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start and run capacitors from Direct Brand

What’s The Difference Between a Start Capacitor and a Run Capacitor?

In air conditioners, there are many parts that can go wrong, and quite often it is either the start or run capacitor. Working together with other components, they deliver cool air into the home when you need it most.

What are HVAC capacitors?

Capacitors are cylindrical containers inside your air conditioner unit that store energy and pass it on to the motor at start-up and to keep it running. Hence why they are called start and run capacitors.

What do capacitors do in HVAC units?

When you first switch on your air conditioner, it will require a lot of energy to get the motor running and quite often your home electricity supply will not be powerful enough. This is where the start capacitor steps in and provides the energy boost required to get the motor running. As soon as it has done its job of getting things moving, the start motor shuts down.

The run capacitor then steps in and provides the power for the motor to maintain its drive. This happens so the air conditioning motor can keep spinning over long periods of time.

Charging the start and run capacitor

As with the start capacitor, the run capacitor will store energy during the process to enable it to provide the power for the next start up.

By connecting the start and run capacitor, you create a dual capacitor system, which is used to drive the motor and compressor together.

As with everything electrical or under immense daily pressure, things do go wrong or get worn out. Capacitors do fail and there are things to look for that will help you decide what is wrong with your air conditioning unit.

Capacitors quite regularly overheat, especially if they are in full sunlight all day long. That extra heat will cause the capacitor to overheat quite quickly. This extra pressure is added to by the fact your air con will be on more and working hard during the hot summer months. As temperatures soar, your unit works harder, causing the capacitor to get even hotter. A never-ending cycle of heat!

What happens when a capacitor goes bad?

If you air conditioner fails to start or starts intermittently, then it could be a sign your start capacitor is on its way out. Also, look to see if the capacitor is looking swollen, rusty, or damaged. If the run capacitor is on its way out, your unit will probably start and stop regularly, or not cool your home as it should.


As you can see, the life of a capacitor is hard. It is a critical part of your air conditioning unit, and quite often goes bad. The start and run capacitors are easily replaced, but you should always seek professional help if you are not too sure how to deal with electrical problems. At least now you will know the difference between the start capacitor and the run capacitor.

You can find our selection of HVAC parts here.