heat transfer area. They also inspect, clean, and test furnace components and operation.

By Anne Fonda

A Content Writer with Trane Technologies, Anne Fonda researches topics and writes for Trane® and associated residential HVAC brands. She works in collaboration with Trane Technologies subject matter experts, offering easy-to-understand, informative content on complex topics. Her goal is to help consumers make informed decisions on the products and services they need.

She has written for HVAC and other service provider websites for over 16 years. Before transitioning to web content writing, Anne had a 14-year stint as an award-winning journalist. She graduated cum laude from the University of Missouri-Columbia School of Journalism. When she’s not working, Anne enjoys playing word games, reading, gardening, spending time with family, and visiting gardens and museums.

Expert review by Kimberly Sexton, Senior Systems Engineer