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Furnace Keeps Turning On and Off Repeatedly?  The Guide to Troubleshooting a Short-Cycling Furnace.

Furnace Keeps Turning On and Off Repeatedly?  The Guide to Troubleshooting a Short-Cycling Furnace.

If your furnace keeps turning on and off repeatedly, it can be stressful at best, and cold or downright uncomfortable at worst.  First, it is important to realize that a furnace which is “short-cycling,” as we call it in the business, probably means that its safety features are working properly, so breathe a sigh of relief.  In fact, in all likelihood, that’s why the furnace keeps turning on and off – it turns on normally, then for some reason, it thinks that an unsafe condition exists, and turns itself off again as a safety precaution.  But let’s not wait for those safety features to fail too…you definitely want to find the source of the problem.  Those of you who have read our articles, know that we are a small, U.S. Veteran-Owned heating and air conditioning company in California, and pride ourselves on giving straight, honest answers; we are straight-shooters, and this will be no different.  Today, we will discuss the most common causes of a furnace turning on and off frequently, and some easy, do-it-yourself troubleshooting steps that you can take to get your furnace up and running properly again.

Furnace Keeps Turning On and Off Repeatedly – But Why?

Furnace keeps turning on and off repeatedlyWhen your furnace keeps turning on and off repeatedly, this is called “short-cycling.”  It turns on for a few seconds to a minute, then turns itself off again.  After a few seconds or minutes, the furnace then clicks on again, and the cycle repeats itself.

Now, you have to realize that a furnace turning on and off frequently, in itself, is a subjective matter.  After all, what does “frequently” mean?  We’ve had our guys called out to houses because the furnace was turning on and off three times in an hour…so, to save your time, and ours, let’s define our terms:

  • A properly sized furnace should turn on and off between THREE and EIGHT times per hour, deepening on the characteristics of your house, outside air temperatures, and other variables.
  • Despite what you are told, if your furnace is turning on and off more than eight times per hour, this does NOT mean that it is short-cycling.  It might just mean that is is cold outside, is sized a little small, or you need some more insulation.

Contrary to hat you might think, however, the frequency with which your furnace turns on and off is not so much what we are concerned with.  We are more concerned with what is causing it to turn on and off so frequently.  Frequency of start-ups is not the same as short-cycling, which is actually characterized by how long the furnace runs for when it does turn on.

A “short-cycling” furnace (when your furnace keeps turning off and on) will typically only run for a minute or two (or less) before shutting off.  Whereas, a properly functioning furnace will run for several minutes per cycle.  In other words, turning on and off frequently is just a symptom of “short cycling.”

So, before you spend a bloody fortune calling up a furnace repairman like me, in the middle of winter, you need to do two things: first, see how long your furnace runs for before automatically turning off, and second, go through the troubleshooting steps below.

Please keep in mind, that this article is about a furnace that keeps turning on and off repeatedly.  If your furnace won’t turn on, or won’t turn off, then you might find these articles useful instead:

Why Your Furnace Keeps Turning On and Off

Below, we will troubleshoot the various causes for a furnace that keeps turning on and off every few seconds.  However, it is important to understand how your furnace actually works (at a basic level) before troubleshooting – here is the 101 course.  In short, the blower motor (the fan that moves air around your house) pushes air over your heat exchanger, which is really just a bunch of tubes that have burning fuel passing through them (making them hot, therefore heating your house).  unlike air conditioners, it’s really that simple.

What’s not simple, are all of the safety features that go into making sure this very simple device doesn’t burn a house down and kill a family of four (joking tone – there’s no reason this can’t be fun).  Your furnace has a multitude of safety features, and this is very likely the cause of your furnace turning on and off, so don’t get too upset – it’s better than starting a fire.  So, let’s take a quick look at the ignition cycle and the safety features included on your furnace – this may be a bit nerdy…sorry.  Here’s everything that your furnace does in order to turn on:

  1. Induced draft motor (IDM) turns on – the IDM is a small fan that blows out all of the air that is inside of your furnace, and circulates it up the exhaust flue.  This ensures that there is no gas build-up inside of your furnace from a small leak.  Otherwise, when you turned your furnace on and the flames ignited, you could have an explosion.
  2. IDM pressure switch (typically a vacuum switch) checks to ensure that the induced draft motor is actually running.  If the IDM is not running, it cannot be removing built up gas inside of your furnace, and therefore, this second safety feature will turn your furnace off before it even ignites.
  3. Assuming that the induced draft motor turned on, and was independently checked by the IDM pressure switch, your furnace will now energize the igniters, create a flame, and introduce fuel into your furnace.  Obviously, if you were just dumping gas into your furnace, but the gas was not being burned, this could be dangerous when the flames finally do come on!  So, your furnace also has a Flame Recognition System, including a flame sensor which detects the presence of a flame.  If no flame is present within 4 second, the system will shut off.
  4. Only after al of these steps have taken place, will your furnace begin to run in normal operation (which is why it take a while for your furnace to turn on after you put it in heat mode).

