Why Does My Furnace Keep Going On and Off?

January 08, 2020
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Worried that your furnace is turning on and off too frequently?

If your furnace is turning on and off frequently, it could mean one of two things:

  1. It’s especially cold outside
  2. Your furnace is short cycling

If it is an especially cold day, your furnace may just be turning on and off frequently because your home is getting colder faster. If that’s the case, there’s no need to worry—your furnace is just doing its job.

However, if your furnace turns on and off and only runs for a minute or two, it’s short cycling. Short cycling is bad news for your furnace because it wears out important components and increases your energy bills.

In this blog, we’ll explain what to do before calling a pro and what causes your furnace to short cycle.

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Before we jump into the reasons why your furnace may be short cycling, we’ll share two things you can do before you call a professional for help.

DIY fix #1: Change your air filter (if it is dirty)

A dirty air filter restricts the flow of air to your furnace. This can cause your furnace to overheat and shut off, ultimately resulting in short cycling (more on that later).

Dirty Vs Clean Air Filter

If your furnace looks like the one on the right, it’s dirty and it should be replaced.

What to do: Check your air filter. If it is dirty, replace it with a new, clean filter.

DIY fix #2: Make sure nothing is blocking your vents

If you close or block your air vents, it can lead to low airflow issues in your furnace. Eventually low airflow can cause your furnace to overheat, which causes short cycling.

An Open Supply Air Vent

An open supply air vent

An Unblocked Return Vent

An unblocked return vent

What to do: Check your air vents. Make sure all supply vents are open (even in rooms you don’t use), and make sure nothing is blocking or covering your return vent.

If you’ve tried the above DIY fixes and your furnace is still short cycling, you’ll need to have a professional inspect your furnace and find the issue. Below we’ll go into detail about what could be causing your furnace to short cycle.

#1: Overheating

Your furnace has a built-in safety switch called a high-limit switch. If your furnace is overheating for any reason, this switch will trigger your furnace to shut off to protect itself from damage.

Once the internal temperature of the furnace returns to normal levels, the furnace will turn back on. However, if you have an overheating problem, your furnace will quickly shut off again. This quick on/off cycle will repeat over and over again, an issue called short cycling.

So, what causes overheating problems?

  • Dirty components
  • Reduced or restricted airflow
  • Malfunctioning blower fan
  • Aging furnace components

Besides changing a dirty filter and making sure all air vents are open, other overheating problems require an HVAC technician’s expertise to fix. They can safely and quickly find what is causing your furnace to overheat and fix the problem.

#2: Thermostat issues

Your thermostat is the brain of your furnace—it tells the furnace when to shut on and off. Just like any kind of electrical equipment, sometimes it can malfunction, causing your furnace to quickly shut on and off when it’s not supposed to.

A professional will need to check the thermostat’s wiring to make sure it is working properly and sending correct signals to your furnace.

#3: Sizing problem

When it comes to furnaces, the biggest size isn’t always the best for your home. An oversized furnace will quickly heat your home then shut off abruptly. Once your home starts to get cold again, it’ll repeat the short cycling process.

Unfortunately, if it turns out your furnace is too big, you’ll need to get a new system that’s correctly sized. A professional can help you calculate the exact size system you need for your home by performing a load calculation.

Learn more about furnace size and load calculations by reading our blog, “What Size Furnace Do I Need? A San Antonio Tech Explains.”

Schedule an appointment today

Our team of trained furnace repair technicians are standing by to help! We offer same-day service and are available 24/7.

Learn more about our repair process by visiting our furnace repair page.


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