Furnace Keeps Running

Separate the blower from the burners

The most important distinction is whether the furnace is actually producing heat continuously or whether only the blower is running. Stand at a supply register: if the air is warm and never stops, the burners are firing continuously. If the air is room temperature, only the circulating fan is on and the burners are idle.

These are entirely different problems. Continuous fan is usually a setting or a relay. Continuous heat points to the thermostat, wiring or gas valve control.

Reasons the blower runs constantly

Continuous blower operation is not usually dangerous, but it adds electrical cost and it can make the house feel drafty. Check the thermostat setting first — it resolves this complaint more often than any repair.

• Thermostat fan switch set to ON rather than AUTO
• Fan circulation feature enabled on a smart thermostat for air mixing
• A stuck blower relay on the control board
• A shorted or pinched thermostat wire keeping the fan terminal energized
• A limit switch that has failed closed in the fan-on position

Reasons the burners run constantly

In Fort Collins, an extended run during a subzero night can be completely normal. A furnace sized to design conditions may run nearly continuously when it is below zero and still be operating correctly. The concern is continuous operation in mild weather, or a house that keeps getting hotter past the setpoint.

• Thermostat setpoint higher than the system can reach during extreme cold
• A thermostat calling for heat continuously due to a failed internal contact
• Shorted thermostat wiring bridging the heat-call terminals
• A gas valve that is not closing properly — a condition that warrants prompt professional attention
• Severe duct leakage or insulation deficits that make the home impossible to satisfy

Safe checks

1. Set the thermostat fan to AUTO and note whether the blower stops within a few minutes.
2. Lower the setpoint well below the current room temperature and confirm the burners shut down.
3. If the burners do not shut down after the setpoint is lowered, turn the furnace off at the service switch and call for service.
4. Check for obvious damage to visible thermostat wiring near the furnace.

Why a furnace that will not shut off matters

A system that cannot stop heating overshoots setpoint, wastes fuel and, in the case of a gas valve that will not close, becomes a safety concern. If lowering the thermostat does not stop the burners, use the furnace service switch and arrange emergency furnace repair. Review furnace safety for the conditions that call for leaving the home rather than troubleshooting.

Efficiency implications

Continuous operation shows up on the gas bill quickly. If your bills have climbed without a change in weather or rates, read furnace high energy bills and the furnace efficiency guide for the full picture, including the envelope and duct factors that often matter as much as the equipment.