Unfortunately, one of the most common issues with furnaces is a cracked heat exchanger. If left undetected, it can cause unsafe levels of carbon monoxide in your home. Even if your carbon monoxide detector has not alarmed you, not all carbon monoxide detectors are created equally and some can be slow to alarm.
First off, let’s explain what the heat exchanger is and what it does. A heat exchanger on a furnace is a piece of metal designed to carry the hot flue gases from the burner to the vent. It keeps the flue gases from entering the air being distributed over the heat exchanger by the blower. As the blower pushes the air from the return air compartment to the supply distribution plenum, it passes over the heat exchanger. As the air passes over the heat exchanger, it picks up heat from the heat exchanger (HE), and the air gets pushed into the conditioned space. As the air passes over the HE, it cools down the HE and keeps the metal from getting too hot. If we don’t move enough air over the HE, over time it will crack or become defective because of excessive expansion and contraction. They can also fail with the age of the system.
A heat exchanger can crack many different ways. In the older types of furnaces (pre-1980) that have a cold rolled steel heat exchanger, the cracks usually develop in the back, close to the location that the factory welded the seam together. They can also crack toward the front by the factory welded seam, or they can develop in the middle of the heat exchanger going up a side usually near a ripple or wave in the metal. Some of these types of cracks can become very large. They can get big enough that you could stick part of your hand through them. These are very dangerous and can cause very high levels of carbon monoxide to escape into the conditioned space. Because these furnaces depend on the natural draft of the vent to pull the fumes up and out of the house, when they crack, they will usually leak some carbon monoxide into the air.
In the newer types of furnaces with an induced draft blower (small blower that blows air into the vent that carries the fumes out of the house), the cracks can develop in many different locations. Depending on the brand of furnace, some have particular problem areas. When these heat exchangers crack, the distribution blower will create a disturbance on the burners. This disturbance is easy to see if you know what you are looking for. The disturbance alone will usually cause the furnace to start producing carbon monoxide.
The good thing about these types of furnaces is the draft inducer motor assembly usually creates a negative pressure inside the heat exchanger and pulls air into the crack then vents it to the outside. They are also usually equipped with some safety switches that can sense this and will shut off the furnace.
When a furnace is burning clean and the vent is drawing correctly, it will produce a very little amount of carbon monoxide. When they start burning dirty, the carbon monoxide level being produced will go up very quickly. As long as there is not a crack in the heat exchanger or a problem with the venting, the high levels of carbon monoxide will be carried out of the house through the vent.
That being said, if there is any question about how a furnace is operating, it should be checked by a professional that has the proper equipment. They should check the draw on the vent, observe the heat exchanger visually, and test for carbon monoxide levels around the furnace and in the flue. There are many other things they should check, but these are some of the most important safety items.