Air Conditioner Evaporator Coil

Evaporator Coils are a key component in a central heating and cooling system of your home. In the warmer months when you need air conditioning, the air conditioning condensing unit compresses the freon in your HVAC system and turns the gas into a very cold liquid. That cold liquid passes through a set of copper or aluminum coils inside the evaporator coil at the same time the blower inside the air handler or furnace unit, is recirculating the warm air inside your home through the small gaps and cooling fins.

As the warmer air passes over the coil, an evaporation process occurs and the colder coils extract the heat from the air, converting the cold liquid back into a gas as it cycles back through the system on its return trip to the outdoor condenser to repeat the process. The resulting cooler air on the supply side of coil is transferred to the duct system of your home, thus cooling it off. This process continues until the temperature of the air inside the home, reaches the point you have it set on your thermostat.

The amount of refrigerant that passes through the indoor coil is controlled by a metering device, or thermostatic expansion valve (TXV). Metering devices are a simple orifice, that regulates the flow based on the size of the hole in the meter. TXV systems are a bit smarter, and they measure the temperature of both the refrigerant and coil to allow or restrict the right amount through the coil, thus providing the perfect combination of thermal efficiency, while preventing ice buildup on the coil itself.

To get a very basic idea of the cooling process, get a foam cup and poke several pen-sized holes in the bottom of it. After you have several holes in the base of the foam cup, fill it with ice cubes and blow directly into the top of the cup, forcing the air down through the ice and out the bottom holes. As you hold your hand at the bottom of the cup, you will feel that the air is much colder.

Types of Evaporator Coils

types of evaporator coils

Evaporator coils are available in several different configurations and the coil on your home hvac system, was designed and matched to the rest of your system. Do not assume that any coil will work for your needs.

  • A-Coils
    An A-Coil, gets its common name from the shape of the evaporator coil. They are available in several sizes and refrigerant types, to match up to your outdoor condenser.
  • N-Coils
    Like the A-Coil, the N-Coil gets its common name by the shape of the coil configuration. N Coils are being used much more often due to the higher efficiency systems available on the market today, in order to reduce the overall size of the coil itself, while still providing ample amount of coil surface area to support the transfer of heat.
  • Slab Coils
    Slab coils, which are not as common as A or N coils, look just like the radiator in your car.

Size and Airflow Considerations for Evaporator Coils

Just like the outside condenser was properly sized for your home, the evaporator coil needs to match the outdoor unit. In other words, you could not have a 3 ton outdoor unit, paired with a 2 ton evaporator coil. The surface are of the coil itself would not be sufficient enough to dissipate the heating or cooling, and your indoor coil would ultimately freeze into a block of ice. The coil MUST at minimum, be exactly matched to the outdoor unit, or sized slightly larger. Ex: A 2 ton indoor evaporator coil will not work with a 3 ton outdoor condenser, however a 3.5 ton coil will work fine with a 3-ton outdoor unit, and may even result in slightly higher efficiency!

While matching the size is very important, the direction of your unit airflow is just as important! Another key component built into the evaporator coil for your system is the drain pan. As the condensation process occurs during the heating and cooling transfer, condensation, or water, will build up on the coils and run off into a built in drain pan. The drain pan MUST be in the proper place to collect and drain this water, or it will simply run to the lowest point, which is usually the floor.


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