Central air conditioners and heat pumps use capacitors. A capacitor keeps an electrical charge on standby to provide it to the motor when it first turns on. Over time, a capacitor will wear out; you must replace it so your air conditioner or heat pump continues to work properly. Before you buy a new one, you must ensure it's the correct size. Your system won't work -- and may even burn -- if you fail to install the properly sized capacitor.
Turn off the electrical disconnect for your air condenser or heat pump. Open the electrical box and pull out the disconnect.
Locate and open the access panel on your air condenser or heat pump. Remove the screws holding the panel in place.
Check the label on the capacitor for the voltage and horsepower range. This is the size the new capacitor must be. If the label is faded, you must size the capacitor by the model of the unit.
Identify the make and model number of the air condenser or heat pump. This information will usually be on a label on the outside casing. Take the make and model number to an HVAC supply store, which will use trade manuals to find the appropriately sized capacitor.