Display shows 'Heat On' flashing and furnace does not start

Safety first: Do not remove furnace panels unless you are trained; internal components carry high voltage and gas.

Steps to troubleshoot:

• 1. Wait the delay: Allow 3–5 minutes for the thermostat's built-in delay to expire.
• 2. Confirm mode and setpoint: Ensure the thermostat is in Heat mode and the set temperature is above the current room temperature.
• 3. Check furnace power: Verify the furnace switch (often a light-switch near the unit) and breaker are ON.
• 4. Reset furnace: If accessible and safe, turn the furnace power switch off for 30 seconds, then back on to clear a possible lockout.
• 5. Review cycle settings: In advanced settings, confirm the heat cycle rate is appropriate for your system type (forced air vs boiler) per the Resideo manual.

When to call a professional: If 'Heat On' flashes indefinitely and the furnace never starts, a technician should inspect the furnace control board and safety circuits.