Many times unfortunately we have to tell a customer that they have a cracked heat exchanger. Usually the first words out of there mouth is we have a carbon monoxide (C.O.) detector and it has never gone off. Not all C.O. detectors are created equally and some are pretty slow to alarm.
First, lets 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 and keep 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 (H.E.) and the air gets pushed into the conditioned space. As the air passes over the H.E. it cools down the H.E. and keeps the metal from getting too hot. If we don’t move enough air over the H.E. over time it will crack or become defective because of excessive expansion and contraction. They will also fail from just being old.
There are many different ways a heat exchanger can crack some are extremely dangerous and others are a little dangerous, (they are all dangerous).
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 or 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. I have seen them big enough that you could stick part of your hand through them. These are very dangerous and can cause very high levels of C.O. to escape into the conditioned space. Because these furnace 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 C.O. out into the air stream.
In the newer types of furnaces with a induced draft blower, (little blower that blows air into the vent that carries the fumes out of the house). The cracks can develop in all different locations some furnaces have particular problem areas. When these H.E. 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. (Yes I said start producing). The good thing about these types of furnaces is the draft inducer motor assembly usually creates a negative pressure inside the H.E. 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 C.O. When they start burning dirty the C.O. level being produced will go up very quickly. As long as there is not a crack in the H.E. or a problem with the venting the high levels of C.O. will be carried out of the house through the vent. I saw one today that the levels of C.O. were over 600 ppm in the flue. This furnace was old and had some problems.
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, test for C.O. 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 to check. Then if everything comes out ok you will be good to go.
Have a great winter, keep your filter clean and stay warm. Winter is on its way !!!!