Furnace runs but produces no heat; blower runs continuously with room temperature air.

Safety: If you smell gas, shut off the gas supply, evacuate, and call your gas utility.

Steps:

• Check the thermostat mode: set to HEAT and fan to AUTO.
• Verify that the manual gas shutoff valve at the furnace is open.
• Observe the furnace through the sight glass during a call for heat: inducer should start, ignitor should glow, then burners should light. If the ignitor never glows, test and replace it as needed.
• If the burners light briefly then shut off and the blower continues, inspect and reset any manual-reset rollout switches once only, and replace a failed high limit if it will not reset.
• If the furnace repeatedly fails to ignite, stop trying and call a technician to check gas pressure and safety circuits.