What should I do if my furnace will not shut off?

What this symptom usually means

A furnace that will not shut off is different from one that will not heat. Here, the key issue is that the call for heat or the blower command is not ending when it should. Sometimes the house eventually reaches temperature but the system keeps running anyway; other times comfort barely improves because the furnace is overheating, cycling on safety, or moving air without delivering steady heat.

What you can tell from what keeps running

Watch whether the burners or heating elements are still producing heat, or whether only the fan is still on. If lowering the thermostat below room temperature stops the heat but the fan continues, the fan setting may be ON or a fan relay may be stuck. If the thermostat is turned down and the furnace still fires back up, that points more toward a thermostat/control issue than an airflow problem.

A high-limit problem often looks like this: the furnace heats for a few minutes, the burners shut off, but the blower keeps running for a long stretch, then the heat starts again and repeats. Homeowners often notice warm air that fades cooler before the next heating cycle, rooms that never quite catch up, or a cabinet that feels unusually hot. That pattern is specific to overheating protection, not a simple no-heat complaint.

If continuous running comes with ice, water, or overheating, the diagnosis changes. Ice or water usually points to a heat-pump/airflow issue where the indoor fan may run during defrost or after a fault, while overheating points toward limit-switch operation or a blower-speed problem. If you smell gas, smell something electrical burning, see smoke, or your carbon monoxide alarm activates, leave the building and call the gas utility or emergency services from outside.

What the technician typically checks

On a service visit, the technician will compare thermostat commands to what the furnace control board is actually doing, then test whether the blower relay, fan center, or sequencer is stuck. They will also check temperature rise across the furnace, limit-switch operation, blower speed, duct static pressure, and whether the unit is cycling off on safety. On heat-pump systems, they may also verify defrost operation and whether auxiliary heat is staying energized too long.

What affects repair cost

Cost depends on whether the problem is a control issue or an overheating/airflow issue. Thermostat replacement often runs about $170-$575 installed for standard models, while a fan relay or sequencer repair may land around $170-$575; an integrated furnace control board is commonly about $500-$1,575 installed depending on brand and availability. If testing shows a deeper cause such as a failing blower motor, duct restriction, or repeated high-limit stress, the final cost can rise because labor and additional parts increase.