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Furnace Not Shutting Off? 7 Common Causes (And What to Do)

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A lot of us think furnaces run continuously when they are switched on at the beginning of winter. However, that isn’t true: they actually run in cycles and will go on and off every 15 minutes or so. That means an overactive heater that stays on and won’t shut off has something wrong – and whenever there’s a problem with your furnace, it’s essential you fix it as quickly as possible.

If your furnace is not shutting off, you most likely have one of these 7 problems:

  • Wrong thermostat setting
  • A malfunctioning thermostat
  • A damaged primary controller
  • A dirty air filter
  • A faulty blower fan motor
  • Leaky ductwork
  • Damaged compressor contractor

A few of these problems can be easily fixed by you, but most of them require the attention of a professional HVAC expert. We’ll tell you which of the problems you can attempt troubleshooting and when you necessarily need to call a professional as we explore the issues that could cause your central heating not to turn off.

Wrong Thermostat Setting 

The correct setting for a thermostat when the heater is on is AUTO. This makes the furnace’s blower run only when the heat is on and shut off when the desired temperature is achieved and the heat is off.

If your thermostat is set to ON, the blower fan will keep going non-stop, which also means higher bills and faster wear and tear.

What to Do with a Wrong Thermostat Setting

Simply move your thermostat switch to the AUTO option if it is on the ON option. This is one of the issues that you can easily fix when your furnace keeps running non-stop. 

If changing the settings on your thermostat does not resolve the issue, proceed to check for other possible causes or save yourself the trouble and call an expert HVAC.

A Malfunctioning Thermostat

An old thermostat or one with wiring faults can fail to send the right signals to your furnace. If the faulty signal is that your heater should stay on, then it will stay running without shutting off between cycles.

What to Do with a Malfunctioning Thermostat

While it might be easy to replace a thermostat, determining if the one you have needs to be replaced or has some wiring issues that need repair requires the expertise of an HVAC pro. 

We recommend you fill the form at the end of the article and let a trusted HVAC expert near you handle the assessment and repairs of your thermostat.

Damaged Primary Controller

A furnace primary controller is an electronic relay that provides the control of burners in response to the room temperature. 

It does this by responding to signals from the thermostat and the limit control to regulate the furnace’s startup, run cycles, and shut down the furnace. Specifically, a primary controller will:

  • React to the presence or absence of a flame.
  • Manage furnace startup by confirming the presence of a flame before activating the igniter, gas valves, and burners.
  • Control the furnace’s shut down once the thermostat indicates that the expected temperature has been achieved or when a limit control opens.

In this light, if your furnace’s controller is damaged, it will not be able to achieve the above functions, and your furnace will continue burning even when it should shut off.

What to Do with Damaged Primary Controller 

Today’s heaters have microprocessor primary controllers rather than simple stack switches. As such, checking and correcting damaged primary controllers should be done by a trained technician with knowledge of their controllers’ advanced features.

A Dirty Air Filter

A dirty air filter is culpable of almost every furnace issue. When it comes to a heater that won’t turn off, a dirty air filter can make your furnace run non-stop due to restricted airflow. 

Here’s how: 

When your heater is turned on, cold air is pulled from your home into the heating chamber through the air filter. 

The filter has the task of cleaning the air of debris before it gets into the heating system. If the filter is dirty, little air is passed to the heating chamber and, consequently, little warm air is produced and sent to your home.

What this means is that the heater will have to work extra to supply the amount of warm air that is required to heat your home, which explains why your heater will stay on even when it’s time to end a heating cycle.

What to Do with Dirty Air Filters

Depending on the type of air filter in your furnace and the time that the filter has served, you can clean or change dirty filters:

  • Replace dirty disposable fiberglass, pleated, electrostatic air filters.
  • Wash/clean permanent electrostatic and high-efficiency pleated air filters.

Remember that the rule of thumb is to clean/change air filters every 3 months or earlier if you suspect that they are dirty and causing issues to your HVAC system. 

If cleaning or replacing your HVAC air filters does not resolve the issue of a furnace that won’t shut off, contact an HVAC expert to assess your furnace for other possible issues.

A Faulty Blower Fan Motor

A furnace blower is crucial in drawing cold air from the home to the heater and sending the heated air to your home.

But the blower should run in sync with the heating cycles of the furnace (run when on and shut off when the furnace is off).

So, if your blower won’t stop running even when the heater is off, then there is a problem. Here are a few possible causes:

  • A wrong thermostat setting (ON instead of AUTO).
  • A faulty thermostat wire.
  • A fan limit switch that’s set on “manual override”.
  • A faulty fan limit switch wire.

What to Do with a Faulty Blower Fan Motor

While a wrong setting on the thermostat or the fan limit switch can be easily corrected by you, electrical faults should be attended to by a professional technician.

Call an HVAC expert to troubleshoot, repair, or replace a faulty furnace blower as the case may require.

Leaky Ductwork

Having leaky furnace ductwork is both a financial loss and a health hazard. According to the University of Florida Institute of Food and Agricultural Sciences (UF/IFAS), the typical ductwork loses 40% of the heating energy.

If your ductwork leaks should cause your system to lose an additional 20%, your heating system will work 50% harder. And here’s why:

Ducts are usually located in non-conditioned spaces (attics, garages, and basements). That means if your ductwork is leaking, it will lose large amounts of heat to the colder air in those spaces.

In consequence, your heater will need to keep working to sustain the hot air supply needs for those places and for your home, which is why it will not shut off.

What to Do with Leaky Ducts

Seal leaky ducts as quickly as they are discovered to restore hot air supply to your home and stop your utility bill from increasing unnecessarily.

Sealing leaky ducts is a job that should be done by a professional so as not to risk leaving any leaky spots. 

Damaged Compressor Contractor 

An HVAC compressor contractor is a small device that controls the flow of power to the parts of the system. 

Depending on your heater’s power needs at a given moment, the HVAC compressor contractor will provide or shut off the power supply. 

As such, if the compressor contractor should malfunction, it may continue to supply power when it is actually expected to cut the power supply, and your heater will keep running instead of shutting off.

What to Do with a Damaged Compressor Contractor

A faulty compressor contractor has to be professionally replaced. You should, therefore, call an expert HVAC technician to do the job and not try to troubleshoot.