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Why does my Furnace Turn Off and On?

Have you noticed that your furnace turns off and on too much? It can be a symptom of a larger problem. Normally, an electric sensor measures the temperature and automatically adjusts the heat. When it senses the air is too cold, it turns the heating unit on. When it senses the correct temperature, the heating unit turns off. This makes your life more convenient because you only need to set the temperature. 

When a heating unit turns on and off too frequently, a serious problem may be festering. HVAC professionals call this short cycling. It's a symptom of problems inside the heating system, and it can damage your furnace and raise your electric bills.

Diagnosing the Furnace Problem

It helps to make sure you're correctly identifying the problem. Short cycling can mean different things. In some instances, the furnace may turn off and on two or three times before engaging fully. In other cases, people complain of the heating unit simply shutting off after five minutes and starting again ten minutes later. 

To be more clear, professional HVAC technicians look at the number of cycles in an hour. The average heating system switches off and on three to six times per hour. If it switches on more often, that's a sign something is wrong. Technicians may also count the number of minutes the heat stays on or off. If too little time goes by, then your heating unit is short cycling. 

Possible Causes of Short Cycling

Multiple points of failure can cause short cycling. Professionals use a process of eliminating the most likely causes first. Some of the more common reasons include:

  • An Old Filter
  • Malfunctioning Thermostat
  • Flame Sensors
  • Improper Ducting
  • Blocked Ventilation

Furnace Filters

The filter is one of the first places an HVAC technician will check. Many home owners forget to change the filter on a regular basis. When a filter becomes full of dirt and debris, the airflow is restricted. This can damage or overheat the furnace causing it to shut off too soon. If the filter is fine, then the next place to look is the thermostat.


A thermostat controls when the furnace turns on and off. Typically, your thermostat is the device on your wall where you adjust the temperature. If the thermostat is at fault, then the good news is that nothing is wrong with the heating unit itself. The solution is usually to replace the malfunctioning thermostat with a new one. 

Replacing the thermostat involves working with electrical wires, which can be dangerous for a homeowner to attempt. Leave it to the professionals, and do not attempt to replace your own thermostat.

Another situation that can cause short cycling is if the thermostat is too close to a heat vent, direct sunlight, heat registers or another source of high temperature. The thermostat works by sensing the air temperature directly around it. If warm air gets too close to the thermostat, it shuts off when it senses the air is at the right temperature. Because another source of heat is too close, there is not sufficient time to allow the warm air to distribute evenly throughout the room. The solution is to remove the heat source or change the location of the thermostat.

Flame Sensors

The next most common cause is at the flame sensor. Over time, oxygen corrodes it. Sensors may also include ceramic parts that crack. If you've owned the unit for a while and not experienced problems previously, then the flame sensor is the likely culprit. The flame sensor is a safety device that shuts down the unit to prevent non-ignited gas from building up inside the home. When it malfunctions, the result can be a series of short cycles in a row before stopping completely. Technicians may clean the sensor or replace it entirely, based on the age and condition.

Other Possibilities for Short Cycling

Other possible causes for short cycling happen less often. Over-sized heating units can produce too much heat or airflow, shutting them down too soon. Objects lodged inside vents or chimneys choke the airflow forcing the heater to work harder. Once the most common causes have been ruled out, then an HVAC technician will troubleshoot the remaining sources.

How to Fix Short Cycling

If you believe your heating unit is short cycling, do not attempt to repair it yourself. A competent technician will troubleshoot and safely identify the source of the problem. HVAC professionals are trained to recognize the dangers of working with fire, gas and heat. In many cases, short cycling can be repaired in one service call.Short cycling can damage your heating unit if left unchecked. Therefore, repairing the problem now costs much less than a major overhaul later.

Safety Issues with Short Cycling

When your furnace turns off and on too much, be aware of the potential complications. If you notice the burner flame is yellow instead of blue, then too much gas may be present. Likewise, strange smells inside your home can be a sign of gas leaks. Additionally, carbon monoxide is lethal in high concentrations. If you notice any of these, then turn the unit off and call a licensed professional at once.

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