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Furnace Not Blowing Air?  A Furnace Troubleshooting Guide from ASM.

Furnace Not Blowing Air?  A Furnace Troubleshooting Guide from ASM.

If you’ve read our articles, then you know that we are a small, U.S. Veteran-Owned heating and air conditioning company, and pride ourselves on giving people honest, straight answers to their questions.  This will be no different – if your furnace is not blowing air on a cold day, it can be concerning.  We get more and more furnace questions this time of the year, as temperatures drop, and people transition from air conditioning to heating.  But what should you do if your problem is a furnace not blowing air?  We have already addressed what to do if you have a furnace blowing cold air, or when your furnace keeps turning on and off repeatedly in different posts, but in this article we will be addressing what to do if your furnace is not blowing air through your ducts at all.  As with many of these furnace problems, there are many reasons for your furnace to stop blowing air, some of which you can troubleshoot yourself and some of which will require a licensed HVAC contractor.  In this article, we will address what to do if you have a furnace not blowing air.  We will do this by explaining the possible causes, in order of likelihood; starting with the most likely causes and working our way down the list.  We will then discuss what troubleshooting steps you can take to get your furnace blowing air again.

Is Your Furnace Not Blowing Air?  Here’s How Your Furnace is Supposed to Work.

furnace not blowing air - how furnace worksIf you’ve read some of our posts, then you know that I don’t like to dive into a topic without first explaining how that system should be working.  For today’s topic, let’s give you a basic understanding of how your furnace works so we can then figure out what might be the likely cause of a furnace not blowing air. It is important now to point out that we are only talking about a furnace today, not a heat pump.  If you have a heat pump, or are unsure of which your system is, first take a look at: Heat Pump vs Furnace.

A furnace works by a very simple principle, it uses some type of fuel (i.e. electrical, oil, gas, etc) to heat a series of coils.  What type of fuel your furnace uses for this combustion won’t be important for today’s discussion.  This series of heated coils (called a heat exchanger) has a fan (called a blower motor) blow your house’s cold air over them and into your air ducts where it is dispersed throughout your house.  This simple process, regardless of what type of fuel you use, is how your furnace is able to blow warm air into your house.  Now this may seem simple enough, but believe me when I tell you that there is a nearly limitless supply of things that can go wrong with this process.  Today we will address the most common causes of a furnace not blowing air, and what you can do to fix it.  For more information on how your furnace works, here is a great article from Popular Mechanics: How your heating system works.

‘Simple’ Troubleshooting of a Furnace Not Blowing Air.

We will work this problem just like I would as your HVAC repairman, going from the most obvious causes to some of the less likely problems.  As we begin to troubleshoot, it is important to check for a couple of things:

1. Check Your Thermostat

It seems like this would be a no-brainer, but I can’t tell you how many times we get called out to a house only to find that the husband keeps turning the thermostat down and the wife keeps turning it up.  Check with your significant other, and make sure your temperature is set correctly!  If it is not, set your thermostat for five degrees higher than the temperature in your house and see if the furnace’s fan comes on.  If this step is unsuccessful, continue to check the other possible causes of your furnace not blowing air:furnace not blowing air carrier furnaces

  • After checking your thermostat’s temperature setting, ensure that it is in ‘heat mode’ instead of air conditioning.  Again, it seems simple but as programmable thermostats get more and more complex, so too will the number of ways that you can mess them up!
  • If you have a programmable thermostat, check that the date and time are correct.  This seems simple, but remember that the point of a programmable thermostat is to save energy.  It does this in the winter by lowering the temperature of your home when it thinks you are at work or in bed.  If the time is off however, then your furnace might think it is 2pm on a Tuesday when it’s actually 10am on a Saturday, causing you to freeze over your morning coffee and suffer a furnace not blowing air all because of a miscommunication between you and your furnace.
  • Replace the batteries in your thermostat.  The most likely cause of an improperly programmed thermostat is a power outage (even a short one) with a dead battery, causing the whole system to reset itself.  In fact, it is possible for your thermostat to dump it’s settings even if the battery is just low so make sure to replace your battery next to ensure that this is taken care of as you move on to more complex problems with your furnace troubleshooting.

