Heat exchangers are made with metal that is formed, then pressed / stamped together. The act of stamping and pressing metal makes the metal stretch where the forms press together. As metal is stretched, some areas become thinner than others. The thin area will be prone to damages as time and use continues to occur.
The items that affect heat exchangers are: temperature, climate, use, gas pressure, dirt, air ducts, pressure, combustion air, and flue stacks. If any of these items are not operating properly. Furnaces that do not operate properly cause the unit to work harder, burn more fuel, struggle with moving air and are less efficient. Consequently this shortens the life span and makes internal components wear out prematurely.
Some visual indications of premature failure of furnace components are rusting, warping, scorching, or discoloration noted on the metal near or on the fuel burners or heat exchanger. All of the mentioned items are indicators that the unit should have maintenance or repairs performed. Rust is a serious condition that should be addressed immediately when noticed, as it will weaken the metal and as an actual condition, rust is decomposing metal.
When a furnace burners is operating, the flame temperature is approximately 1,400 degree Fahrenheit, this extreme heat can severely affect the internal condition of the heat exchanger. Metal expands when heat is applied, and shrinks as the metal is cooling. Minor cracks will expand and continue to grow during normal operation and use. Once cracks occur, carbon monoxide will leak into the breathable air space and be blown throughout the home, causing a dangerous situation to exist in the home.
Rust on metal is a natural occurrence, moisture slowly decomposes metal. Once metal is affected by rust, flaking occurs; the area that is flaking becomes thinner and is prone to pressure and temperature changes, including cracking. All distortion, discoloration, cracks, rust, and other damages should be evaluated by a National Association of Technical Excellence (NATE) Heating and Cooling Specialist.