9 Reasons Why a Gas Furnace Short Cycles

Service technician testing a furnace.

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Short cycling is a malfunction that causes your furnace to rapidly turn on and off for short periods of time, normally between four and seven seconds per cycle. If you notice constant, short bursts of activity, you’re experiencing short cycling. The causes of short cycling vary from minor malfunctions to major problems that will require new parts or in rare cases even a new furnace. Often, short-cycling is the result of overheating in the furnace, which leads safety features in the furnace to automatically shut it down.

Identifying and addressing an overactive, short-cycling HVAC system will help you avoid increased energy costs and long-term damage to your home and furnace. Here’s are nine reasons why your furnace may exhibit short cycling.

Thermostat Problems

A broken or poorly placed thermostat can cause short cycling. A thermostat positioned near a heat source or in direct sunlight can warm too quickly, signaling your furnace to shut off before the entire home is adequately warmed. Your thermostat should not be placed directly over a heat register, as this can cause the furnace to cycle very rapidly as the thermostat quickly heats up. Nor should it be placed near a drafty window or door, as it will cool down too fast.

You should also test your thermostat to ensure it’s working correctly. Though the current is usually low voltage, testing a thermostat can involve handling live wires, so call a pro if you’re unsure about performing the tests yourself.

Many homeowners forget to switch their thermostats over to heat mode at the start of the heating season. This sometimes causes short cycling, depending on your temperature setting. Make sure your thermostat controls are set to the "heat" position before making any other changes.

Dirty Air Filter

A dirty air filter restricts airflow and can cause a pressure backup. The blocked warm air that remains in your furnace raises the internal temperature, causing it to shut off. The lack of warm air entering your home lowers the temperature and leads to extra furnace cycles. Replace your air filter monthly to avoid blockage-related short cycling.

Always consult your owner’s manual before changing your air filter, and make sure to install the right type and size. If you have any questions, don’t be afraid to call a furnace professional. Mistakes in installing your new filter can lead to continued short cycling.

Oversized Furnace

A furnace that is overpowered for the size of the home may heat the home quickly but unevenly. This results is rapid short cycling in order to maintain consistent temperatures throughout your home.

Poor insulation will also cause short cycling with an oversized furnace. Furnaces designed for larger homes produce ample heat but will lose most of their energy through cracks, holes, or faulty seals. The massive amount of energy produced and lost via sub-par insulation causes your furnace to short cycle to maintain a constant temperature.

What Is a Faulty Seal?

A faulty seal can refer to either a broken seal or one that was installed incorrectly in a furnace. A faulty seal can be quite dangerous if you don't address it right away. For instance, faulty seals may lead to carbon monoxide leaks, which can be fatal.

Blocked Heat Register

If the registers or dampers on the forced air ducts are closed or blocked, it can cause the furnace to overheat and shut down repeatedly as safety features in the furnace turn it off.

Duct registers can sometime be accidentally closed by playing children. Furniture blocking registers can also cause overheating. Also check the dampers along the ductwork to make sure they are in the proper position. If the air coming through the registers seems unusually hot, this can be a sign that air flow is being obstructed.

Blocked Exhaust Vent

One of the more serious causes of short-cycling is an exhaust vent that has been blocked with an obstruction, such as a bird's nest or snow and ice. When this happens, exhaust gases build up in the furnace and cause it to overheat and shut down repeatedly. This is especially dangerous because it can cause toxic carbon monoxide to back up into the home.

If the exhaust vent is in the side wall, examine it to make sure it is free of debris—fitting a screen over the vent can prevent birds and other animals from entering the vent. Similarly, roof vents can be fitted with screencaps to keep animals and snow from clogging them.

Malfunctioning Blower Motor

If the furnace blower is not circulating air properly, the furnace is likely to overheat, which will caust the limit switch to shut down the furnace to prevent damage. You will normally hear a furnace blower motor make uncharacteristic sounds if it is beginning to fail. Replacing a blower motor is almost always a job for a professional, who may well suggest that it's time to replace the old furnace entirely. Replacing the motor only can cost $500 or more, an investment you may want to avoid with an older furnace with a limited amount of remaining life.

Faulty Limit Switch

The fan limit switch is a device that prevents the heat exchanger from becoming hot enough to start a fire or getting so hot that it cracks. This is the component that interprets the signals from the thermostat and monitors the furnace temperature and turns the burners on and off. If this component is not working properly, it may shut the furnace off prematurely, leading to rapid cycling as the system repeatedly attempts to meet the thermostat's demands for heat.

Replacing a fan limit switch requires wiring connections, so most homeowners will want to all a furnace technician to handle this job. It is not, however, beyond the ability of skilled, experienced DIYer.

Malfunctioning Flame Sensor

The flame sensor is a safety device that has a probe that makes sure a flame is present while gas is flowing to the burners. If corrosion or damage causes the flame sensor to malfunction, the furnace may repeatedly cycle on and off because the flame sensor is providing inaccurate signals.

Cleaning or replacing a flame sensor is usually a job for a professional, as it requires shutting off electrical power and gas supply. However, a skilled and experienced DIYer may be able to handle this job.

Damaged Heat Exchanger

Although it is rare, the heat exchange chamber in your furnace may become cracked or otherwise damaged, usually due to repeated overheating because of problems with other components. When cracks develop, it allows superheated exhaust gases to enter other areas of the furnace, which overheats them and causes the furnace to shut down and restart, again and again.

A damaged heat exchanger is a very serious problem that can allow toxic carbon monoxide gas to back up into the home. It usually requires complete replacement of the furnace.

Why It's Important to Fix Short Cycling

It’s important to pinpoint the exact cause of your short cycling furnace before making any changes. In most cases, it’s best to hire a pro to examine your furnace. A professional HVAC technician will identify your problem and offer the appropriate solution. Fixing a short cycling furnace requires working with electrical and gas systems, which can result in serious injury to yourself, your furnace, and your home if handled incorrectly.

The advantages of eliminating short cycling fall into three categories:

  • Energy saving: Short cycling results in your furnace being in almost constant use. Non-stop running means reduced energy efficiency and increased utility bills. Additionally, if your home suffers from poor insulation, the hike in energy costs can be severe.
  • Safety: Short cycling can be the sign of a larger, more serious problem. Some persisting furnace issues can threaten the safety of your family and home. It’s important to immediately address any short cycling to avoid critical problems in the future.
  • Convenience: A short cycling furnace will result in unsteady temperatures in your home—temperatures that are too hot or too cold are uncomfortable. A malfunctioning furnace will also cause additional financial burdens with extra energy costs and repairs.