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What Is An HVAC Start Capacitor, Why Does Your AC Need It, And What Malfunctions Can Thwart Your System?

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A central air conditioner contains two main segments, the outdoors condensing unit and indoors air handler, and each segment contains a variety of parts that work together to move a chemical fuel through to provide the cooling your home needs. The process starts at the thermostat, which sends a signal to the condensing unit and powers up the compressor. The compressor squeezes out the refrigerant gas that will move through the whole system in various phases.

The compressor depends on a couple of capacitors, which include the start capacitor, to function properly and reliably. What is an HVAC start capacitor, why does your air conditioner need it, and how can the part malfunction and thwart your system?

Start Capacitor Use and Function

The start capacitor is a small, cylindrical electrical storage device that can provide a small burst of electricity when the compressor is starting up. If the compressor experienced an electrical interruption when starting and the capacitor wasn't present, the compressor couldn't start properly and would simply stop and the air conditioner wouldn't provide any cooling.

The start capacitor sometimes works in conjunction with a run capacitor, which keeps the stored electricity on hand for any interruptions the compressor experiences while already up and running.

Potential Malfunction: Failing Start Capacitor Causes Compressor Instability

If a start capacitor begins to fail, the part can provide an unreliable boost for the compressor when it starts up. The unreliable boost means that the compressor can start up normally sometimes but not every time.

If the compressor runs sporadically, the system won't receive the proper amount of gas refrigerant that's needed to move into the condenser coils, become a liquid, and then moves inside the house to turn back into a gas and create a cold surface to serve as the cooling source for your indoor air. Your system might still run properly most of the time but you could see a loss of efficiency.

Potential Malfunction: Failed Start Capacitor Prevents Compressor from Starting

If the start capacitor completely fails, your compressor will fail to start up any time there's not enough electricity, which will likely happen somewhat often. A compressor that doesn't start won't put out any gas into the system so the unit won't provide any cooling at all.

If you suspect that the capacitor is failed, you can test the part yourself if you have electrical knowledge and a multi-meter that includes AC setting for draining the stored charge and an Ohms setting for testing and comparing the reading to the range printed on the capacitor. If you don't want to test the part yourself, call in the best heating and air conditioning repair service in your area for help.