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3 reasons your furnace cycles on and off frequently

Reasons why your furnace cycles on and off

Have you ever thought "Why does my furnace keep shutting off?" You are not alone. This is a very common problem that if not diagnosed and corrected immediately could result in higher utility bills and major furnace repairs. Or even a furnace replacement if it’s left for too long.

When your furnace is cycling on and off, it's called furnace short cycling. Below we go over some of the more common causes why this might be happening.

1 - Low Airflow

If your furnace keeps shutting off, it could be due to low airflow. There are several indirect issues that cause low airflow.

  • Dirty Air Filters. If you don't change your air filters often enough, the filters will become dirty and clogged, which means the heat exchanger retains heat and eventually causes it to overheat. It's imperative you change your air filters about every 90 days unless you have pets or allergies. If you have pets or allergies, you should be changing air filters every 60 days, or less if your allergies are severe.
  • Dirty Blower Wheel. If your furnace goes on and off too quickly, it could be a dirty blower wheel. The blower wheel has blades that push the air and if it has dirt on it, it won't work as efficiently and might ultimately not work at all. You can prevent dirt from building up on your blower wheel by changing your air filters regularly. If you don't, this could end up being expensive to repair.
  • Closed or Blocked Air Supply Vents. If your furnace keeps shutting off, check your air vents to see if any of them are closed or blocked in any way. You should keep all your vents open, even in the rooms you don't use. If your heat exchanger doesn't get enough air, it can't transfer enough heat. Then that heat builds up and could end up causing your furnace to short cycle.

2 - Faulty Thermostat

Defective or broken thermostats can cause your furnace issues

Your thermostat is what controls your entire heating system. So, if your furnace keeps turning off and on too quickly, your thermostat might be malfunctioning or broken. There are several things that could be causing your thermostat to not work properly. The wiring could be old and need replacing, it might need new batteries, or it could be the location of your thermostat.

If your thermostat is placed close to a heat source such as direct sunlight or a heat register, it will skew the temperature reading causing the thermostat to turn on and off irregularly. Unless your thermostat just needs batteries, you will need to contact a professional to either replace any old wiring or relocate your thermostat to a more appropriate location.

3 - Flame Sensor

If your furnace shuts off too often, you could have a corroded or dirty flame sensor. A flame sensor lets your system know there's a flame when the gas valve is open. If there isn't a flame, the sensor will shut off the gas valve to keep the gas from getting into your house. If the flame sensor is dirty or has become corroded, the flame won't register and will turn off the gas valve, which, in turn, causes your system to shut down. You will need to contact a professional to check this, especially if your furnace is cycling on and off too quickly.

Other Possible Causes of a Short Cycling Furnace

If your furnace is short cycling, it could also be any one of the following issues:

Blower Motor
The blower motor is what circulates the air throughout your house. It's possible your furnace will run when the blower motor is off; however, that's rare. But on the rare occurrence it does, there wouldn't be any air blowing over the heat exchangers which would cause your furnace to go off. The best way to check this is to hold your hand up near the vents to see if there is any air coming out. If there is no airflow or the airflow is weak, the blower motor is likely the problem.

Flue Pipe
The flue pipe, also known as the exhaust vent, is located on your roof and can also cause your furnace to short cycle. This happens when dirt, leaves, sticks, or other debris like a bird or animal nest ends up inside the pipe resulting in a blockage. This blockage will cause hot gases to build up in your furnace causing it to overheat. One of your furnace's safety features includes a mechanism that will trigger your furnace to shut down if it gets overheated.

Over-sized Furnace
If your furnace is too big for your home, it could cause it to short cycle. Unfortunately, in this instance, you will have no other choice but to replace your furnace if you want to correct the issue. What happens when your furnace is too big for your home is it will heat your house too quickly then abruptly turn off. Then once your home starts to get cold again the cycle is repeated.

If you just had your furnace installed and it's already short cycling, the technician probably didn't perform a Manual J Heat Load to determine the proper size furnace for your home.

A furnace cycling on and off too quickly isn't something that should be ignored. As soon as you realize there's a problem, contact a professional HVAC contractor to inspect your system. If you don't, you could be looking at major repairs or quite possibly the replacement of your entire system. Another great way to avoid expensive repairs or a replacement is by performing regular preventive maintenance and scheduling a tune up at least once a year.

If you would like more information about a short cycling furnace, or to talk with someone about scheduling a service call, please contact Petro Home Services today. You can rely on our team of experts to do everything to keep your heating system up and running efficiently and effectively all year round.

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