Furnace Keeps Turning On and Off: Understanding the Reasons

It’s frustrating when your furnace keeps turning on and off. You’re probably wondering if it’s a sign of something serious. That constant cycling can leave you worrying if you’re about to face a bigger problem or a high repair bill.

This article breaks down the possible reasons behind your furnace’s on-off cycle so you can get a clear picture of what’s going on. We’ll go through the common issues, what they mean, and when you might need a pro to step in.

How a Furnace’s Cycle Works

A furnace runs in cycles to keep your home at a comfortable temperature. Here’s how it generally works: the thermostat detects the room temperature and signals the furnace to start when the air needs heating. Once the room reaches the set temperature, the furnace shuts off until the room cools down again. Then, the cycle repeats.

Typically, a furnace runs for about 10 to 15 minutes in moderate weather. But in freezing temperatures, you might notice it turns on more often to keep up with the cold.

Now, what’s known as short cycling is when the furnace turns on and off much faster than it should. Instead of running smoothly and shutting off after reaching the right temperature, it’s constantly powering up and shutting down in quick bursts. This issue can lead to some big problems:

• Wasted Energy: A furnace that keeps cycling uses way more energy than necessary, which will show up on your energy bills.
• Increased Wear and Tear: Constant starting and stopping means extra strain on the furnace parts, which can lead to faster breakdowns or expensive repairs.
• Uneven Heating: Short cycling doesn’t give your furnace enough time to fully warm up the space, so some rooms might feel colder than others.

Reasons Why Your Furnace Keeps Turning On and Off

If your furnace keeps switching on and off, it signals underlying issues that need attention. Some of these furnace problems are simple to fix, while others may call for a technician’s help. 

Let’s break down some common reasons behind this issue:

Dirty or Clogged Filters

One of the most common reasons for short cycling is a dirty or clogged air filter. When the filter is blocked, airflow is restricted, causing the furnace’s components to overheat. 

The furnace triggers a safety shutoff through the high-limit switch to prevent any damage. This stops the furnace from running too long but also means it’ll keep starting and stopping.

Ask yourself:

• When did I last change my furnace filter?
• Do I notice any unusual sounds or smells from the furnace?
• Is the airflow weak from my vents?

The Environmental Protection Agency offers comprehensive information on maintaining indoor air quality, including the role of clean air filters.

Solution: Replace your air filter every 60 to 90 days to prevent airflow problems. If you have pets or live in a dusty area, you might need to change it more often.

Thermostat Issues

The thermostat is the control centre of your heating system, so when it has issues, the whole system can act up. Two common thermostat-related problems often lead to short cycling: a malfunctioning thermostat and improper placement.

Malfunctioning Thermostat

If your thermostat isn’t working properly, it can send mixed signals to the furnace, causing it to short cycle. When the thermostat misreads the room’s temperature, it might turn the furnace on or off at the wrong times, creating a constant on-off pattern.

Solution: Have a professional inspect your thermostat. They can repair or replace it, ensuring accurate temperature readings and steady furnace operation.

Improper Placement

Thermostat location plays a huge role in your furnace’s performance. If it’s placed in direct sunlight, near a heat register, or in the kitchen where temperatures fluctuate, it may not read the room’s actual temperature correctly. 

Ideally, your thermostat should be mounted about 1.5 metres above floor level—not too high, as that can also lead to temperature reading errors.

Solution: Move your thermostat away from direct heat sources or cold drafts. Placing it at a moderate height in a central room can help you get a more accurate sense of the overall room temperature, keeping your furnace from short cycling.

Note that some smart thermostats claim that they automatically adjust for this to prevent sunlight from distorting temperature sensing.

Overheating

When a furnace overheats, it shuts off as a protective measure to prevent damage. This can happen due to several reasons, like shut or blocked air vents, or a blocked exhaust vent.

Shut or Blocked Air Vents

When air vents are closed or obstructed, airflow is restricted throughout the system. This forces the furnace to work harder to distribute heat, and the components inside can overheat as it struggles. Once it reaches a certain temperature, the furnace shuts off to cool down, only to turn back on shortly after, leading to that annoying short cycling pattern.

Solution: Go around your home and check each vent. Ensure they’re fully open and free of any furniture, curtains, or other items blocking them. Keeping vents open and clear helps balance airflow and reduces strain on the furnace, preventing it from overheating.

