Smart thermostat short-cycling the HVAC — San Francisco, California

When an HVAC system rapidly turns on and off in cycles shorter than five minutes after a smart thermostat installation, the thermostat rather than the furnace or AC is usually the culprit. Common causes include incorrect wiring — particularly a missing or improperly connected C (common) wire that starves the thermostat of power, forcing it to steal power from the heating or cooling call wire and prematurely ending the cycle. Other triggers are wrong system-type settings (heat pump configured as conventional, or single-stage set to multi-stage), a thermostat mounted on an exterior wall or near a heat source that gives false temperature readings, or incompatibility between the thermostat's minimum load requirements and a variable-speed HVAC system. A smart home installer can verify wiring against the HVAC wiring diagram, add a C-wire adapter if needed, reconfigure system settings, and relocate the thermostat to a neutral interior wall for accurate readings.