HVAC contactors are essentially on/off switches, though not the kind people turn on or off. That said, when you turn your air conditioning unit or heat pump on, the contactor goes down, allowing the flow of electricity that powers the unit. When you turn your unit off, the contactor goes back up, blocking electrical power from the unit and shutting it down. Contactors are used to control the flow of electricity to HVAC system condensers, compressors, and motors. They in turn can be controlled by on/off switches and by thermostats. As an electrical component, contactors can burn out due to power surges and other electrical anomalies and they can get stuck in either an up or down position. Note also that in our southwest Florida region ants tend to be attracted to them, which also leads to malfunctions.