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Why Does My Furnace Keep Shutting Off?

A tech wearing a blue collared shirt, gray overalls, safety glasses, and red work gloves is taking a part a furnace

This might sound familiar: your furnace turns on, runs for a minute or two, and then shuts off. Soon after, it turns back on, and the process repeats.

When this happens, your furnace is probably “short cycling.” While not uncommon, you shouldn’t let it continue. When your furnace is short cycling, it will struggle to heat your home, increase your energy bills, and put stress on the system.

So, what exactly is short cycling, and how can you fix it?

The most common problems that cause a furnace to short cycle include:

Below, we’ll cover these issues in more detail in order to help you find a solution.

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Overheating Heat Exchanger

A furnace may short cycle when the heat exchanger is overheated, likely due to a lack of airflow.

The heat exchanger is where the air gets heated. It will trip the furnace’s safety switch when it gets too hot, which shuts down the operation to protect the system from dying. If the safety switch breaks, the furnace will overheat, and the heat exchanger will crack.

Fortunately, fixing restricted airflow is relatively easy. Try the following steps:

  • Replacing your furnace’s dirty air filter every 1-2 months
  • Closing any air supply vents
  • Cleaning the furnace’s blower wheel

We’ll cover the logic behind these steps next.

A dirty air filter reduces the amount of cold air your furnace pulls in to push over the heat exchanger. Furnaces need cold air for the heat exchanger to heat up. If there isn’t enough coming in, the system will shut itself off as a safety measure.

Once the internal temperature within your furnace stabilizes, it will kick back on again. However, the dirty air filter forces the furnace to repeat this short cycling. This can cause unnecessary wear and tear on your furnace.

Another problem is any closed air supply vents in your home. Too many closed air vents can slow down your furnace’s blower from pulling in cold air over the heat exchanger, reducing the airflow, and causing short cycling. Make sure you open all air supply vents—including those in unused rooms.

Dirty Flame Sensor Rod

Closeup of a hand holding the flame sensor rod pulled out of a furnace

Check how many times your furnace tries to turn on before stopping for a while. If it’s rapidly short cycling multiple times before taking a break, the problem is probably a soot-covered flame sensor rod within the furnace.

What is a flame sensor rod? It’s a safety device that detects a flame coming from your furnace’s burners. If it doesn’t detect a flame, it will shut off the gas valve to prevent gas from leaking into your home.

A soot-covered flame sensor rod can cause short cycling because it can no longer accurately tell if there’s a flame or not. This rod is quite sensitive—for a good reason—and will shut off the gas valve even if there is a flame.

Got a furnace with a pilot light? It still has a flame sensor rod; it’ll just detect a flame coming from the pilot light instead of the furnace burners.

Contact a heating technician to clean or replace the flame sensor rod.

Blocked Flue Pipe

The furnace’s flue pipe vents any exhaust outside of your home. It’s also called an exhaust vent or vent pipe. Any blockage in the pipe causes hot gases to build up with no way to escape. Your furnace then overheats, triggering an automatic shutoff.

You can check if your flue pipe has debris stuck in it by turning off your furnace and heading outside to inspect it. The flue pipe will likely be on the side of your house near the furnace or on your roof. Take a look inside to see if there are leaves or other particles inside of it. Contact a heating technician for help if you can’t locate the flue pipe or if you notice a blockage that needs cleaning out.

Malfunctioning Thermostat

A woman's right thumb, painted with a pink nail, pressing the cool button on a white thermostat showing 74 degrees

Your thermostat directly communicates with the furnace and controls when it starts and stops. If it’s malfunctioning, it might be telling your furnace to shut off earlier than needed.

What are some ways that the thermostat can malfunction? It may have a faulty temperature sensor, wiring, or relay switch. All of the above can send the wrong message to your furnace.

If the thermostat’s temperature sensor is faulty, it may be in the wrong location to get an accurate temperature read. For example, if it’s near a heat source like a supply vent, it will assume that the home is warmer than it actually is. A skewed temperature reading will lead to the furnace turning off sooner than it should. Then, once heat stops blowing from your supply vents, the thermostat thinks your home is too cold and turns the furnace back on—the cycle repeats.

Contact an HVAC technician to inspect your thermostat and its location. They may recommend relocating it to a more neutral location, repairing any broken parts, or replacing it entirely.

Also, make sure that your thermostat is on the right settings: “HEAT” and “AUTO.” A thermostat set to “ON” will continually keep the furnace on regardless of whether there is a current heating cycle.

Oversized Furnace

Two red gloved hands pulling of a front piece of a furnace

Do you have a new furnace? Or has your old furnace consistently short-cycled over the years? If yes to either question, it may not be properly sized for your home.

Without the right furnace sizing, short cycling is a common occurrence. Oversized furnaces, in particular, heat up homes too quickly. Due to the immediacy, the furnace then shuts off once it’s done its job, and the short cycling continues. This frequent pattern overworks your furnace, shortening your system’s lifespan. It also uses more energy, increasing your utility bills.

Suspect that your furnace may be oversized? Don’t fret; you have a few options, no matter your furnace’s age.

Homeowners with brand-new furnaces can contact their HVAC installers about this issue. Many reputable, high-quality contractors will provide labor warranties with their installation, and they will return within 1-2 years of your installation date to fix any errors due to the installation.

Homeowners with older furnaces may need to replace their systems entirely. Most furnaces last around 10-12 years in the Phoenix area, but a short cycling one tends to have a shorter lifespan. So, if your furnace is reaching the 8 to 10-year mark, replacing it with a more efficient and properly sized one may be more cost-effective. Ensure that your contractor performs a Manual J Calculation during their installation estimate, which considers multiple factors to help them find the optimal furnace size for your home, budget, and heating needs.

While a new furnace may be an upfront investment, it can help you save on your long-term heating bills.

Read more in our blog, ​​"How Long Does a Furnace Last in the Phoenix Area?"

Need A Furnace Repair or Replacement? Talk to Phoenix Heating Experts: George Brazil HVAC!

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Contact George Brazil HVAC. We offer same-day service for fast furnace repair turnarounds. Plus, we provide free same-day furnace replacement estimates and prioritize installing your new furnace within 24 hours afterward.

No matter your furnace request, we provide honest diagnostics, upfront pricing, and flexible financing options. Phoenix homeowners know they can rest assured that we back up our work with labor warranties and 100% satisfaction guarantees.

Call us today at (602) 842-0009 or schedule with us below to experience 5-star heating work!

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