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.
Possible causes include a broken thermostat; the thermostat being set too low; an out pilot light; a blown fuse or circuit breaker; a closed gas valve; or electronic ignition problems.
If your gas furnace won't turn off, it could be due to a faulty thermostat or a problem with the furnace's limit switch. Start by checking the thermostat settings and making sure it's not set too high. If that doesn't work, try turning off the furnace at the breaker and then back on again.
Your heat (generation) does turn off when the room the thermostat is in reaches the desired temperature. However, the fan will run for some time after that to pull the residual heat from the heating element or manifold (depending on your type) to one not waste it, and two to prevent damage to the system.
When a furnace is short cycling, it's working hard to produce heat but isn't making any, causing strain on your furnace. If left unchecked, this could cause major issues for your furnace resulting in hefty repairs.
If your furnace keeps turning off before reaching the intended temperature, the culprits are likely rust and corrosion. Rust is a result of invasive water damage like corrosion, humidity, or leaks in your furnace system.
Thermostat Issues
Older thermostats often have issues where they no longer correctly sense the temperature or they're unable to correctly do so. This can lead to the thermostat signaling the furnace to shut off before it has reached the correct temperature.
The simple solution: Turn your furnace off and check the filter. If it's dirty or it's been 3 months or more since you changed it, put a new, high-quality filter in place.
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.
It could be the wrong thermostat setting is being used, a clogged air filter or your thermostat itself is no longer working properly. When you have a heating system not shutting off, this issue makes it difficult to maintain the desired temperature inside the home.
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.
Radiators might not be getting hot due to trapped air requiring bleeding, a malfunctioning boiler or central heating pump, thermostatic radiator valves that might be stuck or set too low, or the central heating system having sludge or rust which causes blockages.
When temperatures drop below zero or even below freezing for extended periods of time, sometimes your furnace can't keep up. If your home is not heating up to the temperature you've set, try lowering it a few degrees. The temperature of the air coming out of your vents doesn't get warmer by turning up your thermostat.
Some of the most common thermostat issues include:
The thermostat is broken, not properly level, or dusty. The thermostat needs to be recalibrated. There is loose wiring causing a thermostat issue.
Oversized furnace
This is the most common cause of furnace short cycling. If the furnace has been short-cycling since it was first installed, it may have been improperly sized for your home.
With an effective air delivery system and a well-insulated home, this should be long enough to establish your preferred temperature throughout the building. After resting for approximately five to 10 minutes, your furnace will likely cycle on again.
One of the most common causes of a furnace shutting off unexpectedly is low airflow. If your furnace keeps turning off, check for issues such as: Dirty Air Filters. A clogged filter restricts airflow, causing the furnace to overheat.
A dirty filter blocks airflow, which can stop your furnace from heating up all the way. Be certain that you are changing your air filter when needed, as well as check that there are no large objects stacked around the furnace that are impeding airflow. Keep clean ducts and a clear path.
Check the Thermostat
Your thermostat controls your furnace and basically tells the unit when to turn on and off. It's often to blame when your furnace won't stop running. Sometimes having the heat turn off is as easy as adjusting an incorrect thermostat setting. Make Sure Your Thermostat is Set to “Auto”.
Slowly turn the thermostat from off to heat and increase the temperature. Both the thermostat and furnace should make a sound. The problem lies in the appliance that does not make a sound.
If your furnace turns on but no heat is produced, there may be a problem with the fuel supply or the ignition system. If something is stopping your furnace from receiving gas, there is no fuel to combust so the furnace will not create any heat. The furnace turns on but no heat comes out.
A furnace cycling on and off repeatedly can be caused by various reasons, from dirty filters and obstructions in ducts and registers to incorrect thermostat settings and faulty limit switches. By following the steps above, you can help prevent your furnace from cycling too frequently and save energy costs.