A furnace that runs without stopping is not always broken, and it is not always the furnace. The first job is to work out which of three things is actually running: the burners, the blower, or both. That one distinction eliminates half the causes before anyone opens a panel.
Burners or blower? Check this first
Put your hand over a supply register.
- Warm air, continuously, and the house is not getting warmer. The furnace is heating but losing the heat somewhere, or it is too small for the conditions. Go to the heat loss section below.
- Room temperature or cool air, continuously. The burners are off and only the blower is running. That is a fan setting or a control problem, not a heating problem.
- Warm air, continuously, and the house is already well above the setpoint. The furnace is not receiving the signal to stop. That points at the thermostat, its wiring or a stuck relay, and it needs attention because it wastes gas and wears the equipment.
The blower runs constantly
The thermostat fan is set to ON
The most common cause by far. With the fan on ON instead of AUTO, the blower runs all the time whether or not the burners are lit. Between heating cycles it moves room-temperature air, which feels cool. Switch it to AUTO. Some people run the fan on ON deliberately to even out temperatures, which is fine, as long as you know that is why it never stops.
A fan limit switch or control board is stuck
On older furnaces a mechanical fan limit switch controls the blower, and it can be set to manual or fail in the on position. On newer furnaces a relay on the control board can stick closed. Either way the blower keeps running after the call for heat ends. Both are routine repairs.
The furnace heats constantly and the house will not warm up
This is less common in Sacramento than in colder places, because our winter design temperatures are mild. Overnight lows sit in the high 30s and low 40s most of the season, which a correctly sized furnace handles without running nonstop. So when it does happen here, the cause is usually one of these:
- Leaking attic duct work. In most homes in Natomas, Elk Grove, Rocklin and newer Roseville the ducts run through the attic. Heated air escaping through disconnected boots and torn flex duct warms the attic instead of the house, so the thermostat is never satisfied. Our article on duct sealing versus replacement covers the fix.
- A clogged filter. Less air across the heat exchanger means less heat delivered to the rooms, so the furnace runs longer to reach the same temperature.
- Closed registers or a blocked return.
- A thermostat in the wrong place. A thermostat on an exterior wall, near a drafty door or in a hallway the heat does not reach will keep calling for heat while the rest of the house is warm.
- A furnace that is genuinely undersized, usually after an addition or a garage conversion was added without recalculating the load.
The furnace heats constantly and the house is too warm
Now the furnace is not being told to stop. The likely causes:
- A thermostat reading wrong. Compare it against a separate thermometer placed next to it. A few degrees off will keep a furnace running.
- A thermostat that has failed or a smart thermostat with a schedule or setting you did not expect. Our article on thermostat problems covers the common ones.
- Shorted thermostat wire. If two conductors touch, the furnace sees a constant call for heat. This is also the usual reason a furnace turns on when the thermostat is switched off.
- A stuck gas valve or control relay. Rare, and the one that matters most, because burners that will not shut off are a safety issue. If the house is overheating and the thermostat is off, turn the furnace off at its service switch and call.
If you have a heat pump
A heat pump running for long stretches on a cold morning is normal. Heat pumps deliver heat more gently than a gas furnace and are designed for longer run times. What is not normal is running continuously while the house loses ground, blowing cool air for long periods, or the backup heat strips staying on all the time. Our article on a heat pump not heating covers those.
Why not to ignore it
A furnace that never shuts off costs more on every gas bill, puts extra hours on the blower motor and inducer, and if the cause is a control fault, it will not fix itself. Our article on why a heating bill is so high covers the cost side. The opposite problem, a furnace that turns on and off too often, is in short cycling.
What we do
We confirm whether the burners, the blower or both are running, check the thermostat and its wiring, test the controls, and look at duct leakage if the house is not warming. We take a carbon monoxide reading on every gas furnace visit. Our diagnostic is a flat $69 and it is credited toward the repair. See heating and furnace services for the rest.
Frequently asked questions
Why does my furnace keep running even after the house is warm?
The furnace is not getting the signal to stop. The usual causes are a thermostat reading incorrectly, a failed thermostat, shorted thermostat wiring or a stuck control relay. If it keeps heating with the thermostat off, turn the furnace off at its switch and call.
Why does my furnace blower keep running but no heat comes out?
Most often the thermostat fan is set to ON instead of AUTO. If it is already on AUTO, a stuck fan limit switch on an older furnace or a stuck relay on the control board is the likely cause.
Is it normal for a furnace to run constantly in cold weather?
In Sacramento, rarely. Our winter lows are mild, so a correctly sized furnace should cycle off regularly. Nonstop running here usually points to leaking attic ducts, a clogged filter, a poorly located thermostat or an undersized furnace.
Why does my furnace turn on when the thermostat is off?
Usually shorted thermostat wiring, where two conductors touching send a constant call for heat, or a stuck relay on the furnace control board. It needs a technician, because a furnace that ignores the thermostat can overheat the house.
Is it normal for a heat pump to run all the time?
Longer run times are normal for heat pumps, especially on cold mornings. It is not normal if the house keeps getting colder, the air feels cool for long periods, or the backup heat stays on constantly.
Does a furnace running constantly use more gas?
Yes, when the burners are running. If only the blower is running because the fan is set to ON, the extra cost is electricity for the blower motor rather than gas.