Why Does My Furnace Keep Going On and Off? A Boise Tech Explains

February 26, 2019

Your furnace turns on, runs only for a few short minutes and then shuts off. After a little while, it turns back on and the process repeats.

Sound familiar? If so, your furnace is likely short cycling.

In this article, we’ll share...

  • 2 quick DIY fixes to stop your furnace from short cycling
  • 3 short cycling problems a professional needs to fix

Need a pro to repair your furnace right away?

2 quick DIY fixes to stop your furnace from short cycling

Before you call a professional, try these 2 quick DIY tips to see if they stop your furnace from turning on and off frequently.

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

clean vs dirty air filter

Clean vs dirty air filter

If your filter looks like the filter on the right in the image above, it’s dirty and should be replaced.

A dirty air filter blocks air from entering your furnace, which causes the heat exchanger to overheat.

To protect the heat exchanger from damage, the furnace will shut itself off until the heat exchanger cools back down.

DIY fix #2: Open your vents

Make sure all of your vents are open (even in unused rooms) and that nothing is obstructing them.

Similar to a dirty filter, closed supply and return vents prevent air from entering your furnace, which can lead to an overheated heat exchanger.

Like we explained above, your furnace will shut itself down if it senses that the heat exchanger is overheating. When temperatures cool off in the heat exchanger, your furnace will turn back on.

If your vents are closed, the this on/off process will continue, which wears down furnace components and can lead to expensive repairs.

3 short cycling problems a professional needs to fix

If you’ve tried the 2 DIY fixes above and your furnace is still having the same problem, it’s time to call a pro.

We’ll go into detail about 3 short cycling issues a professional will need to fix.

Problem #1: Airflow issues

Earlier we mentioned that a lack of airflow can lead to short cycling.

The reason low airflow can lead to short cycling is because the heat exchanger (the part that actually heats the air) can overheat when little or no cold air is blowing over it.

furnace diagram with heat exchanger

Diagram of a furnace

To prevent your heat exchanger from damage, your furnace will shut off until the temperature inside the exchanger decreases.

Aside from a dirty air filter and closed vents, the most common causes of low airflow include:

  • A dirty blower fan
  • Dirty ductwork
  • A broken blower motor

While most homeowners can change the filter and open vents, a professional will need to handle the problems we list above because they are more complicated.

When you contact a pro to fix your furnace, they’ll inspect heat exchanger to make sure it’s clean make sure the blower motor is working properly.

Problem #2: A dirty flame sensor rod

The flame sensor rod’s job is to detect when there is a flame in your furnace burners.

If the sensor rod does not detect a flame, it will close the gas valve so that gas doesn’t leak into your home.

However, if the flame rod becomes coated with soot, it loses its ability to accurately detect a flame, which can cause your furnace to shut off (and then turn back on later).

A professional will need to replace a broken or dirty flame sensor rod.

Problem #3: Oversized furnace

Have you had this short cycling problem ever since you got your furnace? If so, your furnace may be too big for your home.

Now you may be wondering, “Isn’t a bigger furnace better?”

Not necessarily. You see, sizing a furnace is like sizing a shoe: You have to find the right size or else you’ll have issues.

The problem with an oversized furnace is that it will heat the home really fast and then shut off (short cycling).

A short cycling furnace leads to other issues like:

  • Increased wear and tear on furnace components (which means more repairs)
  • Uneven temperatures throughout the home

A furnace technician can verify if your furnace is the right size for your home via a heat load calculation.

Unfortunately, if your furnace turns out to be too big, you’ll likely need to get a new one. But it’s better to invest in a new furnace than pay for the expensive repairs caused by short cycling.

Need a furnace repair by a Boise pro?

We’ll send one of our trusted techs to inspect your furnace and fix whatever is causing it to shut on and off.

Learn more what to expect when you hire us by visiting our furnace repair page.


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