Why Is My Air Conditioner Freezing Up?  How to Fix Your Iced Up Air Conditioner

Why Is My Air Conditioner Freezing Up? How to Fix Your Iced Up Air Conditioner

We are often asked the question, why is my air conditioner freezing up? Figuring out what is causing your air conditioner to ice up can be frustrating, but it is not an uncommon problem. If you’ve read any of our articles, then you know that we are a small, U.S. Veteran-Owned company in Southern California, and pride ourselves on giving honest, straight answers. In fact, most of our posts address either one of two things: first, common air conditioning problems that people have, or second, the questions we get asked most frequently.  There are a number of reasons for an air conditioner to freeze up, and it really doesn’t matter whether it is a window AC unit or a central air conditioner – the causes of, and solutions to, an air conditioner that keeps freezing up are exactly the same. It is important to address this issue – a frozen air conditioner will not only cool improperly, but can actually be severely damaged if it keeps freezing up. If you have this problem, you have to address it immediately – it won’t solve itself. In this article, we will tell you what causes a central air conditioner freeze up, and what you can do to fix it.


The Joule-Thomson Effect – How Your Air Conditioner Freezes Up

air conditioner freezing up

Standby for a small high school physics lesson. We don’t need to go too in depth, so bear with me, but understanding this principle is the key to figuring out why your air conditioner is freezing up. The Joule-Thomson Effect is a principle in thermodynamics which states that if everything else is kept constant, then as a gas expands, its pressure decreases, and its temperature will also decrease. Likewise, as a gas is compressed, its temperature will increase.

This is because what we sense as temperature is really just a measurement of the amount of energy a molecule has. Simply put, if air molecules are bouncing around rapidly, then you would feel this air as hot. If air molecules are moving around slowly and sluggishly, then you would feel this air as cold. It’s that simple.

So what? This is important to understand because it forms the basis for how your air conditioner works. Your system works by allowing your evaporator coil to expand refrigerant inside of it, causing the coil to cool rapidly. As this rapidly cooled coil comes into contact with your home’s air (which your air conditioner blows over this now cooled coil), the refrigerant absorbs the heat from the air inside of your home.

This newly heated refrigerant is then transported outside of your home where it is compressed, causing it to heat up and transfer the heat that was once inside of your home to the outside air. The cycle then repeats itself over and over again, cooling your home in the process. For more information on the Joule-Thomson Effect, visit this page on Wiki: Joule-Thomson Effect.

air conditioner freezing up diagram of how it works

Why is this important? Well because this effect is used to allow your air conditioner to cool the air inside of your house. But any time you change part of the equation, you might throw the system out of whack. So to keep this simple, your air conditioner is freezing up because something has altered your system in such a way that it is cooling your evaporator coil more than it should, causing the unit to freeze up. Now that you know how, what you have to do is figure out why your air conditioner is freezing up, and to do this you have to figure out what it was that threw your AC system out of whack. We will do this in the next section.

If you want to delve more into how exactly it is that your air conditioner cools air, take a look at: The Joule-Thomson Effect – Why Your Air Conditioner is Freezing Up, or just watch the video below:

Why is My Air Conditioner Freezing Up?

Now that you have a basic idea of how your air conditioner uses expansion and compression to heat and cool its Freon (what is Freon?), you can figure out why your central air conditioner is icing up. Your air conditioner can freeze up for a variety of reasons, so we will discuss them from the most common to some of the more rare causes of ice up.

All of these causes are rooted in the Joule-Thomson Effect discussed above, so if you skipped past it, I’d recommend that you take a quick look or just watch the video above. Simply put, your air conditioner is freezing up because something is either causing the Freon in your system to expand more than it is designed to, which in turn is making your coils colder than normal, or something is preventing the air in your house from transferring its heat into your system. Either way, this causes your evaporator coil to drop below freezing and your air conditioner to freeze up. Here are some common causes:

window air conditioner freezing up

1. Insufficient air flow across your air conditioner’s evaporator coil can cause it to freeze up.

In a split air conditioner (the most common type – see: What is a Split Air Conditioner?), our air conditioner is designed to drop the temperature of your house by taking heat from it. If your air conditioner is not properly blowing your inside air over the evaporator coil, then this heat exchange cannot take place and the Freon that should be removing heat from your home is not. Over time, its temperature will continue to drop and your air conditioner can freeze up.

