HVAC Capacitors and What They Do

Posted by Sara Udell on Apr 19th 2023

HVAC Capacitors and What They Do

So we know from our previous blog post how to identify and replace a bad A/C capacitor, but what is a capacitor and what does it do? 

What is a Capacitor

A capacitor is like a battery that helps to start up the Motor and/or Compressor because it takes a more energy than what the typical household has as a constant. You can see this when you start a vacuum and the lights dim for a second, the same can happen with your furnace or air conditioning. As your Unit cycles on and off the Capacitor is in use storing and releasing electricity to help take that load off the houses grid so that you don't see any diming lights or electricity being pulled away from other parts of the house. When replacing a motor that uses a capacitor it is a good idea to also replace the capacitor so there is less chance of having to do so in the future. A Bad or old capacitor can damage a new motor or compressor. 

Single Run Capacitor

A single run capacitor will typically be used for an indoor blower motor. These will only have a single Microfarad rating and 2 terminals on top. When installing a new single run capacitor it does not matter what terminal is hooked up to which wire as they do not have a specific polarity. Single run capacitors can come in round or oval shapes.

You will also find that on some large roof top units on the condenser side that have multiple fan motors will use single run capacitors for the motors as opposed to a dual run capacitor for the motor and compressor.

The last place you may find these are in very old condensing units. They will have a single run capacitor for the fan motor listed and another single run capacitor (with a higher MFD rating) for the compressor.

They have moved away from having 2 separate capacitors to having 1 dual run capacitor as it cuts down on costs and space when building the units.

Some single run capacitors will have a voltage of 370 and some will say 370-440. Most manufacturers are moving away from 370V and moving to 440V as it has better durability and longer life span. You can always use a 440 in a 370 application as you can go up in voltage but not down.

Most motors will tell you what capacitor is required on the motor tag. 

Dual Run Capacitors

Dual run capacitors are typically used on the outside unit whether its a heat pump or condenser. They will have 2 microfarad ratings, one for the fan motor and one for the compressor and will have 3 terminals and need to be hooked up according to their polarity.

C = Common
Fan = Fan Motor
Herm = Compressor (Hermetic compressor)

Most dual run capacitors will be round, there are some that are oval but they are not as common anymore. The shape of the capacitor does not effect the performance. 

To be able to determine the correct dual run capacitor for a unit we will need the model number to find the parts breakdown. 

Hard Start Kit - Start Capacitor

A Start Capacitor is not the same as a single or dual run capacitor. They are also sometimes referred to as a Hard Start Kit.

A Hard start kit stores energy and uses it to help the compressor and fan motor initially start up but do not keep the unit running. Where as the single and dual run capacitors, along with helping with the initial start up, help to keep them running after start up.

Example: If your air conditioner starts up in the summer and your lights in the house still dim for a second. The hard start kit will help take that load off the houses electrical system and keep your lights from dimming and give your unit the extra jolt of voltage to get started. 

These will have a range of microfarads that they work in and also different voltages.

Kits will sometimes come with a relay included.

These are most often considered an accessory or aftermarket item and most times will show up in a parts breakdown, so we will need a model number to find the correct one. 

Soft Start Capacitor

A soft start capacitor works much the same as a hard start but instead of sending the full voltage to the unit to jump start it the soft start will reduce the in-rush current by 60% and slowly start up the compressor. This will minimize any damaged that can be caused by the jolt of voltage to start the unit. Another benefit of a soft start capacitor is if you are using a generator to start your heat pump or condenser they will reduce the size of the generator needed to start the system by up to 60%. This makes them a great option for RVs and anything that may regularly be started by a generator.

Take a Look at Our Capacitors 

Still Not Sure or Would Like Some Assistance?

After learning about the different types of capacitors and their functions, if you are still unsure what capacitor you need you can contact our Part Department or give a call and talk to us directly. We offer a wide range of Capacitors that are in stock and ready to ship to keep your HVAC unit running smoothly as it is important to have the right sized capacitor for your unit so that it does not cause any damage to the motor or compressor. 

Don't hesitate to contact us with your model number to find the correct parts and ensure your unit runs at its best!

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