What if my furnace heat exchanger is bad?

1. What is a heat exchanger?

The heat exchanger in a furnace separates the combustion process from your breathing air. It is a combination metal chamber and passageway that starts at the burner assembly and ends approximately where the chimney vent connects to the furnace. Air is heated as it is blown across the hot metal surface of the heat exchanger. The heated air is then distributed through the house to warm the house.

The heat exchanger must have an air (and gas) tight seal to separate the gasses in the flue products inside the heat exchanger from the breathing air passing over the outside surface. This is because the flue gasses can be poisonous – such as deadly carbon monoxide – and contamination of the breathing air by these gasses pose a health risk and can be fatal.

2. Why do heat exchangers fail?

All heat exchangers fail eventually. This is because of metal fatigue. Metal when it is heated up expands, and when it is cooled off contracts. This expansion/contraction cycle is part of the normal furnace heating process. Over time this constant expansion and contraction has the same effect on a heat exchanger that bending a paperclip back and forth: it breaks. And when that happens contamination occurs and it is no longer safe.

While heat exchangers are typically manufactured to last between 10 – 20 years, many factors can accelerate the process of heat exchanger failure. These factors usually fall under the categories of poor maintenance, poor initial system design and installation, or poor equipment design by the manufacturer. Any one or a combination of these factors can result in a heat exchanger failing in a few short years.

3. How can you know when a heat exchanger has failed?

A heat exchanger must be visually inspected on a regular basis. Visual observation of light or water passing through the breach is positive confirmation of a crack or hole in a heat exchanger.

4. What tools are needed to determine if a heat exchanger is bad?

The only absolute way to determine if a heat exchanger is bad is to see it or visually confirm it. The old stand-by method of a mirror and a flashlight has been replaced by high tech infrared video inspection systems. This new technology has advanced the heating industry like arthroscopy has advanced medical surgery. The technician can now see places that are impossible with a mirror alone.

5. What about a carbon monoxide test?

A test for carbon monoxide (CO) can be inconclusive. A test for CO reveals whether a furnace is producing CO. A furnace creating CO is a symptom of bad combustion in a furnace because unlike a car, CO is not a regular by-product of the furnace combustion process. Therefore, a heat exchanger can be breached and if the furnace is not producing carbon monoxide the breach will remain undetected.

6. What are the options if a heat exchanger is bad?

There are only two options if a heat exchanger is bad:

Replace the heat exchanger or replace the furnace. If the heat exchanger is under warranty, this option is a good way to go unless it is unavailable in the time frame needed, which can be immediate in cold weather.

The other factors are energy efficiency and cost of service which can make replacing the furnace a preferable option even if the furnace is under warranty and available. If a furnace is out of warranty the preferable option is to replace the furnace.

7. What about a carbon monoxide alarm?

Relying on a CO Alarm is not an acceptable solution for a bad heat exchanger. This would be as unsafe as driving a car that has a leak in the brake line – you might be able to brake a few times but you wouldn’t want to bet your life on it.   Further it is against the Mechanical Code, Fire Department regulations, and SEMCO policy to allow a furnace to operate that has a bad heat exchanger. A representative from any of these organizations would shut the furnace down.

8. Is a heat exchanger inspection foolproof?  

Use of a mirror and a flashlight is adequate for the accessible spots, however the majority of the heat exchanger can’t be seen. If the technician uses an infrared video inspection system you get as close to foolproof as possible short of totally dismantling the furnace and removing the heat exchanger.

9. How much does it cost to replace a heat exchanger?

Plenty!  The most costly part of replacing a heat exchanger is the labor.  That’s the primary reason it is rarely done.  For most furnaces, the part itself is under lifetime warranty – or 20-year warranty on some models.  The part is covered, but since most furnaces come with a 1-year labor warranty and most homeowners don’t buy extended coverage, the cost of labor must be paid.

Replacing a heat exchanger involves disassembling much of the furnace.  It is right at the core of the unit. It’s an 8-10 hour job for most teams of HVAC technicians.  When you factor in the cost of labor for most heating and air conditioning companies, it gets expensive.  Labor rates vary, but most are between $75 and $125.  Total cost can be in the $600 to $1200 range for replacing a heat exchanger.

10. Repair or replace the furnace?

If you’ve got a furnace less than 10 years old, and it’s a high-performance model – high efficiency, staged heating, variable-speed blower – it may be worth repairing.  Get estimates on both the repair and the replacement and weigh your options.  Have a qualified contractor inspect the furnace and make a professional recommendation.  Get 2 recommendations/estimates if you’ve got the time.  It might be worth it to replace the heat exchanger.

However, if your high-quality furnace is more than 10 years old – and the older it gets – replacing it starts to make more sense.  If the furnace is a basic model to begin with, the cost of the repair may approach or exceed the purchase price. In that case, no decent furnace professional would recommend repairing it. Your money is much better spent on a new furnace.


In most cases, your heat exchanger won’t fail.  If it does, the $600 to $1200 bill should give you reason to consider replacing the entire furnace.  Perhaps the information here will help you make your decision easier.