Why do my filters need to be changed and how often?

Air conditioning equipment is designed to operate with a specific quantity of air passing over its indoor coil surface. When air filters are not replaced, they clog and become coated with dirt. Similarly, the indoor coils get coated with dirt. This dirt reduces the amount of air flowing in the unit below the design limit, the unit struggles to keep up and can wear out sooner.

In the cooling mode, if there is not enough air over the indoor coil, the coil temperature drops. When it drops below the freezing point, ice forms on the coil, which further reduces the airflow, which further reduces the coil temperature. The compressor within the unit is a pump, which is designed to pump a vapor. As the airflow through the indoor coil drops, there isn’t enough heat being removed from the air passing over the coil to vaporize the liquid refrigerant inside the coil. Thus, instead of receiving a vapor, the compressor receives liquid refrigerant.

This is called “liquid slugging”. The effect of “liquid slugging” is similar to the effect of pouring liquid into the cylinders of a gas engine. As liquids are not compressible, cylinder pressure exceeds the design limits, of the cylinder, and the valves, connecting rods, pistons, or other internal components are destroyed. The air conditioning unit starts out requiring that its filters be replaced. Now it needs a new compressor. Preventive maintenance or a new compressor? Ultimately, it’s your choice.

In the heating mode, low airflow causes the heat exchanger to overheat. Heat exchangers are designed to operate at temperature between 120°F and 200°F. At higher temperatures, the heat exchanger oxidizes, its life-span is reduced, or it cracks and breaks. In either case, It makes far more sense to replace air filters regularly than to replace a heat exchanger costing far more, right.