Now, many of you must be thinking after reading about all of these safety features, that your furnace keeps turning on and off because one of these has failed.  This, however, is not true.  If any one of these features were true, then your furnace wouldn’t even be turning on in the first place.  But it is – after all, this is about a furnace that keeps turning on and off, isn’t it?  The reason I told you about these features was to show you that your furnace is built to be inherently safe.  Next, we will discuss the most likely culprit for a furnace turning on and off repeatedly – the High Limit Switch.

Furnace keeps turning on and off repeatedly - high limit switch

Furnace High-Limit Switch

High Limit Switch – the Likely Cause of a Furnace Turning On and Off Repeatedly.

“Why all this technical jargon, Tim?  Just give me the answer.”  Hold your horses sparky – there is a method to my madness.  After all, I used to be a Navy flight instructor…so trust me, knowing the basics of how this thing works will help you a lot when we start troubleshooting – I digress.

A high-limit switch is a switch that turns your furnace off if it gets too hot, and it is the most likely cause of your furnace turning on and off repeatedly.  We’ll discuss more on this later.

Flue Limit Switch – When Your Furnace Keeps Turning Off and On

A Flue Limit Switch is a switch that turns your furnace off if air is not properly ventilating out of the exhaust flue.  Why would this be a problem?  A couple reasons: first, it means that hot, newly combusted gasses are building up inside of your furnace, hindering its performance, and possibly introducing a dangerous situation; second, heat will continue to build up in the furnace, tripping the high limit switch.  Again, redundancy is the key to safety, so this is another feature to help aid the High Limit Switch in keeping you and your family safe.  If this is obstructed, it will also turn your furnace off – more on that later.

Troubleshooting Guide – Furnace Keeps Turning On and Off Repeatedly

So, we’ve taken a look at the fundamentals of how your furnace works, and what safety features are present to keep you and your family safe.  But let’s put that information to use.  In this section, we will go through our troubleshooting process step-by-step, addressing when a furnace keeps turning on and off frequently.  The order we go in is important, as this will be written in order from the most likely causes, to the least likely causes of a furnace repeatedly cycling on and off.

Step 1 – Check Your Air Filter if Your Furnace is Cycling On and Off Repeatedly

furnace turns on and off - air filter - old vs new

On the left, a brand new air filter. On the right, the same type of air filter that is only 3 months old, and being replaced at the proper service interval of 90 days.

Look, I know that most of you think that your furnace air filter is to make the air that you breath more healthy and comfortable, but this just isn’t the case.  Sorry, but you’re not that important (kidding).  Although this is definitely an ancillary benefit, the primary reason for a furnace air filter is to keep the internal components of your HVAC system (including the furnace), clean.

Imagine, for a moment, that your furnace heat exchangers – the actual coils that heat up the air for your house – are caked in years of dust and debris from your home.  Aside from creating a disgusting smell in your house, it can actually be a source of combustion.  No, it won’t burn your house down (I don’t think…), but nevertheless, you should be changing your air filters every three months at a minimum, and more often in dry, arid climates where there is a lot of dust in the air.

Why a Clogged Furnace Air Filter Can Cause Your Furnace to Turn On and Off Repeatedly

If you remember from our orientation up above, the most common cause for your furnace to keep turning on and off is the High-Limit Switch, which turns the unit off if the temperature inside gets too high (as a safety precaution).  Now, since I know that most of you just scrolled past the first section and went straight here – to the good stuff – the instructor side of me is telling you: “no…do it the right way, and read the above part first…I wasn’t writing it for my health.”  In all truth, it is worth the time and will help you better understand the troubleshooting process.

Furnace keeps turning on and off repeatedly - clogged air filter

A 2-year-old air filter, severely clogged and blocking proper air flow to the furnace. Click to enlarge.

If your air filter gets too clogged – such as when people don’t replace their filters often enough – the filter then becomes kind of like a wall, blocking proper air flow to your furnace heat exchangers (the things that get hot).  “So what, Tim, who cares?”  (Don’t be a comedian – I care).  You have to realize, that the furnace is tuned to inject a specific amount of fuel into the combustion tubes, and that the blower motor – the fan that actually blows your household air over these tubes to warm your house – should be providing a constant flow of air over these tubes.  However, when your filter clogs up, then not enough air gets to the tubes, they don’t get the opportunity to shed their heat into your house, overheat, and eventually turn your furnace off via the High-Limit Switch.

So, in essence, the most common causation for when a furnace keeps turning on and off, is a clogged air filter.  Turn the furnace off, replace the air filter (with something quality), and turn it on again.  90% of the time, this will fix the problem.

For more on the importance of air filters, try:

Step 2 – Check Your Thermostat if Your Furnace Keeps Turning On and Off Repeatedly

The second most likely suspect when your furnace keeps turning on and off, is your thermostat itself.  Thermostats, just like cell phones, computers and other electronic devices, can go bad with time.  As such, we will need to check it.  If you aren’t that handy, that’s ok – this is easy.  All you’ll need is a small flathead screwdriver.

