Furnace runs but produces no heat; blower operates, but supply air is cool or only slightly warm.

Safety first: Turn off power and gas if you suspect a gas leak or repeated ignition failures.

• 1. Check thermostat settings: Ensure the thermostat is set to HEAT and the setpoint is above room temperature. Confirm the fan is set to AUTO, not ON (ON will run the blower continuously even without heat).
• 2. Verify gas supply: Confirm the manual gas shutoff valve to the Thermia furnace is open. Check other gas appliances for operation.
• 3. Observe ignition sequence: Through the sight glass, watch for the igniter glowing and burners lighting when there is a call for heat. If the igniter never glows, it may be faulty. If it glows but burners never light, the gas valve or safety switches may be at fault.
• 4. Inspect safety switches: Check high limit and rollout switches on the Thermia furnace for manual reset buttons. If tripped, do not simply reset; investigate airflow or combustion issues as described in other entries.
• 5. If you cannot identify the cause: Contact a qualified technician to test the ignition system, gas valve, and safety circuits.