Check the ductwork. As air flows through your ductwork, seals and joints loosen. That can create leaks. Other leaks may also develop. Leaky ductwork makes it hard for your furnace to reach and stay at your set temperature, meaning it will run longer and more often.
Check the thermostat. If it's set to a high temperature, the furnace may run almost continuously. Try setting it low and see what happens. Can the furnace keep up? It'll run constantly if it's significantly undersized, the ducting is poor, or the house can't hold heat in because doors and windows are open.
It could (likely) be one of two things. Either the thermostat is sticking and not disengaging or you have a short in the wiring.
Seals and joints can loosen in your home's ductwork over time. That and other leaks mean your furnace will have to work harder, longer and more often to reach and stay at your set temperature. There are also some things you can try before you call your heating equipment service technician to come out and fix it.
Furnaces typically run for about 10 to 15 minutes in moderate weather. Extreme cold can result in longer cycle times.
If your AC unit is experiencing electrical problems, it can cause the system to run continuously, even after reaching the desired temperature. Faulty wiring or a malfunctioning thermostat system fan can be the culprits.
Your furnace could be running constantly because either your thermostat isn't set correctly or is malfunctioning and not properly signaling the furnace when to turn on and off.
Most furnaces will turn on and run for several minutes before reaching the desired temperature and shutting back down. This can happen anywhere from three to eight times an hour, depending on the condition of the furnace and the outdoor temperatures.
Check Your Furnace Filter – The most common cause of short cycling is a dirty or clogged air filter. If your filter is dirty, air cannot pass over the heat exchanger, causing the system to overheat and shut down prematurely.
The AC sensor on your thermostat is located near the evaporator coils. Indoor air moving toward the return vents passes by the sensor and coils. In turn, the sensor reads the temperature and checks if it matches what you've set on the thermostat.
If your heating doesn't turn off, the issue probably lies with the programmer/timer/thermostat, which may be different units or part of the same unit. If you have a complex modern programmer, check the settings thoroughly.
When the coils freeze, they can't efficiently transfer heat, causing your air conditioner to run continuously. This can be due to low refrigerant levels, poor circulation, dirty coils, power issues at the outdoor unit, setting the AC unit temperature too low at night, or power issues at the condenser.
A bad thermostat can be detected when your AC or furnace is not turning on, your HVAC systems are not turning off, the thermostat not responding, your HVAC system is short-cycling, the room temperature not matching the temperature displayed on the thermostat, and the programmed setting resetting themselves.
The short answer is your furnace won't turn off due to some kind of error or malfunction within your furnace, thermostat, or HVAC system. It could be the wrong thermostat setting is being used, a clogged air filter or your thermostat itself is no longer working properly.
As long as the furnace is running efficiently and not making strange noises, the constant operation is likely nothing to worry about. But in general, your furnace should not constantly be running. Otherwise, it could indicate a problem with your heating system.
Start by making sure that your thermostat is calibrated correctly. A miscalibrated thermostat can lead to inaccurate temperature readings. This will cause it to overshoot the desired temperature. Look at your thermostat's manual or contact the manufacturer to recalibrate it.
Check the ductwork.
As air flows through your ductwork, seals and joints loosen. That can create leaks. Other leaks may also develop. Leaky ductwork makes it hard for your furnace to reach and stay at your set temperature, meaning it will run longer and more often.
If your heater won't turn off, the first thing you do is check the thermostat. The thermostat controls how the heating system runs. It detects the indoor temperature and signals the furnace to start or stop. So, if the thermostat malfunctions, it can signal the unit to run continuously.
Short cycling is a common sign of a bad flame sensor. The sensor thinks that there isn't a flame, so it shuts off the gas as a safety mechanism. Except, there is a flame and without the gas to supply it, it dies. This is a frustrating problem, but an HVAC technician can fix it.
With all that being said, if there are no extreme factors involved, the average run time for a furnace is 10-15 minutes per cycle. Your furnace may need to run two to three times an hour to maintain your desired temperature if your home has average insulation.
Incorrect settings on your thermostat or fan limit switch can cause the fan to run continuously. Faulty wiring or a broken fan limit switch can also cause your fan to keep running. You can diagnose and fix some furnace fan issues on your own. Faulty wiring or a broken switch will require professional repair.
Running your furnace constantly can put extra strain on the system, potentially shortening its lifespan. It is important to have your furnace regularly maintained to ensure it is in good working condition and not being damaged by constant use.
If this is the case, then you'll need to find the fan limit switch and move it back into automatic mode. This will ensure that your furnace fan runs only when necessary and not continuously. The fan limit switch is typically located near the blower motor or in the control box.
The cost to replace a furnace control board is $500 on average, but it can cost between $300 and $700 or more depending on the complexity of the control board and the brand. Furnace control board replacement costs anywhere from $50 to $900 on average, but most people pay $500.