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Why Is My Furnace Cycling On and Off?

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You may have HVAC problems if your furnace cycles on and off without you fiddling with its controls.

A furnace short cycling indicates a problem somewhere in the heating system. Its heating cycle may be compromised due to dirty filters and air leaks. 

However, there may be more severe possibilities affecting your furnace. Overlooking minor furnace issues may lead to permanent damage and costly repairs.

You should strongly consider keeping up with quarterly maintenance and inspection. A consistent and professional inspection can help pinpoint problems before they compromise your comfort and surge heating costs.

Back to the problem at hand, let's deal with that pesky short-cycling furnace.

Your Furnace Cycles ON AND OFF By Itself: WHY?

Dust, dirt, residue, tripping breakers, faulty switches, poor insulation, and more; take your pick.

There are at least a dozen possible reasons behind a furnace short cycling. It can be annoying, persistently hearing clicks and freezing. 

Unfortunately, without getting to the root cause, that's exactly what you can expect from your furnace.

Your Furnace Turns OFF By Itself: HOW?

How exactly does your furnace turn ON and OFF by itself? A furnace short-cycles due to obstacles in the way of its automated transmission.

A blower motor automatically switches ON or OFF depending on its temperature reading.

  • The furnace turns ON when temperatures start dropping below your thermostat settings.
  • When indoor air achieves the set temperature, the furnace turns OFF, shutting heat exchangers and fuel intake pipes down.

This two-step process is the typical automation of most furnaces. Older furnaces may need to be set manually, while newer furnaces are fully automated.

Some furnaces have additional safety configurations, such as different sensors.

While these extra safety measures help prevent hazardous malfunctions, they use more energy and give furnaces more vulnerable components.

If your new furnace is acting up, its safety sensors may be defective. Depending on its warranty, a factory defect may entitle users to free furnace repair or replacement.

The Why, How, and When your Furnace is Short Cycling Matters

A new furnace short-cycling may be attributed to factory defects. A furnace may also struggle to achieve your set temperature against the cold winter wind.

However, there may be underlying causes if your furnace is short cycling due to inexplicable reasons.

Consider the when in your equation if you can't tell why your furnace is short cycling. Does it cycle ON and OFF during winter or summer?

Does it only falter when you set the thermostat to a specific temperature? Did it only start behaving unusually after a certain occurrence?

Take all factors into consideration to help you understand your short-cycling problem.

The Most Common Reasons Behind Your Furnace Short Cycling

Your furnace is struggling to achieve your desired temperature, but it's newadequately sized, and has the capacity to deliver your heating needs.


It still can't do its job properly, shutting down a few minutes after firing up its heating cycle.

Don't worry; a furnace short cycle is a common problem. So common that there are various proven ways to resolve it.

Before we can remedy the situation, we need to know what's causing the short cycling.

Damaged Heat Exchanger

If your furnace has a damaged heat exchanger, your furnace shuts down due to automatic safety measures.

The flue limit switch is an additional safety switch that automatically turns your furnace OFF when there's too much hot air.

The wasteheat coming off damaged heat exchangers with nowhere to go triggers the switch to prevent overheating.

Internal safety systems may also shut down the flame sensor when the heat exchanger is compromised. The flame sensor controls the gas valve and is connected to the pilot light.

Without the flame sensor instigating heating cycles, all motors cycle off to prevent components from triggering engines to generate heat.

Furnace Blower Motor

The blower motor powers fans, facilitating air coming through supply lines and going out exhaust vents. They also distribute warm air through air vents.

If the flame sensor shuts down, blowers and motors should stop working. That may seem like an obvious thing to say but read the next part carefully.

If your heat exchanger is damaged, they shouldn't be any air coming from your air vents, not your indoor handler or exhaust vent pipe.

If there's still cold air going through your ducts or vents, that's another story.

Defective Flame Sensor

Yes, we just mentioned flame sensors' role in short cycling. However, for flame sensors (AKA the thermocouple), shutting everything down is the proper response to safety measures involving compromised heat exchangers.

defective thermocouple doesn't respond accordingly to built-in automated commands, for safety or otherwise. A furnace's flame sensor that isn't functioning properly can cause short cycling or uneven temperatures.

Cold air may come from your vents even when the thermostat is set all the way up when the flame light isn't lit.

The lack of hot air from your vents means the rest of your furnace is performing the normal heating cycle, leaving your thermocouple as the odd man out.

