8 Causes of Furnace Short Cycling and How to Fix it

HVAC system problems like furnace short-cycling can be a nightmare, especially when they pop up during the icy throes of winter. It may not seem like a big deal, but short cycling could leave your home feeling too cold, while the furnace sounds like it is frequently running.

If you think your home’s furnace is short-cycling, several things could be causing it. To learn how to diagnose a short-cycling furnace and what you can do to fix it, continue reading!

A HVAC technician inspecting the cause of furnace short cycling

What Is Furnace Short Cycling?

Before we jump into the causes of short cycling, let’s explain what it is and why it is a problem. 

Furnace short-cycling is when your furnace constantly turns on and off in small bursts and doesn’t reach the thermostat’s setpoint.

When this happens, you’ll notice several unwanted side effects:

  • Your home will almost always be too cold
  • Your gas bill will skyrocket
  • Your furnace will experience undue wear and tear, leading to future mechanical failures

Why it Matters

While a short-cycling furnace may not seem like a big deal, it is far from ideal. It takes considerably longer to heat your home since the system kicks on and off repeatedly without actually reaching the set temperature. In many cases, it never heats your home to a comfortable temperature. 

Additionally, the frequent on-off cycles cost more money because the system uses more energy; Starting up and turning on uses over 50% more energy than running steady for a lengthy period. 

8 Reasons for a Short Cycling Furnace and How to Fix Them

Your home’s furnace could be short-cycling for several reasons. While some issues are quick and easy to fix, others are more serious and require considerable amounts of work. Some cases will even need help from an experienced HVAC technician. 

If your home’s furnace is short-cycling, here are potential reasons and how to fix them:

Thermostat Problems

an HVAC technician replacing the thermostat

Believe it or not, your thermostat may be the culprit of your home’s furnace issues. It is probably apparent to you, but if your thermostat is set at the wrong temperature, your home will not be at the correct temperature. 

Thermostat placement is also essential. If your thermostat is located incorrectly, your furnace may run less efficiently. 

Note: Ensure the thermostat isn’t located in a spot that experiences frequent temperature fluctuations, such as next to a drafty window, open doors, or close by a heat source (like an oven or fireplace).

Your thermostat measures the room’s temperature, so if it is exposed to heat fluctuations, it will think your home needs more or less heat based on temperature swings from open windows or hot fireplaces.  

Additionally, if your thermostat has technical issues, the furnace may run in short cycles. You may need to have an HVAC technician come in to check and test the thermostat.

If your thermostat is set at the wrong temperature, readjusting it is easy enough. On the other hand, if your thermostat is poorly placed, you can minimize drastic temperature fluctuation by keeping the window or door shut. 

Wrong Furnace Size and Installation

a furnace in the basement

An improperly-sized or incorrectly-installed furnace may also cause short-cycling. An oversized furnace will heat your home rapidly and unevenly. 

It will blast lots of heat into all of the rooms of your home. The sudden flux of warm air on the thermostat will cause it to think the house has reached the setpoint. Then, the thermostat will turn the furnace off before the temperature in your home has equalized. 

At this point, the furnace is now off, and your home will have hot spots near the exhaust vents. It will be much colder in areas further away from the vents (known as cold spots). 

Once these hot and cold spots mix, your thermostat will realize that the indoor temperature is now lower than the setpoint and kick the furnace on again.

This short-cycling process will then repeat indefinitely. 

Unfortunately, if you have this problem, the best solution is to replace the furnace

When the furnace is too big, it creates too much heat for the space, which causes the thermostat to cut off the system early, telling it to shut down. This repetitive process isn’t suitable for the furnace and will eventually cause the system to break down.

Inadequate Airflow

Poor airflow can result in short cycling. If your air vent registers are blocked, it will negatively affect your furnace. Things like furniture, boxes, pet beds, etc., covering the air vents cause airflow restrictions.

To prevent this problem, walk around your house and ensure all air vent registers are unobstructed and open. If there is furniture over top of the register, move it to allow adequate airflow. 

Poor Insulation and Lots of Leaks

Poor insulation throughout your home can also cause short cycling. If there are numerous gaps, holes, cracks, or faulty seals throughout your home, heat will seep outdoors. 

