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What is a heat exchanger?
A heat exchanger is a system in a gas furnace that heats air to warm your home during cold days. It is the core of a gas furnace and generates heat through an exchange process. They are comprised of a set of metal tubes or coils that are S shaped and wrapped around the inside of the heat exchanger. The heat exchanger takes the hot gasses that the furnace makes when it burns fuel and exchanges these gassed through metal tubes, coils, fins or plates, which then heats the metal of the exchanger. 

Maintenance
Heat exchangers should be checked annually to ensure that they are functioning safely and properly.

Are Cracked Heat Exchanger Dangerous?
Yes, a cracked heat exchanger is dangerous.

Just how dangerous and what are the dangers?

While many cracked heat exchangers do not cause the occupants of the home to become ill or die from carbon monoxide poisoning, but some do. 

Customers with homes that have a furnace with cracked heat exchangers have complained of shortness of breath and having flu like symptoms when they are home. The systems seem to go away when they are awhile from the home. This can be a sign of carbon monoxide poisoning. 

CO poisoning isn't the only worry when you have a crack in your heat exchanger.

When the heat exchangers are cracked the air from the blower compartment gets pushed into the burner chamber and this causes what we call flame rollout. This happens when flames in the burner compartment get pressurized from behind and as the crack grows (and they all grow) the flame gets pushed out in the open area of the furnace where the gas valve and electrical components of the furnace are located and can start a fire.

Cracked heat exchangers are dangerous and get more dangerous as the cracks continue to grow.

How do I know I have cracked heat exchangers?
There are 2 ways for us to find that you have cracked heat exchangers.

The first is being able to physically see or feel them.

If the technician finds cracks in the heat exchangers, they will show you the cracks.

The second way is when the technician tests the system with a combustion analyzer and finds more than 100ppm of carbon monoxide in the exhaust, we can confidently conclude that there are cracks in the heat exchangers. They will probably recommend a heat exchanger diagnostic to better determine the exact reason for the high CO output.

If the technician finds the heat exchangers are cracked, they will disable the furnace due to liability issues. Because of the danger of CO poisoning and fire, we are not allowed to leave with furnace able to operate. They will have you sign that you acknowledge that the furnace has been shut down due to cracked heat exchangers being cracked.

What do you do if you have cracked heat exchangers?
If you have cracked heat exchangers, then it is time to replace them.

The heat exchangers may be covered under warranty. That means that the customer will only pay for labor to replace the heat exchanger and not for the heat exchangers themselves.

If the furnace is not still under warranty, you can still replace the heat exchangers, but the cost is significantly higher, and it might be a better idea to replace the furnace.