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Why Is My Furnace Short Cycling?

Furnace Filter Image

My Furnace Keeps Short Cycling. What Is Going On?

A furnace that is short cycling can be a frustrating issue when you are trying to keep warm.

Does your furnace keep turning on and back off in a short period of time? This is what is called short cycling.

A short cycling furnace can be a frustrating issue to have.

Unfortunately, after not being used all spring and summer, some furnaces have some issues when it comes to firing up for the first time in the fall.

Common causes of short cycling furnace include:

  • Overheating due to a dirty filter
  • Oversized Furnace
  • Faulty Thermostat
  • Blocked Exhaust Vents
  • Blocked Air Vents
  • Bad Flame Sensor

Overheating From a Dirty Filter

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Dirty air filters are one of the top causes of furnace and air conditioner break downs.

If your filter hasn’t been changed in a while, go ahead and take a look at it. Keep in a mind a brand new filter should be white in color.

If you have a clogged air filter, not much air will be able to pass through it and it can cause your furnace to overheat, tripping the limit switch which is a safety feature built into your furnace to prevent a fire from occurring.

An overheating furnace can also create potential cracks in the heat exchanger which can lead to carbon monoxide leaking into your home.

Be sure you have the correct size filter in your furnace as well. A filter that is the wrong size can damage the furnace and sometimes even get sucked into the blower motor causing major damage.

Checking your filter is a simple fix that could prevent a future furnace repair.

Furnace Is Oversized

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Bigger is not always better when it comes to HVAC equipment. A properly sized furnace and AC is critical for your home.

This type of issue is unfortunately only fixed by having your current furnace removed and the correctly sized furnace installed.

If you have an oversized furnace for your home, it will end up using much more energy than needed.

This ultimately means that the warmer air will not be properly distributed throughout your home.

Your home will heat for a very short period of time, and then the furnace will kick right back on again, continuing to short cycle.

This is not good for your furnace’s overall health.

Not only will it increase your monthly energy bills, but it will also cause unneeded wear and tear on your furnace’s expensive parts.

A furnace that is oversized will wear down its heat exchanger much faster than one that is properly sized.

If you believe your furnace is too big, you will need to have a professional HVAC company out to take a look and verify its heating capacity.

Faulty Thermostat

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Some thermostats can be finicky.

Your furnace only does what your thermostat tells it to do.

If your furnace is short cycling on a continual basis, the fix may be as simple as replacing your current thermostat with a new one.

You can either buy one yourself at the store, or have a professional install one for you.

Also be sure to check the location of your thermostat in your home. Is it in direct sunlight during the day?

If so, this can cause to think it’s warmer in the home than it actually is.

The ideal location for your thermostat should be on an interior wall in a highly occupied area.

It should not be located near any windows where sunlight or drafts can affect it’s temperature reading of the home.

Blocked Exhaust Vent


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If you have a high efficiency heating and cooling system, it will vent out the side of your home via white pvc piping.

During the fall and winter months, it is sometimes possible for this exhaust vent to become clogged with leaves and snow, preventing the furnace from venting properly and causing it to short cycle.

This problem is not only bad for your HVAC system but can be dangerous to your health as carbon monoxide can begin to leak into your home instead of venting outside correctly.

Blocked Air Vents

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Sometimes air vents are put in inconvenient locations in homes and people just cover them up with couches and other furniture.

If you have some air vents in your home covered up, this could be the cause of your furnace short cycling.

Blocked air vents will prevent warm air from heating your home properly and will also shorten its overall lifespan.

Be sure to keep the area around them clear of furniture and also make sure the dampers are open to allow air flow.

Bad Flame Sensor

If your furnace starts and then shuts off almost immediately, a bad flame sensor may be the culprit.

A flame sensor’s job is to tell the furnace that a flame is actually present when the gas is being released.

This part is actually a safety component of your furnace in that the furnace will not run if a flame is not present.

Without a flame sensor, your furnace would just leak gas into your home even though no flame is actually present.

This would be a very dangerous situation!

How Long Should My Furnace Run?

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The cycle time for your furnace can vary by home and also the temperature you set your thermostat to.

If your home is not very energy efficient and you have the temperature set high, you will notice that your furnace runs longer than more energy efficient homes.

The heating cycle in an average home usually lasts about 10-15 minutes from the time you thermostat sends a signal to the furnace to when it finally turns off.

Why does my furnace turn off after a few minutes?

Your furnace may turn off after a few minutes because:

  • Your flame sensor is malfunctioning.
  • Your pressure switch is broken.
  • Your furnace’s draft inducer motor is broken.

Your furnace has many components that will work in sync with one another to maintain the safety and operation of your unit. When one of the components breaks, this can cause your furnace to turn on for a few minutes and then turn off. To get your furnace repairedcontact us today.

Why do I have to keep restarting my furnace?

A few reasons you may have to keep restarting your furnace include:

  • A blocked flue.
  • A dirty flame sensor.
  • A broken or faulty ignitor.

If you have to keep resetting your furnace, that indicates there is a deeper issue that is preventing your furnace from properly functioning. Give Sears Heating & Cooling a call today to get your furnace repaired.

Can a bad thermostat cause a furnace to short cycle?

Yes, a bad thermostat can cause a furnace to short cycle. Some reasons your thermostat may be causing your furnace to short cycle are:

  • Your thermostat is broken.
  • Your thermostat is placed in a location with too much direct sunlight.
  • Your thermostat is malfunctioning.

If you believe that your thermostat is the cause of your furnace short cycling, you should look inside your thermostat to check if there is dust, dirt or loose wires. To get your furnace working againcall us today.

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