Thermostat won't turn the heat off

Symptoms

• Furnace runs continuously regardless of temperature
• System doesn't stop even when switched to Off
• House gets uncomfortably warm

Likely causes

• CommonThermostat sensor reading incorrectlyA sensor reading colder than actual room temperature will never satisfy the call for heat.
• PossibleStuck heat relay in the furnaceA physically stuck relay can keep the furnace firing regardless of the thermostat's signal.
• PossibleWiring fault bypassing thermostat controlCertain miswiring can cause the furnace to run independent of the thermostat's commands.
• PossibleThermostat set to a very high temperature by mistakeAn accidentally high set point will keep the furnace running longer than expected.

What to do

1. Confirm the thermostat's set temperature and mode, since an unusually high set point will simply extend the runtime.
2. Try switching to Off directly on the thermostat and see if the furnace responds.
3. If it keeps running in Off mode, turn off the furnace switch or breaker immediately to stop it.
4. Check the thermostat's reading against a separate thermometer for accuracy.
5. With power off, avoid touching internal furnace components, but note if you can see or hear anything unusual like a relay stuck closed.
6. Leave the furnace off until the cause is identified if it ignored the Off command, since a stuck heat relay is a fire and overheating risk.

Tools and parts

• Separate room thermometer

When to call a pro

Call an HVAC technician promptly if the furnace won't turn off from the thermostat or its own switch, since a stuck relay can be a safety hazard.

Safety