Menu Close

If you have a crack in your heat exchanger, your furnace will not be able to continue to perform at the level that it used to. Since a heat exchanger is where all the combustion gasses are collected, a crack will allow them to enter your residential or business areas.

The heat exchanger is a main component of the furnace that heats up by the means of gas, oil, or wood being burned. This warm air (that is being created around the heat exchanger) gets blown into the living areas that we all get to enjoy during cold winter months.

The gasses that were produced as a result of this heating process, will be safely disposed of outdoors from the flue pipe or chimney. As the air gets heated inside the heat exchanger, the metallic frame will get softer and start to expand.

When the heating cycle is complete, your heat exchanger’s tubes or clamshells will go from the hot state to the cold state. The cycle starts again when your thermostat will send a signal to the control board and the burners will turn ON or OFF.

Over the wintertime, this cycle repeats many, many times over. Heat Exchangers are usually built very sturdy, and this is why they can handle such stress (unless your furnace is on the cheaper side).

Overheating is the main cause of the cracked heat exchanger in furnaces! If the furnace is not adjusted properly or gas valves get worn out, the heat exchanger may get hotter than it is supposed to and it will develop cracks.

If you did not change filters on time or the airflow inside was restricted by some other means (like undersized ductwork), this can lead to overheating as well. The lifecycle of heat exchangers is usually very high and if you are having problems, it is highly likely due to:

  1. Poor maintenance
  2. Installation and configuration mistake
  3. Bad manufacturer design

Cracked heat exchanger myth

“Cracked heat exchanger” diagnosis has a bad reputation at this moment due to many scammers that are trying to sell you new furnaces. On the other hand, this is something that you should NOT ignore since heat exchanger cracks will allow toxic gasses to enter your living area and affect the air that you breathe.

So, what do we do? First of all – “Don’t Panic”, turn your furnace OFF and ask for proof. With the latest technologies that we have available today, the person inspecting your furnace should be able to show you a crack with no problem.

Here is a nice video about that:

This is another way to check if you have cracks in your heat exchanger:

There are two common ways that your HVAC technician determines that you have a  cracked heat exchanger:

  1. Excess Carbon Monoxide (or CO) in your ducts
  2. He had a visual of the actual crack

According to Grayfurnaceman on YouTube, you cannot make a visual inspection of the heat exchanger crack in 5-10 min, because there is some disassembly involved. This means that:

If your diagnosis is only 5-10 min old (visual) and there is no image to prove it, it could be false!

If the technician found some Carbon Monoxide in your duct, with his portable exhaust gas analyzer tool, he should be able to show you the readings as well. There is a simplified version of this tool from Amazon sold by Vividia (paid link).

Here is a full video on that:

Also, don’t forget that the heat exchanger comes with its own warranty. This basically means if your furnace is old and way out of warranty, your heat exchanger may be still covered!

Do NOT just get a new furnace because they told you so! Unless you want to get rid of your old furnace anyway, there is no way that you should be replacing it without checking your warranty first.

Warranty agreements can be different and it is more common that they cover just the part and not labor. Here is an example of a heat exchanger warranty from PVI:

Follow the link for the full version in PDF format.

Is a cracked heat exchanger dangerous?

** Warning! Carbon Monoxide (or CO) gas poisoning is not a joke! If you start feeling a headache, nausea, and other poisoning symptoms, get into the fresh air first and contact your physician!

Carbon Monoxide along with other gasses inside your air vents is one of the signs of a cracked heat exchanger. Placing carbon monoxide alarms around your house is a general requirement, but you may want to take it a step further and get a device with a digital display.

This neat device goes inside your wall socket, but if you unplug it (and even start walking around), it will give you the exact Carbon Monoxide level that you have around your house! If the device detects a Carbon Monoxide problem, it will trigger the alarm. Very good life-saving device Nighthawk (paid link) is conveniently sold on Amazon.

There could be other reasons for CO inside your house besides the broken heat exchanger, like other combustion appliances in your house (water heater, for example) and damaged chimneys. Regardless of the reason, you should not underestimate this “silent killer”:

CO level Health problems
0-9 ppm None
10-29 ppm Long-term exposure can cause headaches and nausea
30-35 ppm Exposure can cause flu-like symptoms plus all of the above (especially in children and the elderly)
36-99 ppm Exposure can cause fatigue, drowsiness, vomiting, plus all of the above.
100-299 ppm Exposure can cause severe symptoms like confusion, intense headaches, plus all of the above.
300-400 ppm Exposure can cause irreversible symptoms like brain damage, coma, or death.

Cracked heat exchanger symptoms

There are certain signs that your technician will notice and can show you as evidence of the heat exchanger problems:

1. Fire rollout and soot

If for some reason your furnace shuts itself down and does not restart, that could mean that there is an overheating problem due to flame rolling out. Furnaces can only take so much heat before getting damaged and this is why in newer versions this safety feature is installed.

There are two main reasons for the flame rollout:

  1. Too much ventilation
  2. Not enough ventilation

The reason behind “too much ventilation” is commonly the cracks or holes inside your heat exchanger. They allow the air (that is not supposed to be there) to enter the combustion chamber and interfere with the flame.

