How to Fix Short Cycling Furnace

How to Fix Short Cycling Furnace

If you have a furnace that’s constantly turning on and off, you may be dealing with a short cycling problem. This can be frustrating, as it not only leads to temperature fluctuations but also puts extra strain on your heating system, which can cause it to break down prematurely.

In this section, we will explore the common causes behind a short cycling furnace and provide step-by-step instructions on how to fix the issue. By following these troubleshooting tips, you can effectively resolve the problem and ensure optimal performance of your furnace.

Key Takeaways:

  • Short cycling can cause temperature fluctuations and put extra strain on your furnace.
  • The common causes of short cycling include problems with the thermostat, air filter, heat exchanger, flame sensor, blower motor/belt, air ducts/vents, and furnace size.
  • Checking and adjusting the thermostat settings, inspecting and cleaning/replacing the air filter, examining the heat exchanger for damage, cleaning or replacing the flame sensor, inspecting and adjusting the blower motor and belt, checking for obstructions in the air ducts and vents, and ensuring the furnace is the right size for your home are all steps you can take to fix short cycling.

Understanding Short Cycling

If you’ve noticed your furnace turning on and off frequently, you may be experiencing what is known as short cycling. This is when your furnace goes through frequent on-off cycles that prevent it from running efficiently. Short cycling can be caused by a number of factors and can lead to higher energy bills, premature wear and tear on your furnace, and an uncomfortable indoor environment.

Short cycling is a common furnace cycling problem that can be caused by various issues. It’s important to address this issue promptly to prevent damage to your furnace and to save money on energy bills. Let’s take a closer look at frequent on-off cycles and how to resolve this furnace cycling problem.

When your furnace is short cycling, it is not running efficiently, and this can lead to higher energy bills. Additionally, short cycling can cause wear and tear on your furnace, leading to costly repairs or even early replacement. If you notice that your furnace is short cycling, it’s important to troubleshoot the issue and fix it as soon as possible.

Check the Thermostat Settings

The first step in addressing a short cycling furnace is to check the thermostat settings.

Make sure the thermostat is set to the desired temperature and mode (heat). Check the temperature differential setting, which controls how much the temperature can vary before the furnace turns on or off. If the differential is set too low, the furnace may cycle on and off too frequently.

Refer to your furnace’s manual for specific instructions on adjusting the temperature differential. Typically, it can be adjusted using a small screwdriver on the thermostat or in the furnace control panel.

If you have a programmable thermostat, ensure that the set program is in line with your needs. For example, if you are away from the house for extended periods, you can program the thermostat to set the temperature lower to save energy and prevent short cycling.

Inspect the Air Filter

If your furnace is cycling on and off continuously, a dirty or clogged air filter could be the cause. Over time, air filters accumulate dust and debris that restricts airflow. This can cause the furnace to struggle to reach the desired temperature, leading to short cycling.

To fix this issue, you need to inspect and replace the air filter. Here’s how you can do it:

Step Action
1 Locate the air filter
2 Turn off the furnace
3 Remove the air filter
4 Inspect the filter
5 If the filter is dirty or clogged, replace it
6 Insert the new filter
7 Turn on the furnace and test it

It’s important to check and replace your air filter regularly, at least every three months. If you have pets or live in a dusty environment, you may need to replace it more frequently.

A clean air filter not only fixes the short cycling problem but also enhances the furnace’s efficiency, reduces energy bills, and improves the indoor air quality.

Examine the Heat Exchanger

If you’ve gone through the previous steps and the short cycling issue persists, the problem may be a cracked heat exchanger. The heat exchanger is responsible for transferring heat from the furnace to the air that circulates around your home. A crack in the heat exchanger can allow dangerous gases, such as carbon monoxide, to leak into your home’s air supply.

To check for a cracked heat exchanger, start by turning off the furnace and allowing it to cool completely. Once it’s cool, remove the front panel of the furnace and look for any visible cracks or holes. You can also use a flashlight to inspect the heat exchanger more thoroughly. Look for any signs of rust, corrosion, or damage.

If you do find a crack in the heat exchanger, it’s essential to address the issue immediately. A damaged heat exchanger can pose a serious safety risk to you and your family. It’s best to leave this repair to a licensed HVAC technician who has the expertise and equipment needed to safely replace the heat exchanger.

