What Is The Compressor, Why Does Your AC Need It, And What Malfunctions Can Thwart Your System?

Posted on: 5 January 2017


A central air conditioner has a complicated system of carefully calibrated parts designed to move a chemical refrigerant that acts as the fuel for cooling. The process starts at the thermostat inside your home, which sends a signal to the condensing unit outside. The first part to start up in the condensing unit is called the compressor.

What is the compressor, why does your air conditioner need it, and what malfunctions in the compressor can thwart your cooling system?

Compressor Function

The refrigerant in the air conditioning system starts out as a cool gas with low pressure. The refrigerant needs to become a liquid before it moves inside to the air handler to finish the cooling process. In order to make it to the liquid stage, the chemical first needs to become a high-pressure, warm gas—and that's where the compressor comes in.

The compressor compresses or squeezes the cool gas until it becomes highly pressurized, which also warms the gas up. The compressor then passes the gas on into the condensing coils, which are able to transform that gas into a liquid, but only if the compressor did its job right in the first place.

Potential Malfunction: Broken Start or Run Capacitor

The compressor requires a constant electrical current to squeeze that gas. If the current becomes interrupted for any reason, the compressor can either fail to compress properly or stop working entirely. There are a couple of parts built into most air conditioners to help with potential interruptions: the start capacitor and the run capacitor.

The start capacitor, as the name suggests, makes sure the compressor has enough continuous power to turn on and start compressing. The run capacitor stands by with a potential boost if the electricity becomes interrupted during the course of operations.

Either capacitor can stop working, but it's fairly easy to tell what one failed based on when the compressor and air conditioner stop working. If it's right after starting up, you know it's the start capacitor; otherwise, the problem is the run capacitor.

You can test the health of the capacitor if you own a multi-meter that has AC and Ohms settings. Turn off power to the AC system, gain access to the capacitors using your manuals, and then carefully drain the charge out of your capacitors. The start drains with the multi-meter probes set to AC – simply wait until it reads zero before testing for Ohms of resistance. Compare the Ohms reading to the number printed on the side of the capacitor to make sure it is in healthy range.

Test the run capacitor in a similar way, but drain the charge using the end of an insulated screwdriver rather than the AC probes. If you don't want to replace the broken capacitor yourself, call in an AC repair service.

Potential Malfunction: Broken Compressor Motor

The compressor is motorized, and a failure of the motor can cause the part to either work less efficiently or stop working altogether. If the latter is true, a lower amount of refrigerant will go into the condenser coils, which will produce a smaller amount of liquid. The liquid will go inside and provide minimal cooling capabilities to the evaporator coils. Your system might still cool for a while but will quickly lose efficiency.

A compressor with a broken, dead motor won't pump out any gas at all. Nothing will happen in your system and you won't have any cooled air. Call in an HVAC technician as soon as possible for a service call.