Furnace Won’t Shut Off With Thermostat

The thermostat keeps running and running even the thermostat setting has been reached. For example, maybe you set your thermostat at 71°F, and the thermostat indicates that the indoor temperature is 72°F, which is 1°F higher. However, your heater continues to run with no signs of slowing down!

Reasons Furnace won’t Shut Off with Thermostat

It could be several things. For one, maybe your worst fears are valid that the furnace is broken. If the furnace is broken, it may fail to take any instructions from the thermostat.

Alternatively, it could be the thermostat. When the thermostat is broken, it may “forget” to communicate to the furnace to stop. Read on to find out the three most common causes and what you can do about each.

1. Wrong Thermostat Setting 

Before you begin to panic, consider that it may just be the wrong thermostat setting. The thermostat is the brain of the heating system. It tells your heater what to do and when.

It will tell your heater when to start heating and when to stop. So, if the heater isn’t stopping, it could point to a minor issue with the thermostat, such as the wrong thermostat setting.

The most common problem is when the furnace is set to ON rather than AUTO. The two settings aren’t the same. Although they keep the heater running and are designed to maintain utmost comfort indoors, one is an automatic setting, whereas the other is manual.

The AUTO setting functions automatically. It will automatically call your heater to action (as long as the furnace is powered on) and automatically tell it when to stop. No further effort is needed on your part.

However, the ON setting is manual. It keeps the furnace running until you turn it OFF manually. Until you do so, it will keep running. The only thing it does is alter heat output to avoid overheating.

Solution: Verify the setting on your thermostat. Is it ON or AUTO? If it’s on AUTO, you have a problem because it should go off when thermostat settings are reached. However, if it’s ON, you need to change it to auto so that it goes off automatically when the thermostat setting is reached.

2. The Thermostat is Broken

If the thermostat is on AUTO, the second possibility is that it’s damaged. As we’ve seen, the thermostat is the brain of the home heating system. Therefore, it may not relay the right signals if it’s damaged or may fail to relay any signals altogether.

For instance, a damaged thermostat may not sense when the setting has been reached. It may not be able to tell that indoor temperatures have risen past the thermostat setting.

So, it won’t be able to send the right signals to the thermostat. Or the calibration could be compromised. It’s just like a clock on your wall. If the battery is depleted, it may slag. So, it may end up pointing to 6.00 pm when it’s already 9.00 pm.

The actual problem could be something as minor as depleted batteries or a loose wiring connection. However, it may also be as complex as a damaged sensor. Sensors are delicate relay systems inside the thermostat that measure temperature readings through electrical signals.

A typical sensor comprises two wires that generate an electrical voltage or resistance to determine the temperature change. If the wires become loose, the whole system falls apart.

Solution: A malfunctioned thermostat will display other symptoms. For instance, the display may go off or become responsive. You likely have a thermostat problem if you see these signs and alongside the furnace’s non-stop running. Try changing the batteries. However, if that doesn’t work, call the experts.

3. Broken Blower Fan Motor

The furnace blower fan motor is a fan system that blows warm air into the ductwork and forces the air into your home. By doing so, it makes sure that hot air doesn’t stagnate inside the furnace. This can cause issues. Additionally, the blowing action ensures even distribution throughout the home.

However, the blower fan can break or become compromised. For instance, blower fans overheat all the time. The fan can also become clogged with dirt. Or, one of the parts, especially the blades, can break. When this happens, the fan can no longer function usually.

For instance, if the fan has broken blades, its capacity to blow air out of the furnace into your home is compromised. This can cause it to run longer, sometimes for many minutes after the thermostat setting is reached.

Another potential problem is the fan limit switch. The limit switch tells the fan when to run and when to stop. Essentially, this means that it indirectly determines when hot air should be blown into your home.

The fan-limit switch also indirectly determines when the furnace goes off, especially when there’s overheating. If the switch’s sensors become dirty or stop working correctly, it may fail to detect overheating. This means the furnace may continue running even after the thermostat setting is exceeded.

Solution: Regular maintenance helps keep the fan and its components in perfect working condition. So, make sure to check the fan regularly to make sure it’s not blocked or damaged in any way.

Additionally, schedule professional maintenance at least once a year. More importantly, always listen out for strange sounds from the furnace. If you hear something abnormal, such as screeching sounds, it could be a damaged fan.

4. Compromised Primary Controller

Finally, the furnace may also refuse to stop even when the thermostat setting has been reached if there’s a problem with the primary controller. The control board is the central hub for circulating power throughout your furnace. It takes electricity from the power outlet and supplies it to every component inside the furnace, including the motor, ignitor, blower, etc.

However, the control board can get damaged too. For instance, the PCB can crack or become bent. When this happens, some of the wiring connections can become loose or pulled off altogether. Since the PCB also has onboard circuits printed deep inside the board, a broken board would disconnect such circuits.

Another possibility is dirt, debris, and moisture buildup. Excessive accumulation of dirt on the circuit board can cause short-circuiting. Meanwhile, moisture may result in corrosion within the board, which may eat up the plastic covers on wires or the board itself. This can alter how the board works, potentially causing it to fail to respond to shutdown requests.

Solution: PCB issues are best left to HVAC experts. The professional will know where to look and what to change. However, there’s no harm in confirming that your circuit board is acting up.

Fortunately, most furnaces have a plastic or glass port with LED lights behind them to help you diagnose issues with the PCB board. Using the user manual as guidance, read the lights and figure out what could be the problem. Then call the HVAC professional.

Key Takeaway

Occasionally, the furnace may fail to shut off even when the thermostat indicates that ideal indoor temperatures have been reached. In many cases, this points to an issue with the thermostat, the blower motor, or the PCB board.

You can attempt to diagnose the first three independently, though you may eventually need a professional opinion. For PCB issues, it’s best to involve an HVAC professional from the start.