Quick answer
A furnace that won't shut off when the house is already warm has a stuck call for heat. The usual causes are a thermostat relay stuck closed, a short between the R and W wires in the thermostat cable, a welded relay on the control board or, rarely, a gas valve stuck open. If the burners stay lit with the thermostat off, close the furnace gas valve and call.
If your furnace won't shut off when the house is already warm, start by working out what is still running. A blower that keeps moving room-temperature air is a fan setting or a fan-control fault, and it is covered in why a furnace fan runs constantly. This page is about the other case: the burners are lit, the air at the registers is hot, and the room is above the temperature you asked for.
Is it a stuck call for heat, or a furnace that cannot catch up?
Compare the room temperature on the thermostat with the setpoint. If the room is still below the setpoint, nothing is stuck. The furnace is running because it has not reached the target, which happens with a clogged filter, closed registers, leaky ducts or a cold snap that outruns the equipment; a house that stays cold with the furnace on covers that.
If the room is a degree or more above the setpoint and the burners are still firing, something is holding the heat circuit closed. A thermostat calls for heat by connecting its R terminal, which carries 24-volt power, to its W terminal. Anything that keeps R joined to W, or keeps the gas valve powered without that signal, keeps the furnace burning.
Three steps that find where the call for heat is stuck
- Step 1: set the thermostat to Off. The burners should go out promptly and the blower should run on for a minute or two to clear the heat. If that happens, the thermostat was calling on purpose, so look at the schedule, a hold setting or a sensor that reads low.
- Step 2: if the burners keep firing with the thermostat set to Off, pull the thermostat off its wall plate. That disconnects it physically. If the burners now go out, a relay inside the thermostat is stuck closed and the thermostat needs replacing.
- Step 3: if the burners still fire with the thermostat off the wall, the fault is in the cable or at the furnace. Go straight to the emergency steps further down.
What keeps a furnace firing with the thermostat removed
- A shorted thermostat cable. The R and W conductors are touching somewhere between the wall and the furnace: a staple driven through the cable, insulation chewed by mice, or bare ends touching behind the wall plate or at the control-board terminals. Older homes still on their original two-conductor thermostat cable are prone to it, because the insulation turns brittle with age.
- A welded heat relay on the control board. The board powers the gas valve through a relay, and contacts that have arced for years can fuse shut. The board may flash a fault code and run the inducer and blower without stopping, because it senses flame it did not ask for.
- A gas valve stuck open. Modern valves have two seats in series so that a single failure does not leave gas flowing, which makes this rare. It is also the dangerous one, because nothing electrical will shut it off.
Burners still lit with the thermostat off: treat it as an emergency
A furnace that will not stop firing overheats the house and leans on the high-limit switch as its only brake. That switch is there to protect the heat exchanger from overheating, not to stand in for the thermostat. Do the following in order:
- Close the manual gas shut-off on the pipe beside the furnace: turn the handle a quarter turn until it sits crosswise to the pipe. How to shut off the gas to a furnace shows where to find it.
- Let the blower run for a couple of minutes to cool the heat exchanger, then turn the furnace service switch off.
- Leave both off. Do not reopen the gas valve to see whether the problem has cleared.
- Call for service on an emergency line. If you smell gas at any point, leave the house and call Enbridge Gas from outside.
What it costs to fix a furnace that will not stop
- Diagnostic visit: $90–$150, which comes off the repair bill when you go ahead.
- Control board: $500–$1,200.
- Gas valve: $500–$1,000.
- Thermostat replacement or a new thermostat cable: quoted in writing on the visit, because the price depends on the thermostat chosen and how the cable has to be routed.
A stuck thermostat can wait a day if someone is home to manage it: seat the thermostat to run the furnace and pull it off the wall to stop it. A board or gas valve fault cannot wait, and the gas stays off until it is repaired. Gas valves and the wiring that controls them are work for a TSSA-licensed gas technician.
ZK Mechanical answers its line 24 hours a day with no overtime, weekend or holiday surcharge. Call (647) 801-1252 for furnace repair, or use the contact form for a thermostat problem that is under control. Related questions are grouped under heating answers.