Why Your Furnace Keeps Turning On & Off (Short Cycling)

Checking the furnace settings

Is your furnace acting like a moody teenager, flipping on and off on a whim?

This common issue is called short cycling, and there may be a simple fix for it. Then again, there may not be, in which case you’ll need to call in a professional.

But let’s talk about why your furnace is short-cycling in the first place.

Understanding Short Cycling

In the HVAC world, short cycling is when your furnace turns on and off quickly and repeatedly — like a bad case of hiccups. Your furnace is designed to heat your home until it reaches a pre-set temperature. Once it hits that point, it turns off.

When the temperature drops again, the furnace turns back on.

But when your furnace is short cycling, it’s not completing an entire heating cycle. This means your home isn’t getting warm enough, and your furnace is constantly turning on to compensate for it.

Possible Causes of Short Cycling

So why is your furnace behaving this way? There may be several reasons:

  • Dirty air filter: An air filter clogged with dust and debris can restrict airflow and make your furnace overheat. This triggers the safety mechanism that shuts off your furnace before damage occurs.
  • Faulty thermostat: Incorrect settings or a malfunctioning thermostat might cause rapid cycling.
  • Blocked vents: Closed or blocked vents disrupt airflow, leading to overheating and frequent cycling.
  • Improperly sized furnace: If it’s too large for your space, it might heat too quickly and then shut off repeatedly. And it will struggle to maintain the desired temperature if it’s too small.
  • Dirty or faulty flame sensor (gas furnaces): A malfunctioning flame sensor can shut your furnace off shortly after lighting the burners.
  • Malfunctioning limit switch: The limit switch turns off the furnace when it reaches a specific temperature. If it’s faulty, it may shut off the furnace too soon.
  • Thermostat location: If your thermostat is near a heat source or in direct sunlight, it may register as warmer than the rest of your home and shut off the furnace too soon.
  • Electrical issues: Defective wiring or electrical connections can make your furnace short cycle.

DIY Troubleshooting Tips

Before calling a professional, here are a few things you can do to try and fix the issue yourself:

  • Check your air filter and replace it if needed.
  • Ensure all vents are open and unobstructed.
  • Double-check your thermostat settings and replace the batteries if needed.
  • Shut your furnace off for a few minutes, then turn it back on. This can help reset any electronic components that may be causing the issue.
  • Inspect the location of your thermostat. Consider relocating it to a cooler spot if it’s near a heat source or in direct sunlight.
  • Examine the flame sensor (if you have a gas furnace). If it appears dirty, try gently cleaning it with an emery cloth.
  • Verify the size of your furnace. If it’s too large or too small for your space, you may need professional help to replace it.
  • Take a look at your electrical wiring. If you observe any loose or frayed wires, it’s time to call a repair pro.
  • Check the limit switch. It might be malfunctioning and causing your furnace to turn off prematurely. This typically requires a professional to fix.

When to Call a Professional

If you’ve exhausted these DIY troubleshooting options and your furnace is still short cycling, it’s time for professional help. HVAC pros have the experience, expertise, and equipment to diagnose and fix the issue or, if need be, replace the furnace.

Never attempt to troubleshoot or fix electrical or gas-related furnace issues yourself. These can be dangerous and should be handled by a trained and certified professional.

The Value of Prompt Repairs

Why the rush to get this fixed, you ask? Short cycling can make your energy bills skyrocket faster than a space shuttle.

Your furnace is working overtime and will charge you for it. Plus, allowing the problem to continue means extra wear and tear on your furnace, leading to costly repairs or even a complete replacement if you’re not careful.

You’re better off dealing with a wayward furnace sooner rather than later.

Proactive Prevention Measures

  • Regular maintenance: Like any other appliance, regular maintenance can help prevent issues like short cycling.
  • Proper installation: Only a certified technician should install your furnace (and not your hobbyist Uncle Joe). Incorrect installation is a guarantee of problems down the line.
  • Opt for a programmable thermostat: This will ensure your furnace operates at optimal levels, reducing the chances of short cycling.
  • Right-sizing your furnace: Ensure your furnace is the right size for your space. An oversized or undersized unit is more likely to short cycle. An HVAC professional can help determine which furnace size, brand, and model is right for your home.
  • Regular filter replacement: A clean filter ensures optimal airflow and prevents overheating, thus reducing the chances of short cycling.
  • Thorough duct cleaning: Regular duct cleaning (every 3-5 years) improves airflow and helps prevent short cycling. This is a job for the pros.
  • Correct thermostat placement: Your thermostat should be in a place that best represents the average temperature of your home.
  • Proper ventilation: Keep all vents open and unobstructed for adequate airflow. Blocked vents contribute to furnace overheating.
  • Routine flame sensor cleaning: A dirty sensor can shut off the furnace prematurely.

Furnace Maintenance & Repair in Los Angeles

Do you live in metro Los Angeles and need a furnace inspection or repair?

Call West Coast Chief for guaranteed furnace service and same-day service visits.

Chief employs only certified technicians and guarantees all repairs. We service all brands, honor manufacturer warranties, and use only certified parts in any repair we perform.

Don’t wait until your furnace becomes a problem or liability. Call us today! 888-832-3599.

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Frequently Asked Questions about Furnace Short Cycling

Q: How long does a furnace cycle last?

A: A typical cycle lasts anywhere from 10 to 15 minutes, depending on the desired temperature and the size of your home.

Q: What’s considered short cycling for a furnace?

A: If your furnace cycles on and off more than three times an hour, you may have a short cycling problem.

Q: How dangerous is a short cycling furnace?

A: While not immediately dangerous, prolonged short cycling leads to increased wear and tear, higher energy bills, and the potential need for costly repairs or replacement.

Q: Can a dirty air filter cause short cycling?

A: A dirty air filter can limit airflow, causing the furnace to overheat and short cycle. Regularly replacing your air filter can help prevent this problem.

Q: Does the size of my home affect short cycling?

A: Yes, the furnace size should be appropriate for the size of your home. An incorrectly sized furnace can cause short cycling.

Q: Can I fix a short cycling furnace myself?

A: While homeowners can do some simple fixes, calling a professional is always safer and more effective, especially for problems involving gas or electrical components.

Q: Is short cycling a common problem in furnaces?

A: Yes, short cycling is a common issue in many furnaces, but it’s also a problem that should be addressed promptly to avoid further damage.

Q: How can I tell if my thermostat is causing the short cycling?

A: If your thermostat is set too high or is located near a heat source, it might cause your furnace to short cycle. Relocating the thermostat or adjusting the temperature setting can often solve the problem.

Q: Are there any signs of short cycling apart from frequent on-off cycles?

A: Apart from the obvious frequent starting and stopping, other signs can include unusually high energy bills, inconsistent heating, and your furnace struggling to maintain the desired temperature.

Q: Can a new furnace short cycle?

A: Yes, a new furnace can also short cycle, especially if it’s incorrectly installed or is the wrong size for your space. Always have a certified professional handle installation.


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