On the other hand, if you don’t see any visible signs of damage, you can perform a simple test to further check the heat exchanger. Turn on the furnace and watch the flames closely. If you see any flickering or wavering, this could indicate a problem with the heat exchanger. In this case, it’s best to call in a professional to take a closer look.

Check the Flame Sensor

Your furnace’s flame sensor is responsible for detecting the presence of a flame. A faulty or dirty flame sensor can cause the furnace to cycle on and off too frequently, leading to short cycling. Here’s how to check and address any issues with the flame sensor:

  1. Locate the flame sensor, which is usually located near the burner assembly.
  2. Turn off the furnace and allow it to cool down completely.
  3. Remove the flame sensor from the furnace using a wrench or screwdriver.
  4. Inspect the sensor for any signs of damage or corrosion. If the sensor looks damaged, it may need to be replaced.
  5. If the sensor appears dirty, use a soft cloth or fine-grit sandpaper to gently clean it.
  6. Reinsert the flame sensor and tighten any screws or bolts.
  7. Turn the furnace back on and observe its performance. If the furnace continues to short cycle, you may need to call a professional for further assistance.

Ensuring a clean and properly functioning flame sensor can eliminate short cycling and enhance the overall efficiency of your heating system.

Inspect the Blower Motor and Belt

If your furnace has a faulty blower motor or a loose blower belt, it can result in short cycling. However, inspecting and fixing these components can be easily accomplished with just a few steps.

First, make sure that the power to your furnace is turned off before starting the inspection process. Locate the blower motor, which should be connected to a fan wheel or blades. Check for any visible damage or wear and tear. If the blower motor is faulty, you may need to replace it.

Next, inspect the blower belt. A loose belt can cause your furnace to short cycle. Look for any signs of wear or damage, and check that the belt is properly tightened. If the belt is damaged or too loose, you may need to replace it with a new one.

Once you have inspected both the blower motor and belt, turn the power back on, and test your furnace to see if it still short cycles. If it does, it’s best to contact a professional technician to take a closer look at your furnace’s blower system.

Check the Air Ducts and Vents

Blocked air ducts or closed vents can cause your furnace to short cycle. When the warm air gets trapped in your system, it causes overheating, which triggers the safety switch to shut off the furnace. To address this issue, follow these steps:

  1. Inspect all the air ducts in your home to ensure that they are unobstructed. Check for any signs of blockage such as dust, debris, or other objects that may have fallen into your ductwork.
  2. Look for any closed or partially closed vents in your home. Ensure that all vents are open and properly functioning to allow the warm air to circulate freely.
  3. If you find any blockages or closed vents, remove them and clean the affected areas. Clean the ducts using a vacuum cleaner with a long hose attachment to reach deep into the ductwork.

By having unobstructed airflow, you can prevent your furnace from short cycling and improve its efficiency. If you’re unsure about any of the steps above, contact a licensed HVAC professional to assist you with the ductwork cleaning and maintenance.

Verify the Furnace Size and Sizing Issues

One of the possible culprits behind a short cycling furnace is having the wrong size furnace for your home. If your furnace is too big or too small, it can disrupt the airflow and cause your furnace to cycle too frequently.

So, how do you know if your furnace is the right size for your home? One way is to check the furnace’s capacity rating, which is usually listed on a metal plate attached to the furnace. The rating should match the square footage of your home and the height of your ceilings.

If you suspect that your furnace is too big or too small, it’s best to consult with a professional HVAC technician to perform a sizing calculation. They can take into account several factors like your home’s insulation, windows, and age to determine the appropriate furnace size for your home.

Correcting sizing issues may involve replacing your existing furnace with a better-suited model. While this may involve some upfront costs, it can save you money in the long run by keeping your energy bills low and prolonging the life of your furnace.

It is essential to ensure that your furnace is the proper size for your home to avoid short cycling problems.


Congratulations, you have successfully troubleshooted and resolved your short cycling furnace issue! By following the steps we outlined in this comprehensive guide, you can now enjoy optimal performance and efficiency from your heating system. Remember to check your thermostat settings, inspect and clean or replace your air filter, examine the heat exchanger, clean or replace the flame sensor, inspect the blower motor and belt, check your air ducts and vents, and verify the size of your furnace to ensure correct sizing.

If you encounter any other furnace problems, don’t hesitate to refer back to our troubleshooting guide or seek professional help. By staying proactive and knowledgeable about your heating system, you can save time, money, and keep your home warm during the cold winter months. We hope you found this guide helpful and informative!

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