A capacitor is an electrical device that stores energy between a pair of conductors. These conductors are usually called plates. There are two types of capacitors used in HVAC equipment. There are start capacitors and run capacitors. Start capacitors give an initial start boost to a motor or compressor. Run capacitors increase efficiency of motors on air handlers, high efficiency furnaces, and pumps for hot and chilled water.
Unless you are familiar with capacitor technology, it can be difficult to tell if yours has failed. If it has, it will be bulged out on the top and sides. Don’t touch it, even if the equipment has been turned off, because you risk electric shock. Have a qualified HVAC contractor check out the equipment and repair it if needed. If you can’t afford the cost of repairs, you can replace the capacitor yourself with proper research and tools.