Thermostat powers on but frequently reboots or flickers when heating or cooling starts.

Safety first: Turn off power to the HVAC system before inspecting wiring.

• Step 1 – Check C wire: Ensure the C (common) wire is securely connected at both the thermostat backplate and the HVAC control board.
• Step 2 – Measure voltage under load: With a multimeter, measure 24 V AC at the thermostat while the system is idle and while heating/cooling is running. A large drop indicates a weak transformer or wiring issue.
• Step 3 – Inspect transformer rating: Confirm the VA rating of the transformer is sufficient for all connected loads (thermostat, zone valves, etc.). Upgrade if undersized.
• Step 4 – Separate power supplies: If multiple devices share the same transformer, consider adding a dedicated transformer for the thermostat and controls.
• Step 5 – Replace thermostat if needed: If supply is stable but reboots continue, the thermostat’s internal regulator may be faulty and the unit should be replaced.