The reason behind “not enough ventilation” is very often the clogged filter. If there is not enough oxygen coming through for the proper combustion process, the flames will start “rolling out” to get the oxygen from outside the furnace.

Problem is that unless you actually see a hole in your heat exchanger, you cannot say for sure that it is cracked. This visual diagnosis can only be performed by an HVAC specialist, and if you need a second opinion (due to the lack of actual evidence), feel free to hire another company.

Soot is a black carbon buildup that you can find on the inside of a furnace. It is produced by incomplete combustion and one of the reasons behind it is a cracked heat exchanger.

And this is what the actual fire rollout looks like:

2. High Carbon Monoxide levels

A crack or a hole in the heat exchanger can cause big problems! A portion of poisonous Carbon Monoxide gas will be allowed to enter your home and be a possible danger to you and your family.

It is highly likely that CO alarms will go off in this case, but think about it this way, how much Carbon Monoxide do you have to breathe in before your problem gets fixed! They are supposed to be a “last resort” and a way to save our life in case of emergency!

Do not wait for high Carbon Monoxide levels to be detected by your CO alarm, be proactive and inspect your furnace frequently!

If high Carbon Monoxide levels are detected in your Air Ducts by the HVAC specialist, they should also be able to show you the reading on their screen.

3. Actual picture of a crack

This is a really difficult question that became a popular topic on the internet. Does your HVAC specialist show your the real crack or they do not? Why do they make a diagnosis so fast?

One reason for making a fast diagnosis could be a very, very experienced and ethical HVAC specialist! Only one look at the corrosion on the flue could give them an indication of the cracked heat exchanger.

This is what happens when you do something for many years and become a true professional. So, do we ask for pictures, or just go with a flow…? What if you encountered simply a salesperson, who is after commission?

There could be a picture of a crack saved on the camera already and they are showing it to you as something that they just took… Some stories are hanging around (or maybe just a rumor) of people that drew a line on a heat exchanger and were showing it to their customers via borescope!

Of course, checking out boroscope images is a great idea, but the problem with a camera is…

… that it works jut like a camera! You have to be at the right angle to actually see the problem!

On the other hand, infrared cameras leave, so-called: “heat signatures” which really helps with identifying cracks in your heat exchanger. Expert HVAC technicians from this company (One Hour) are using infrared cameras for their diagnostics and are quite successful.

This is what they say:

An infrared camera detects minuscule — but important — cracks in your heat exchanger

To make your life easier, I found this guide for heat exchanger inspection in PDF. It shows EXACTLY what HVAC specialists should be looking at to determine a crack in your heat exchanger.

Honest companies will stand behind their diagnoses and should be able to supply you with visual proof of the crack, even if the only way to show it to you is to disassemble the furnace…

What causes a cracked heat exchanger?

The number one cause for a cracked heat exchanger is overheating, but the reasons for overheating could be rather different. If a furnace was not sized properly for the house in the first place and it has air ducts that are too small, the heat exchanger will overheat. The clogged filter (or insufficient airflow) is another common reason for the overheated and eventually cracked heat exchanger.

Airflow is essential for absorbing the extra heat and maintaining proper internal temperatures. When this temperature rises, it will speed up the degradation process of your heat exchanger.

Furnace gas valves can also get worn out and create too much pressure within your heat exchanger. This is why regular inspection of your furnace and filter replacement is crucial to your comfort and well-being.

Even though your heat exchanger is meant to outlive your furnace, the repeated cycles of heating and cooling will also cause it to start deteriorating. The expanding and contracting of the metal plates or tubes can cause cracks over time (in about 10-20 years).

Frequent Questions and Answers (F.Q.A.)

Here are some common questions that people ask about cracked heat exchangers:

  1. Can you weld a cracked heat exchanger? NO, welding a heat exchanger crack is not a good solution to the cracked heat exchanger.
  2. Can a cracked heat exchanger cause a fire? Yes, it can cause a fire within your furnace.
  3. Is a cracked heat exchanger dangerous? Yes, besides being a fire hazard, it can release a poisonous Carbon Monoxide gas (or CO).


The heat exchanger in the furnace is designed to last you for a long, long time. This is a reason for very long warranties on heat exchangers, with some of them being a lot longer than on a furnace itself. Overheating will get your heat exchanger cracked prematurely.

A key to a long-lasting heat exchanger, of course, is proper maintenance. Sounds simple and basic, but that requires some attention and financial commitment.

We don’t want our furnace to break in a middle of a cold winter night, do we?

Attention! This article is for informational purposes ONLY and is NOT a replacement for a professional advice! You will need to visit school's website for details and updates, as well as consult your local HVAC specialist for appropriate solution to your problem.


Related Posts

1 Comment

  1. Green Leaf Air

    I have already learned that CO2 poisoning in Cracked Furnace is responsible for a huge number of deaths each year. The lives of so many people are caused by a slight negligence. If we regularly maintain our hvac unit at the right time, it will not only save money but also our health and safety of life.

Leave a Reply

Your email address will not be published. Required fields are marked *