The heat exchanger is an essential component to any furnace, and a broken or damaged heat exchanger makes it impossible to run your furnace safely. If something is wrong with the heat exchanger, it will have to be replaced before you can use your furnace. First, you should know what your heat exchanger does and how to spot problems.
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The heat exchanger is a large metal container in which hot combustion gases circulate after being created by the furnace's gas or oil burner. This causes the heat exchanger to get hot. As this happens, the furnace blower blows air across the heat exchanger, which warms the air before it enters your home. The gases inside the heat exchanger are drawn up the flue or chimney and expelled outside your home, but if the exchanger is damaged, these potentially harmful gases can mix in with the air in your house.
Be sure to completely shut down your furnace before you attempt to examine the heat exchanger. If you know it to be leaking, a visual inspection of the heat exchanger may turn up the cause. The metal surface can become cracked, warped or burned through. These issues may be visible on the surface, but you may need to check inside as well. Use a mirror and flashlight to check the interior surface, and use your hands to check the outside for roughness or deformation. The heat exchanger should have a perfectly smooth surface; any unusual texture indicates a problem.
If you find a problem of any kind, do not run your furnace until you have the heat exchanger replaced. Only you can judge your own ability to remove and replace this component, so contact a professional furnace technician if you need assistance. If you decide to do it yourself, remember that different furnace models are built differently -- consult the owner's manual that came with your furnace for instructions.
A visual inspection alone cannot always detect heat exchanger leaks and problems, but professional service technicians have a number of other tools at their disposal, including flame tests, water spray tests, penetrating dye and carbon monoxide testing instruments. If your heat exchanger is damaged, inspect your furnace's fan limit switch as well. This component is supposed to shut down the burner in your furnace before it overheats and damages the heat exchanger. In many cases, a damaged heat exchanger can be traced back to a malfunctioning limit switch.