In order to service and troubleshoot an air-source heat pump system, a service technician must understand the unit's sequence of operation. This is the order of events the system undergoes to cycle itself on and off. Knowing how the unit operates properly aids in determining where to start troubleshooting when the system doesn't operate properly. Where the system varies from its normal sequence is a major clue to any problems.

1. The compressor pumps out high-pressure, superheated refrigerant vapor.