The heat exchanger on a furnace is the section that keeps the combustion chamber and the breathing air separate. A heat exchanger is made of thin metal, and as it heats up from the combustion of the furnace, it transfers the heat to the air being distributed through the house by the blower. When a heat exchanger becomes damaged and the combustion fumes and gases mix with the clean air, serious consequences can result. Inspecting your heat exchanger regularly for damage is important to keep everything running safely and smoothly.
Visual Metal Cracks
The easiest way to tell if a heat exchanger is damaged is to inspect it and actually see cracks that have formed in the metal. Many companies use infrared light to detect cracks, but a flashlight is sufficient for more noticeable cracks. If you don't notice damage from a visual inspection, it doesn't mean there is no damage. Regular inspection by a professional is a wise idea.
Buildup and Discoloration
Often when cracks are present in a heat exchanger, soot from the combustion process will seep through and discolor the metal. The result will be a buildup of soot around the crack site and/or spots that are a darker color than the rest of the metal.
Carbon Monoxide Detection
Although you should certainly not wait until your carbon monoxide detector sounds to determine if your heat exchanger is broken, if you do get an alarm, it is a strong sign that damage is present. Carbon monoxide is a byproduct of the combustion process, and it can seep through the cracks of a damaged heat exchanger. Contact your local fire department right away if your carbon monoxide detector goes off.
Difference in Furnace Flame
Sometimes, if the heat exchanger is damaged and the fresh, breathing air mixes with the combustion air, the flame in your furnace can change. If you suspect damage, have someone turn the thermostat up to initiate the furnace, then sit and observe the flame. A damaged heat exchanger may produce a flame that jumps and dances after the blower fan has been on.