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”:

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).

Conclusion

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?

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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.

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