HVAC 101: Airflow's Effect On Compressor Longevity

An easily understood concept for most people is the car radiator. A water pump pushes water around the engine block to pick up excess heat. The water then travels to the radiator. Airflow through the radiator cools the water off and the process repeats itself.

An air conditioner does a similar thing with refrigerant (Freon). The refrigerant picks up excess heat from inside the house and transfers it outside. But unlike a car it adds the additional step of boiling the liquid Freon into vapor as it absorbs heat from the house. Then it condenses the vapor Freon back into liquid as it rejects heat to the outdoors.  This video explains it rather nicely. The refrigeration cycle is the process that enables us to take heat from an 80 degree house and transfer it to the outdoors where it's 100 degrees, something a car's radiator could never do. In the next few paragraphs we're going to focus on how airflow affects this liquid to vapor process and how that affects your compressor's longevity.

An air conditioner delivers liquid Freon to the cooling coil. The furnace fan blows hot air from the house through the cooling coil. The heat in the air causes the liquid Freon in the cooling coil to evaporate into vapor. As such the cooling coil is sometimes referred to as an evaporator coil. You must deliver a minimum amount of heat to the cooling coil so that all of the liquid Freon will turn into vapor. Anything that reduces airflow will also reduce the amount of heat that the Freon is absorbing.

The evaporation of liquid Freon to vapor is critical for the compressor. Within the compressor's steel shell you normally have nothing but vapor Freon and oil. Liquid Freon doesn't normally make it back to the compressor. But if a little of it does it will typically drop to the bottom of the compressor and mix with the compressor's oil. Liquid Freon is a very good solvent and mixes readily with oil. The diluted oil won't lubricate the compressor properly and the compressor's lifespan will be reduced.

Make the problem severe enough and even worse things can happen. A dirty evaporator coil can reduce airflow. A dirty filter can do the same. Throw in some undersized and/or damaged ducts and you can potentially reduce airflow so much that a tremendous amount of liquid Freon makes it back to the compressor. If enough liquid Freon gets into the compressor then some of it will get into the compressor's compression chamber. On the more modern scroll compressor the liquid can be passed with minimal damage - though there's no guarantee of that. On reciprocating compressors the result can be catastrophic. Liquid Freon can't be compressed. But a reciprocating compressor will try anyway and something will give. (The reciprocating compressor in a typical home air conditioner is a lot smaller and looks different than what's shown in the link.)

This type of damage happens more often than you might think. But few customers ever know it. The typical reaction a customer has to a prematurely failed compressor is to blame the air conditioner manufacturer. They assume that brand X makes lousy equipment. But the reality is that an extraordinarily small percentage of compressors fail because of manufacturing defects. Compressor failure is almost always due to improper installation and maintenance. In fact, there's a saying within the industry that speaks to the true cause of compressor failures: "Compressors don't die. They're murdered."

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