Without the thermocouple triggering the pilot light, your furnace may never reach your desired temperature.

Dirty Air Filter

A dirty flame sensor can stem from poorly maintained air filters. Your furnace cannot generate enough heat when the air filter is clogged.

Your furnace filter isn't like other HVAC air filters. Your furnace has internal air filters to prevent debris and pollutants from getting into the gas valve, exhaust flue pipe, or heat exchanger that may hinder heating cycles.

A clogged flue pipe also impacts airflow and bars distribution from going in or out of the furnace. When enough air or heat builds within the furnace, it may result in hazardous pressure buildup. 

A surplus of hot gases can impact sensors, causing unreliable and erratic flickering.

Faulty Thermostat

The set temperature indicated on the thermostat may be lower than you are comfortable with and is easily adjustable. However, inspect your thermostat for any visible signs of damage, such as dents, frayed wires, or ineligible digital displays. 

A faulty thermostat may trigger the wrong responses from your furnace. It may shorten heating cycles by limiting supply and hindering burners.

The thermistor is a device connected to most thermostats used to measure temperatures. Average wear and tear may lead to heat damage, separating the resistor from an accompanying lead element.

Temperature Readings

If the thermistor is faulty, your furnace may believe the room is already too hot.

An inaccurate temperature reading leads to short heating cycles. The flame light won't burn fuel intake if the thermostat says your home is already at the temperature you desire.

There may not be anything wrong with the rest of your furnace after all. Your furnace may only be responding to a faulty control board. 

However, without any visible signs, you may never know that your thermostat is to blame.

How to Prevent Short Cycling

It's best to keep your heat source steady and reliable before winter rolls around. You can prevent short cycling by ensuring all your furnace systems are well-maintained.

A reliable HVAC professional can prevent your furnace from sustaining damages that may cause short cycling and other heating concerns.

Remember, short cycles may be minor at first. Still, they may lead to worse problems over time.

How a Furnace Short Cycling Affects Your Entire Heating System

A malfunctioning thermostat can hinder internal components and cause others to overcompensate. A short cycle is only one of many problems heating cycles may go through.

While a short cycle may prevent warm air from making your ambiance nice and cozy, heating cycles can also go into overdrive.

Hidden Dangers: Carbon Monoxide Poisoning

You can expect indoor temperatures and monthly bills to spike when your furnace is thrown into overdrive.

However, there are things more dangerous than being uncomfortable. If you were shocked by this month's utility or energy bills, have your heating systems checked immediately.

The internal pressure within your furnace can burst through supply lines while clogging every air vent. Leaking gas lines and blocked air filters or vents allows hazardous emissions to seep into your living spaces.

If you smell rotten eggs or feel stale cold air blowing through your vents, a dangerous gas leak is saturating your home. Quickly and safely evacuate your entire household and report it to the local fire department or gas company. 

How to Deal With a Short Cycling Furnace

Furnace tune-up and repairs are your best bet. We should only dismantle and handle a furnace with the proper tools and techniques.

You may end up doing more harm than good, both to your furnace and yourself. Call a reliable HVAC professional to help remedy your heating problems.

Sunset Heating & Cooling | Electrical

Sunset Heating & Cooling helps residents around Oregon keep things cozy at home, even at the height of winter. Call Portland's finest if you need HVAC maintenance, repairs, and installation services.

Don't hesitate to call If you want an upgrade; our furnace replacement and installations are efficient and reliable.

List of heating services in Portland, Oregon.

Call us now at (503) 500-5866

Observing proper furnace maintenance ensures everything is working properly. Reliable routine HVAC maintenance services can be costly, but they are necessary.

If you want to avoid unwarranted surprises, consider signing up for a membership that you can tap into every time you need a tune-up or repair .

Find out more: Sunset Heating & Cooling Membership Plan for HVAC Services

What Our Clients Say

Sunset went out of their way to help with my project and David was amazing. Great job and look forward to future projects.

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Roman did an awesome job. I had the trifecta, new AC unit ,furnace and hot water heater. Courteous, and competent service. I highly recommend Sunset Heating & Cooling.

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Eduardo provided excellent service when he repaired our AC. I felt his effort was over and above expected. Polite and professional at all times. Good job!

-William G.

Caring, professional and helpful. Thanks for making things happen quickly and getting my Dear friend's heating back up and running in such a quick manner. You went above and beyond for a family in need and that is very much appreciated. Thanks from the whole group!

-Kristina K.

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