The Thermostat Is Off. Why Won't My Furnace Turn Off?

When the weather gets cold, your furnace keeps your home and family warm and toasty, but it needs a thermostat to do the job. This small device tells your furnace how much heat to deliver so your house stays at a comfortable temperature.

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If something goes wrong with the thermostat, you could find yourself freezing — or sweltering. If you're starting to sweat because your furnace won't shut off, even when the thermostat is off, don’t panic! Here's everything you need to know.

Why Does My Furnace Keep Running When the Thermostat Is Off?

When your thermostat is off, your furnace should stop running. If it doesn’t, there could be a few reasons why:

• The fan control is set to ON.

  Your thermostat’s fan setting controls the blower, which pushes heated or cooled air through the vents in your home. If it’s set to "ON," the fan will run continuously even if your home's temperature has reached the number set on the thermostat. This could make it seem like your furnace is still running even if it isn't.

• The thermostat's batteries died.

  If you have a battery-operated thermostat, battery failure can sometimes lead your furnace to run continuously.

• The furnace filter is dirty.

  Over time, dirt, dust and other airborne particles can clog your furnace’s filter. This can restrict airflow, causing your furnace to run continuously just to push air through.

• A furnace switch is stuck.

  If a switch on your furnace gets stuck in the On position, it can cause the unit to run continuously, even if the thermostat is off.

• The limit switch is faulty.

  Your HVAC system’s blower has a limit switch, which tells it when to turn on and off. If this switch fails, your furnace fan may not shut off when the thermostat goes off.

• The thermostat has loose or faulty wiring.

  Over time, electrical wiring can wear out or pull loose, which can cause connection problems between your thermostat and furnace. This can cause your furnace to run continuously because it doesn't receive the signal to shut down.

Can a Bad Thermostat Cause a Furnace to Run Continuously?

Your thermostat registers your home’s current temperature and tells the furnace when to turn on and off so it keeps your house at the desired temperature. If the thermostat malfunctions, it can cause the furnace to run continuously. This can include issues such as a faulty heat sensor, an incorrect thermostat calibration or loose battery connections. Although you may be able to recalibrate your thermostat by following the manufacturer's instructions, other problems may not be fixed so easily, and you may have to replace your unit.

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What Should You Do When Your Furnace Won’t Shut Off Even When the Thermostat Is Off?

Even on the coldest day, your furnace should shut off regularly for brief breaks. If it doesn't, it can strain your whole HVAC system, which can lead to major problems. However, you may not need to call in a pro just yet. First, you can troubleshoot the problem by following several simple steps:

• Check the thermostat settings.

  Make sure your thermostat is set to "HEAT," and the fan control is set to "AUTO.:

• Replace the batteries.

  If your thermostat is battery-operated, change out the batteries. Use fresh batteries that haven’t reached their expiration date.

• Visually inspect the thermostat's wiring.

  Depending on your thermostat, you may be able to inspect the wiring by lifting off the unit's cover. Look for loose, frayed or broken wires. If you find loose wires, gently reconnect or tighten them using pliers and a screwdriver. However, you should always prioritize safety, so don't touch wiring or other electrical components without first turning off the power at the breaker box. If the wires are frayed or broken, call in a licensed electrician or trained HVAC technician to execute the repair.

• Reset your thermostat.

  Sometimes, simply resetting the thermostat can solve the problem. The procedure can vary among models, so follow the manufacturer’s instructions for your unit.

• Check your furnace or HVAC filter.

  If the problem persists, move on to your furnace and begin by checking the filter. To prevent damage to your furnace, set it to OFF, and then locate and open the access door. If the filter appears dirty or clogged, replace it with a new filter according to the manufacturer’s instructions.

If your furnace still won't shut off, it's time to call in a professional HVAC technician who can accurately diagnose and repair the problem.