Blocked Exhaust Vent

Your furnace has an exhaust vent to expel gases safely outside. If this vent becomes blocked—by leaves, snow, or other debris—the furnace may struggle to release these gases, leading to overheating. This triggers the safety shutoff, which then causes the furnace to turn on and off frequently.

Solution: Check your furnace’s exhaust vent outside your home to make sure it’s clear. Remove any obstructions, and clear it away if there’s a snow build-up. Regularly inspecting the vent, especially in winter, can prevent overheating and help your furnace run more smoothly.

The NFPA offers guidelines on preventing overheating and potential fire hazards in heating systems.

What to Do When Your Furnace Won’t Turn Off

If your furnace just won’t stop running, it can quickly get uncomfortable and even raise your energy bill. Here’s a step-by-step guide to help you troubleshoot the issue before calling in a technician.

1. Check the Thermostat Settings: Start by looking at your thermostat. Make sure it’s set to “Auto” instead of “On.” When set to “Auto,” the furnace should only run when the thermostat calls for heat. If it’s set to “On,” the fan will continue running, even when the furnace isn’t heating.
2. Adjust the Temperature: Lower the thermostat setting by a few degrees to see if the furnace responds and shuts off. If it keeps running, there might be a thermostat or wiring issue.
3. Inspect the Air Filter: A dirty air filter can cause airflow restrictions, making the furnace work harder and potentially causing it to stay on longer. If you haven’t replaced your filter in a while, try putting in a new one to see if it helps.
4. Check for Blocked Vents: Walk around your home and make sure all air vents are open and unblocked. Closed or obstructed vents make it hard for the furnace to heat evenly, leading it to run longer than necessary.
5. Reset the Furnace: Sometimes, a simple reset can fix a stuck furnace. Turn off the power to the furnace at the breaker panel, wait about 30 seconds, and then turn it back on. This may reset any issues causing it to keep running.
6. Inspect the Blower Fan: If your furnace still doesn’t turn off, it could be a problem with the blower fan. Look at the fan’s switch on your thermostat; it should be set to “Auto.” If it’s already on “Auto,” there may be a mechanical issue with the fan itself.
7. Contact a Technician: If you’ve gone through these steps and the furnace is still running nonstop, it’s time to call a professional. They can check for issues like wiring problems, thermostat malfunctions, or faulty furnace components that require specialized tools and expertise.

Why Does Your Furnace Shut Off Before Reaching the Set Temperature?

When your furnace shuts off before hitting your set temperature, it’s likely not working right. One of the biggest reasons for this is short cycling. Short cycling is when the furnace turns on and off without running long enough to heat your space fully. 

This can happen because of airflow issues, like a dirty filter, or because the furnace is too big for your home, which means it heats up too fast and shuts down before it finishes the job.

Another possible cause is a faulty thermostat. If the thermostat misreads the room temperature, it might tell the furnace to shut off even though it isn’t warm enough. Also, if it’s in a spot where it gets hit by direct sunlight, near a vent, or in a drafty area, it may not accurately read the room’s actual temperature.

Maintaining a Healthy Furnace Cycle

Here are some key steps to help your furnace cycle properly:

• Clean the Flame Sensor: A dirty flame sensor can cause the furnace to shut off too early. Cleaning it regularly helps the system run in longer, smoother cycles.
• Seal Air Leaks: Gaps around doors, windows, and ducts can mess with your furnace’s ability to heat your home evenly. Sealing these leaks can help your furnace maintain a regular cycle and avoid extra work.
• Check Thermostat Batteries: Low batteries in your thermostat can prevent communication with the furnace, leading to abnormal cycling. Swapping in fresh batteries might be a simple fix.
• Clear the Flue: Clogged flue pipes can block exhaust gases from leaving your home. This backup can cause the furnace to cycle on and off unexpectedly. Make sure the flue is clear of debris.
• Consider Furnace Age: Furnaces are usually around 15 to 20 years long. If your furnace is nearing this age, it may be time to replace it for more reliable cycles. Here’s how to find out how old your furnace is.

It’s best to schedule a maintenance check for your furnace once a year, ideally in early fall, before the heating season starts.

Conclusion

If you’ve gone through every step and still can’t figure out why your furnace keeps acting up, it’s probably time to call in a technician. Sometimes, the issue is buried in the wiring or in a component that only a pro can safely handle. But let’s face it—finding a trusted HVAC expert can be tricky.

We’re here to make that part easy. You can get FREE, no-obligation quotes from local experts right in your area. Get a free quote today!

Get Quotes

How soon are you looking to buy?*

Urgently!

Next few weeks

Not sure yet