HVAC For Beginners

Capacitor testing

How to Test Your HVAC Capacitor

This capacitor testing guide will show you how to tell if a run capacitor requires replacement. Although the parts are relatively inexpensive, calling a repairman to diagnose and replace it would cost over $100. With this guide, you can keep that money.

A capacitor is an electrical device that stores an electrical charge. They are used on fan motors and compressors in hvac systems. They have two electrical ratings. The first rating is the unit's capacitance which is given in microfarads (mfd). This is a measure of the amount of charge the unit can store with a specified voltage applied. The second rating is a voltage rating which tells what supply voltage the unit is rated for. This is usually 370vac or 440vac in hvac systems. This is important to know because, if a significantly higher voltage is applied, the capacitor will prematurely fail.

Capacitor Testing

Types of capacitors

There are two different types of capacitors which are used in hvac units. The first type is called a run capacitor and is used in furnaces, air conditioners, and heat pumps. These capacitors are used with fan motors and compressors. They maintain a relatively constant voltage supply to these motors and increase their torque at startup. The second type is called a start capacitor. They are used in some air conditioners and heat pumps. They provide extra torque at startup for the compressor. A potential relay automatically disconnects them from the compressor after the compressor starts.

Capacitor Testing

For capacitor testing, a multimeter is required. Some digital meters have a capacitance setting. First the power supply should be shut off then the two terminals on the capacitor should be shorted across with a screwdriver. This will discharge the unit so you do not receive a shock. Disconnect the wires from the capacitor. Then, the leads of the meter are placed on the terminals and a reading is obtained. The reading you get should be within 6% of the rating on the capacitor. If your reading is more than 6% lower than the rating, the capacitor should be replaced. If you have an analog type meter, the capacitor is checked with the meter set to measure resistance. Shut off the power, discharge the capacitor, and disconnect the wires attached to it. Then with the meter on the highest resistance setting, put the leads on the capacitor terminals. The resistance reading should start at zero and go up to maximum. Some obvious signs that the capacitor is bad are bulging of the unit or an oily substance leaking from it.

**NOTE** Only you can assess your ability to perform this task. This is a guide and cannot provide all of the details for every situation.

Replacing a faulty capacitor

To find the proper replacement part, you can look for the ratings on the existing part. You should find a voltage rating (either 370 or 440) and a micro-farad rating. After your capacitor testing has shown that you have a faulty part, the replacement procedure is simple. If you have a dual run capacitor (the capacitor has three terminals on it), then you should mark the wires and write down where they go. Then simply disconnect the wires, replace the faulty part and then reconnect the wires.