So, in its most basic function, all a thermostat does is touch two wires together.  If it’s too cold, it touches the two wires together that turn the furnace on until it isn’t cold anymore.  If it is too hot, it touches the two wires together to turn the air conditioner on.  That’s it.  It is basically a thermometer with the ability to touch wires together.  So, the easiest way to test if your thermostat is bad, is to take it out of the equation.

  1. Furnace keeps turning on and off repeatedly - White and Red wires

    Touching the WHITE wire to the RED wire will turn your furnace on without the thermostat.

    Remove the face plate of your thermostat.  This is usually done by pulling on it firmly…it takes some strength sometimes, but be careful not to pull it out of the wall.  Look at the sides or bottom and see if there are any clips required to remove the front of your thermostat.

  2. Next, let’s talk about a few safety things: A. these wires are low-voltage, so they will give you little bite if they touch your skin, but aren’t going to hurt you.  If you aren’t used to working with wiring, get a pair of rubber gloves, get someone who is, or stop being such a baby…unless you have a medical condition, if they shock you, it won’t really hurt you – but take precautions anyways; B. Never, EVER touch the RED wire to the BROWN wire.  The center of the sun will implode, and we will all be sucked into an inescapable gravitational vortex that even Stephen Hawking couldn’t have predicted…or, you’ll blow the fuse inside of your furnace which is a huge pain to replace…so, basically the same thing.
  3. Unscrew the RED wire, and the WHITE wire only (sometimes they are attached by clips on some of the newer thermostats, and you can just push the clip and pull on the wire to remove it).
  4. Carefully, touch the WHITE wire to the RED wire and hold them together for a few minutes (5 minutes).  You are checking to see if the furnace still turns on and off repeatedly.
  5. If the furnace turns on and runs smoothly, then you have a bad thermostat.  Get a new one and replace it.  If it still does the same short-cycling routine, then reinstall your existing thermostat wires and thermostat face plate – your thermostat is just fine.  NOTE: the best way to touch the two wires together is with a set of alligator clips.  If you don’t know what these are, then you don’t have them – just touch them together.  However, just realize that you need to do this firmly – if they disconnect – for even a second – it will turn your furnace off, voiding the test, and you’ll have to start all over again once there is a 4 minute safety delay.

NOTE: if you only have four wires, then your thermostat is battery powered – make sure that you replace your battery to check for proper operation.

If your thermostat was the problem, you might want to read these articles before buying a new one:

Step 3 – Check the Furnace Blower Motor if Your Furnace Keeps Turning On and Off

Although uncommon, it is possible for your furnace to run while the blower motor is off (blower motor is the fan that circulates air around your house).  If this were the case, air would not be blowing over your heat exchangers, and your furnace would be going off on High-Limit, just as with step 1.  The easiest way to check this is to put your hand up to the vents and see if there is air coming out when it is operating.  If the air is weak, or nonexistent, then this is likely your problem.

The only solution to this, unfortunately, is to call out an HVAC technician to examine it in more detail.   Although this is more common in older furnaces (modern ones don’t usually allow this to happen for safety reasons), it is still worth checking.

Step 4 – Check Your Flue if the Furnace is Still Turning On and Off Repeatedly

furnace turns on and off frequentlyThis one will be difficult for a novice, but if you are a veteran do-it-yourselfer, you can pull this off.  When the furnace is operating, go on your roof (carefully) and see if there is gas coming out of the flue for your furnace.  The gas will be extremely hot, so be careful – don’t just stick your hand on it.  If the air feels weak, or there is no air coming out at all, then turn the furnace off, and take a peak down the tube to see if there are any objects blocking the exhaust.

The most common culprit is a mouse, or other critter, that has deposited some branches in there.  If so, clear them out.  Your furnace is going off on the Flue-Limit Switch and should work properly after being cleared out.  NOTE: always be careful when walking on your roof or working with hot gasses.  If you are not 100% sure how to do this, we’d recommend calling a professional.

Step 5 – Your Furnace Might be Too Big

The final culprit, unfortunately, is going to be the most expensive.  If your furnace keeps turning on and off repeatedly, it might just be sized improperly.  A furnace that is too large will kick on, heat your house rapidly, and turn off again.  Or, it will overheat because it has nowhere to push the hot air to – it was made to do a house much larger than yours.  For this, unfortunately, you’ll just have to call in a professional.

Step 6 – If Your Furnace is Still Turning On and Off, the Last Thing to Check is the Flame Sensor

If you remember our introduction section above, we talked about a lot of the safety features that your furnace has.  The last thing I would consider checking before calling in a professional HVAC technician is your flame sensor.  Here is a good video on just how to do that:

Final Thoughts on When Your Furnace Keeps Turning On and Off Repeatedly

In the end, there are a multitude of things that can cause your a furnace to turn on and off.  When your furnace keeps turning on and off repeatedly, it can be frustrating, but hopefully, we were able to help you out today.  For more topics like this, check out our ASM furnace blog.  If you live in Southern California, you might be in our service area, so don’t hesitate to give us a call – click below for more information.

Click Here to See If You Are in ASM’s Service Area.

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