If your thermostat is shot, consider spending a bit more and go for a new Smart Thermostat – here are links to some reviews of a few of our favorites:

  1. Nest vs Lyric Review
  2. Ecobee 3 Review

2. Check Furnace Circuit Breakers

I can’t emphasize this enough, but I’m honestly not trying to treat you like an idiot.  I am going off of experience here as to the most likely causes and some of these might not even be your fault. Have you had electrical work done lately, or a new water heater installed?  It is possible that your furnace circuit breaker was turned off so go to your main power distribution panel (circuit breaker panel) as your next step.

Check that the circuit breaker to your furnace is not off, but also understand that it is not uncommon for your blower motor (the fan) to have a separate circuit breaker, so check this as well.

Next, go to your furnace and check on the furnace itself and surrounding walls for another on/off switch or circuit breaker and ensure that it is on as well.

If any of these are are off, turn them on once (and only once, for safety) and allow your system to run.  If they trip off again then this is a sign that there is a problem with the wiring of your system and you will need to call out your local HVAC contractor.  Do not keep resetting faulty circuit breakers to your furnace as this is dangerous and could start an electrical fire.

heater not blowing air3. Check Your Furnace Air Filter

Your next step should be to check your furnace’s air filter as this can cause a furnace to stop blowing air for a couple of reasons.  First, your filter could be so saturated with dust and debris that air is not making it into your ducts causing you to think that you have a furnace not blowing air when in fact, it is blowing air but your clogged filter is blocking it!  This is a bit less likely, but some people don’t change their filters as regularly as they should.  You should be changing your furnace air filter every three months or so – for more information on this, take a look at: Air Conditioning Filter Change.

The second problem with a clogged furnace air filter (and far more likely) is that restricted air flow out of your furnace can cause your furnace to overheat.  Every modern furnace is equipped with a safety shutoff (which will be discussed later) that will be tripped if your furnace gets too hot.  Simply put, a clogged air filter can cause your furnace to overheat, shut itself off and give you the illusion of a furnace not blowing air when, in fact, it is your air filter that is causing your furnace to turn itself off as a safety precaution.

Unless it is brand new, your next step in troubleshooting a furnace that isn’t blowing air should be to replace your furnace’s air filter.

Advanced Troubleshooting of a Furnace Not Blowing Air

Don’t let the terms ‘simple’ and ‘advanced’ throw you off, I’m not talking about your experience level.  You have to perform the above troubleshooting steps before progressing to this section otherwise it might not work!  Working your way from easy to hard will ensure that you’ve covered some of the bases necessary for advanced troubleshooting.  Then continue on by utilizing the following steps in order:

furnace not blowing air blower motor

Furnace Blower Motor (Fan)

1. Check Your Furnace Blower Fan

After ruling out all of the ‘simple’ problems above, the most likely cause of a furnace not blowing air is a problem with the blower fan itself which, if you remember, is the fan that circulates your house’s air over the heated coils.  To check your furnace’s blower fan for proper operation, start by turning your furnace to the ‘off’ position.  Next, you will turn your thermostat to the ‘fan only’ mode.  There are a couple of ways to do this depending on your thermostat, but the two most common are:

  1. Turn heat to ‘off.’  Then change fan from ‘auto’ to ‘on.’
  2. Or, some thermostats will have a ‘fan only’ position.

Wait for at least 2 minutes as some thermostats have a decent delay between input and execution.

If your fan turns on, then you now know that your blower fan is not the problem (because it’s running well by itself), and can move on to the next step.

However, if your fan does not turn on then go back to your circuit breaker panel and check if the circuit breaker has tripped.  If it has tripped, then there is likely a problem with your wiring and you will need to call out an HVAC technician.

