Inspect the heat exchanger within the furnace for cracks, openings or excessive corrosion. Typically the blower will need to be removed and the inspection performed through the blower compartment with mirrors and cameras. While visibly inspecting the heat exchanger you will want to look for the following possible issues: Cracked cells, missing eyelets, corrosion, and holes.
Easily the most common problem that heat exchangers experience, cracks are the result of repeatedly overheating and cooling the exchanger. This process of unrelenting heating and cooling stretches and stresses the metal of the exchanger. These stressed points crack open and cause further problems.
Eyelets are formed during the manufacturing process of the heat exchanger. They are used to seal the two parts of the heat exchanger together. When they fail they typically crack and fall off, preventing the heat exchanger from fully sealing.
Corrosion and holes typically occur due to an excess buildup of moisture.
90% furnaces from the mid 1980s through the mid 2000s are known to have the secondary HEX fail. The plastic lining in the interior warps away from the steel portion of the HEX and the corrosive condensate eats away at the coil from the inside. The blower must be removed in order to inspect the secondary heat exchanger. Often times there is a visible leak.
