A relay is an electromechanical or solid-state switch that uses a low-voltage control signal to operate high-voltage electrical circuits in HVAC equipment. Relays enable control devices like thermostats to safely manage powerful motors and compressors. They provide electrical isolation, allowing low-voltage thermostat circuits to control high-voltage equipment without direct contact.
Types and Functions
Single-pole single-throw (SPST) relays control simple on-off functions, while double-pole double-throw (DPDT) relays manage complex switching operations. Electromagnetic relays use coil windings to create magnetic fields that activate mechanical contacts, while solid-state relays employ semiconductors for silent operation. Relays typically handle voltage ranges from 24V to 240V with switching capacities up to 30 amps.
System Integration
Relays control compressors, fan motors, heating elements, and other major equipment based on thermostat commands. Defrost relays in heat pumps switch system operation between heating and cooling modes. A malfunctioning relay causes intermittent operation, failure to start, or continuous running. Relay replacement typically costs between 75 and 150 dollars.