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Causes of Furnace Short CyclingAn average furnace will operate two to three times in an hour to maintain a warm temperature in your house. A standard heating cycle ranges from 10 to 15 minutes depending on a couple of variables. However, the total amount of time your furnace operates is dependent on factors such as its AFUE rating and the speed of the blower motor. For example, the average time of a heating cycle will vary in furnaces with variable-speed blower motors and single-stage fans. Regardless of the type of system, it is not normal for a furnace to activate and deactivate every couple of minutes. To help you diagnose the problem, we have compiled a list of the 5 most common causes of furnace short cycling and how to prevent it.

Causes of Short Heating Cycles


A Furnace is engineered to shut off when it becomes too warm to prevent damage to internal parts such as the heat exchanger and gas burners. If your furnace is short cycling, the source of the problem may be restricted airflow. Ordinary causes of restricted airflow include blocked furnace exhaust pipes and clogged air filters. An easy way to stop your furnace from overheating is to inspect your air filter once a month to confirm if it’s filled with dust, dirt, or hair.

If the furnace in your household is filled with dirt and contaminants, you will start to have problems with your blower motor. Once the collection of dirt becomes severe, the blower motor will struggle to ventilate and will begin to overheat. Eventually, the blower motor will become inoperable and you will be required to replace it. If your furnace is turning on and off after a few minutes, this is a sign of blocked vents. Check the supply and return vents in each room of your house to confirm they are open. To prevent your furnace from overheating, it’s important to remove objects such as tables, chairs, curtains, dressers, or rugs near the supply vents.

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Furnace Heating CyclesFurnace Is Too Large

While a large number of homeowners believe bigger furnaces are better and more efficient, this is a myth that should be ignored. If your furnace is oversized, it will provide warm air to your household at a rapid rate and perform short heating cycles of 5 minutes or less. A standard heating cycle varies from 10 to 15 minutes depending on your insulation and weather.

Once the temperature of the interior air becomes too low, the thermostat will distribute a signal to activate the furnace. Since the furnace is an incorrect size, it will provide too much heat in a given period of time to each room in your house. This will cause the furnace to deactivate quickly. Unfortunately, the rapid heating cycles will negatively impact the even distribution of heat in each area of your space. You may start to notice cold spots in rooms or spaces in combination with high utility bills. If your furnace is turning on and off, you will need to call a licensed technician to confirm if your furnace is oversized.

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Dirty Flame Sensor

If the flame sensor is contaminated, it will not be able to detect the flames or natural gas in your furnace. The sensor is engineered to prevent a gas leak by closing the gas valve if it does not sense flames. As your furnace reaches the middle of its life cycle, it is not uncommon for a thin layer of dirty and dust to form on the exterior of the flame rod.

Once the sensor becomes contaminated with dirt, it will be unable to detect the presence of flames and will shut the furnace off. The easiest way to fix a furnace that is short cycling is to remove the flame sensor to clean it. Use a sponge or brush to remove the dirt from the sensor. After the part is clean, follow the manufacturer’s instructions to insert the flame sensor into the furnace. If you are still experiencing issues, you will be required to replace the furnace flame sensor to restore the flow of warm air in your household.

Malfunctioning Thermostat

The thermostat should be installed in a central location of your space. Do not install a thermostat near a window, door, or appliance. If your thermostat is installed near a heat generating appliance such as an oven, it will sense an incorrect temperature and cause the furnace to shut off prior to the end of the heating cycle. On the other hand, a thermostat that is located near a window will detect heat from the sun.

This will cause the thermostat to determine it’s too warm in your household and deactivate the furnace. If your furnace is igniting and shutting off at a rapid rate, inspect the location of your thermostat to determine if it’s sensing heat from an appliance or sunlight. To fix this problem, move the thermostat to a location that is not near windows, supply vents, appliances, or computers.

Short Heating CyclesObjects in Exhaust Vent

The exhaust vent is engineered to remove poisonous carbon monoxide created by the furnace throughout the combustion cycle. However, the distribution of gas to the outdoor air can be disrupted by blockages in the vent. If the exhaust vent becomes clogged by a birds nest, rats, bugs, or ice, the carbon monoxide will become trapped and the furnace will overheat. Failure to address this issue will result in a gas leak in your household. If your furnace is turning on and off every couple of minutes, this is an indicator of a blockage in your exhaust vent.

Heating & Air Repair Servicessy

Our team of trained technicians have the tools and skills necessary to eliminate clogs in exhaust vents and replace damaged flame sensors. Is your furnace constantly igniting and shutting down? We offer furnace repair and installation services to homes in Pennsylvania. Our staff of certified contractors also offers maintenance and replacement services for furnaces, heat pumps, boilers, and air conditioners. Give our team a call by phone at (302) 367-7915 to receive help with a furnace problem. We are committed to keeping your household safe throughout the winter season.

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