What is Furnace Short Cycling – Causes And How To Fix

Posted by: Mas Broto
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Furnace Short Cycling Causes And How To Fix It

If your furnace turns on and off repeatedly for a short time before reaching its proper temperature, it is short-cycling. Furnace short cycling is a common issue that must be dealt with immediately.

Below we go over some of the furnace short cycling causes and how to fix the problems.

What is Short Cycling?

Short cycling happens when your furnace does short bursts of activity and intermittently turns on and off every few seconds. A normal furnace operation generally turns on and off between 3-8 times per hour.

Short cycling can also be used to describe a furnace that turns on and off too often.

Importance of Fixing Furnace Short Cycling

Furnace startup is the most strenuous sequence of a furnace’s operation. When your furnace cycles on and off repeatedly, it is harder for the system to endure leading to parts and eventually a system breakdown.

If left unchecked, it can cause serious and long-term damage to your overall heating system. This causes even more trouble and money for repairs.

Hinder Increase Energy Use

Furnaces use more energy when starting up than when it is already running. So when it keeps turning on nonstop, a whole lot of energy is wasted for unnecessary startups. This leads to lower energy efficiency and higher energy consumption, hence, a hike in electricity bills.

Maintain Safety At Home

A furnace that short cycles may be a mere effect of a bigger and more serious problem. Some of which, if ignored, can pose a great threat to your home and family’s safety.

Maintain Comfort

Suppose your furnace keeps on turning on and off. In that case, it will result in indoor home temperature being too hot or too cold, which can be really annoying and uncomfortable. If the unsteady temperature persists, it can even affect your family’s health and promote poor sleep.

Prevent System Breakdown

Furnace short cycling will cause more parts of your furnace to collapse because of the extra strain experienced during multiple startups. This will eventually lead to a bigger problem which is your entire furnace breaking down.

So instead of just a minor malfunction that can be easily fixed, now you’ll deal with an expensive repair or, worse, a system replacement.

Causes Of Furnace Short Cycling and How to Fix

There are several conditions that can cause furnace short cycling, including:

Faulty or Dirty Flame Sensor

A dirty or faulty flame sensor is the most common cause of short cycling. This happens because a dirty, rusted, or faulty flame sensor won’t be able to sense the presence of heat and will command your furnace to shut down every time you turn it on.

Most often than not, if your flame sensor experiences problems, it is not broken but just dirty. Check out this blog for a complete guide on how to clean your furnace flame sensor.

Should you be facing a damaged flame sensor, here is a helpful blog for you: Furnace Troubleshooting Guide.

Thermostat Issues

There are a couple of probable reasons for your thermostat to cause your furnace to short cycle. One is when it is placed in the wrong position inside your home.

For example, suppose your thermostat is placed near a heat source or in direct sunlight. In that case, it can sense heat all too quickly and may signal your furnace to shut down even though the correct indoor temperature is not yet achieved.

Also, failing to program your thermostat to heat mode can cause your furnace to short cycle. Before assuming that your thermostat might be broken, first make sure that it is in the “heat” function.

Finally, your thermostat may really be damaged. A broken thermostat may signal your furnace to switch on and off regardless of the indoor temperature. Set the heat anticipator level on your thermostat one calibration mark closer to the “longer” setting to rule this out.

Give it some time, but if you notice that the short cycles persist, a professional HVAC technician should check your thermostat wiring or have your thermostat replaced.

Clogged or Dirty Air Filters

A dirty or clogged furnace filter restricts system airflow, so warm air that should be vented out to warm your home is retained inside your furnace. This makes your furnace’s temperature rise up and causes it to shut off.

And since your home is not warm enough yet, it triggers your thermostat to turn on the furnace for another cycle.

Fortunately, cleaning and replacing your furnace air filters is easy and can be done DIY. We have the complete guide on cleaning the air filter; click here.

System Overheating

Another common reason for short furnace cycling is constant system overheating. This can be caused by your furnace’s temperature-monitoring devices being faulty. Usual built-in furnace components that monitor the temperature are the limit switch and the heat exchanger.

A damaged or cracked heat exchanger, for example, can release heat inside your furnace which can cause overheating and frequent short cycling.

Oversized Furnace

Oversized furnaces in a small home will deliver heat in all rooms more quickly but unevenly. This will lead the furnace to a short cycle as it tries to achieve a consistent indoor temperature in various rooms throughout your home.

Suppose you’re still experiencing uneven temperature between rooms after your furnace short-cycle issue is fixed. In that case, you might have an unbalanced duct.

Moreover, poor insulation in an oversized house can also cause frequent furnace cycles on and off. Because most of the heat produced by furnaces is lost through cracks, holes, or faulty seals. Oversized furnace causes the system to short cycle to keep up with the missing heat and maintain a constant temperature.

Other possible causes of a short cycling furnace:

  • The blower motor problem
  • The exhaust vent problem
  • Air leaks


A short cycling furnace can cause your home heating system to break down prematurely. If your furnace is short cycling and you can’t fix the problem by yourself, contact a professional HVAC technician immediately. They will diagnose your furnace issue and offer the best solution for your home heating system.

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Mas Broto

Have been in the heating and air conditioning (HVAC) industry for over 20 years. He is person that will grow and thrive to learn more about the HVAC industry throughout his career. Mas Broto is also a blogger, who's dedicated to bringing you the best knowledge to get ahead in the game of life.

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