If your circuit breaker is not tripped, then the problem is likely to be either your thermostat itself or your blower motor (fan) has completely failed and consideration should be made to calling in an HVAC technician or replacing the thermostat with a new one (I’d start with a new thermostat because it is the cheaper option, so don’t let an HVAC contractor talk you into a new blower motor right off the batt.  Always work from cheap to expensive if you have no way of ruling one out as more or less likely).

On a side note, if you are spending the money, I’d recommend you take a look at the newest generation of programmable thermostats – smart thermostats, which are thermostats that will actually learn your behavior throughout the week based on your location and adjust your temperature for comfort and energy efficiency.

furnace not blowing air limit control switch

Furnace Limit Control Switch

2. Limit Control Switch

Remember that you should not be jumping straight to this section!  This is a latter step in a multi-step troubleshooting process.  In fact, skipping straight to this step is the equivalent of a doctor diagnosing you with a rare West African disease without first ruling out simpler diseases like the common cold.  If you have followed the steps above, and have now gotten your blower fan to operate by itself but it still won’t run with your furnace in ‘heat’ mode, then I’d suspect that the culprit is the limit control switch on your furnace.

What is a Limit Control Switch and How Can it Cause Your Furnace to Not Blow Air?

The limit control switch is one of those parts that can have several different names (aka, fan limit switch, furnace fan limit control switch), which can be frustrating.  Rest assured that they are all the same thing.

In simple terms, a limit control switch is just a thermometer attached to a switch, and it is the switch that tells your blower motor (fan) to turn on and off.  When your furnace is off (if you turn it off, or if it is warm enough in your house that your furnace doesn’t need to run), this is what senses that there is no heat being generated by your heat exchanger and tells your fan to turn off (because if it didn’t, it would be blowing unheated air throughout your house and wasting energy).  If it gets warm from your furnace turning on, then it kicks your fan on to circulate the warm air.  This is how your limit control switch works in it’s normal function.

However, it also provides a secondary safety roll in that it is also designed to sense when your furnace is too hot and shuts it off as a safety precaution, including your fan (this is why you replaced your air filter in the ‘simple troubleshooting of a furnace not blowing air’ section).  So whether your limit control switch is shutting down your furnace because it thinks your furnace is overheating, or because the limit control switch itself is damaged and needs to be replaced, I’d say that replacing this switch is your next best step in troubleshooting a furnace that is not blowing air.

Here is where you are getting into the realm of just calling your local HVAC contractor.  I would say that it is possible to replace this part yourself and it will save you around $150, but if you aren’t reasonably handy then I’d skip it.  If you are set on doing it yourself, for more information on how to replace your limit control switch, take a look at this short video: Limit Control Switch Change – Furnace Not Blowing Air

If you have replaced your limit control switch and your furnace is still kicking the blower motor (fan) off, then you have a more serious issue with your heating coils and should call out an HVAC technician.  Don’t freak out – it isn’t the end of the world and it doesn’t necessarily mean that you need a new furnace, but we’ve already proven that your blower motor (fan) is working, and you just replaced your limit control switch, so it’s time to call out the pros.

Final Thoughts on Troubleshooting a Furnace Not Blowing Air

I hope this guide has helped you troubleshoot your furnace.  In the end, there are several things that can lead to a furnace not blowing air.  At the end of the day, it may just be time to replace it.  If this is so, here’s an article that will help get you started: Top 10 Furnace Brands.  Start with the basics like I showed you above, and work your way down the list to more complex problems.  If you work your way through this troubleshooting guide and your heater still isn’t blowing air then I’d suggest you call your local HVAC contractor.  I realize that it is unlikely that you live in our service area of Santa Clarita and Greater Los Angeles, so I’ve included a guide to finding a reputable HVAC contractor in your area: How to Choose a Contractor.  For more information on other related articles and troubleshooting guides, take a look at our ASM Air Conditioning Blog.  If you live in the Santa Clarita, San Diego, or the Greater Los Angeles area, click below to contact us:

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

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