You’ll end up losing vast amounts of heat. In turn, your furnace will short cycle to keep the temperature consistent. You’ll have to seal off cracks, holes, and leaks to fix this issue. 

Dirty Flame Sensor

An HVAC technician checking the flame sensor

As your furnace burns fuel during its heating process, the system produces minimal amounts of water. As time passes, the water may cause metals in the system to corrode and rust, leading to issues. 

The corrosion of the flame sensor can trigger short cycling. When corroded and dirty, the flame sensor in the furnace may not detect the burner flame, and the furnace will prematurely shut down. 

If a dirty flame sensor is an issue, you’ll be able to solve the problem by simply cleaning it. 

On the other hand, fixing issues with corrosion will likely require a flame sensor replacement. In that case, it’s best to have an HVAC professional handle the flame sensor inspection due to the presence of electricity and flammable gasses.

Clogged Flue

a person inspecting the flue

Birds, debris, pests, and other unwanted objects may reside in your flue or vent pipe. When this happens, it clogs the flue. This is not only problematic, but it is also dangerous. 

Your furnace will respond by shutting down to prevent carbon monoxide poisoning to people in the home. 

This can create short cycling issues, which need to be addressed immediately. If you are comfortable doing so, check the flue for debris. Remove the flue cap and visually examine the flue for damage or debris. 

Clean out debris, nests, or other objects in the flue. Alternatively, you can call an HVAC technician to examine the flue for you. If there are issues with the flue, they can help you handle it. 

Clogged Air Filter

a person removing a clogged air filter

One of the most common causes of poor airflow is a dirty and clogged air filter. A clogged air filter causes inadequate airflow, thus leading to short cycling. Luckily, the solution to this problem is simple: replace the air filter. 

Generally, you need to replace the furnace filter every two months when the furnace is in operation. However, how often you should replace it depends on the type of fitler and the air quality of your home. 

Filters that are too restrictive to airflow may also result in short-cycling, so ensure you use the recommended filters for your furnace model. 

Overheating 

an HVAC professional disassembling an overheating furnace

If your furnace has faulty temperature-monitoring components, there is a good chance the short cycling issue is caused by overheating — this is one of the most common culprits of the problem. 

While most modern furnaces come with a built-in safety device that regulates the temperature, faulty components can cause issues. When the system overheats, the furnace will shut down, thus causing short repeating cycles. 

If you know your way around HVAC systems, you can examine the system. Otherwise, it is best to call in an HVAC professional to inspect. 

How Long Should a Heating Cycle Be? 

When the furnace runs in an ideal setting, it should only run 10 to 15 minutes in each cycle. Generally, your furnace should run between 3 and 5 cycles per hour to provide adequate heat for your home. However, several factors can impact the run time. 

For example, the outside temperature may cause your system to run for more extended periods than if the temperature were slightly higher. Although the insulation lining in your home should minimize heat loss, frigid temperatures will cause the temperature in your home to drop. 

This causes your furnace to work overtime to bring your home up to the set temperature on your thermostat.

Additionally, the temperature setting on your thermostat plays a role. When you set the temperature to high or close to the furnace’s operating limits, your furnace has to run longer to maintain the temperature in your home. 

Signs of a Short Cycling Furnace

If your home’s furnace is short cycling, it will exhibit several symptoms that indicate the problem at hand. You may notice that the furnace kicks on and off repeatedly in a short time frame yet doesn’t run long enough to reach the proper temperature. 

Your home may be cooler than usual, regardless of whether your furnace is frequently running. In that case, it’s time to look into the components that could be causing the furnace to short cycle. 

Stop Selling Your Furnace Short 

Short-cycling furnaces can be the result of several different things. While it may not seem like a big deal in some cases, some scenarios can be deadly, especially when the flue is blocked.

Contact an HVAC technician today if you can’t figure out what is causing the short cycles. It’s no fun having to deal with a cold home in the winter months, so it’s better to get it fixed sooner rather than later!

About The Author

Jonathon is a mechanical engineer with over ten years of experience in the HVAC industry. He has hands-on experience with all types of HVAC systems.

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