2. If your air conditioner is low on refrigerant, then your air conditioner can freeze up.

This is due to lower pressure inside the system itself. This is because now there is less Freon in your system, but it is still being forced to expand the same amount – more expansion equals a cooler temperature. As your air conditioner’s evaporator coil drops below freezing, the moisture in your air will freeze as it comes into contact with your coil causing it to ice up.

3. Air conditioners are designed to function in a very specific temperature range.

If you are running your air conditioner when the outside air is below around 62 degrees, then the pressures inside of your system will drop and this can cause a freeze up of your central air conditioner.

4. Mechanical failures can cause your air conditioner to freeze up.

If there is a kink in your refrigerant lines, a blower fan that is not running or is dinged and out of balance, or a clogged filter dryer? These too can change the pressure in your system and make your central air conditioner freeze up.

But which is it, and how do I fix it? How to troubleshoot an air conditioner that is freezing up will be discussed in the next section. However, if you’ve already determined that it’s time to replace, make sure that you don’t overpay. This program will save you money, and accept or reject bids with confidence:


How Do You Fix a Frozen Air Conditioner? AC Troubleshooting.

air conditioner keeps freezing up - AC troubleshooting chart

Just because your air conditioner is freezing up does not mean that you have to call your HVAC contractor! Some contractors will tell you that they are the only ones that should touch your AC unit (I have never believed that), and if you don’t feel comfortable around your air conditioner then you should definitely call out a contractor. Either way, the first thing you need to do is turn your air conditioner off and allow it to defrost. Ice can harm your air conditioner! Next, if you do feel comfortable and if you have even a basic touch of the handy-man bug, then there are a few things that you should take a look at before you call your local air conditioning repair service. In order to troubleshoot a frozen air conditioner, I’d recommend you do the following steps in order:

I would start by ensuring that you never run your air conditioner when the temperature is below 62 degrees outside. If you are, then stop it and see if this fixes the problem. This is the simplest thing to fix but can cause big problems. Next, if your air conditioner is freezing up from restricted air flow, then you need to rule this out – the best way to rule this out is by checking the following:

air conditioner freezing up outside

1. Check your air conditioner’s air filter, if your AC is freezing up.

This is one of the most common causes for restricted air flow through your air conditioner. You should be using a high quality air filter and replacing it every 2 to 3 months. If your air conditioning filter gets clogged, then the restricted air flow can cause your system’s temperature to drop below optimal levels, causing your air conditioner to freeze up. 

For more information on which filters to choose and how often to change them, see: 

2. Check your evaporator coil if your air conditioner is freezing up.

A dirty evaporator coil (the inside one) can cause a lack of airflow across it, dropping the temperature and causing your air conditioner to freeze up. It is also a recipe for several other unpleasant problems with your AC, like Dirty Sock Syndrome. I would recommend cleaning your evaporator coil as your next step in troubleshooting an iced up air conditioner. You can learn about how to do that in this article:

3. The next thing to check is a restriction in air flow from your ductwork.

air conditioner freezing up frozen evaporator coil

Start by opening all air conditioning registers (the vents in your ceiling). Any restriction in air flow can cause your air conditioner to freeze over, so make sure that you also inspect your ductwork for leaks, bends or disconnected portions. To do this, you will likely have to go into your attic. Be careful – if you’ve never walked in your attic before, then ask a friend who knows how to and make sure you only walk on the joists!

4. Check for extremely dirty ductwork if your air conditioner is freezing up (as in so dirty that it blocks air flow – a little dust is okay).

If all of your ductwork is in order, then remove one of your registers and look inside. I am not a big fan of having your air ducts cleaned, but it can be necessary at times if you or the previous owner didn’t properly use high quality air filters. Personally, I’d just have the ductwork redone – cleaning your ducts doesn’t really work that well, and can damage your ductwork. For more information on whether you need your ducts cleaned, try:

You might also be interested in:

If your air conditioner is still freezing up after the above steps have been taken, then I’m afraid you’ve done all that you can in the do-it-yourself category. Call out an HVAC contractor. The next likely causes are a low pressure system due to low refrigerant levels, or too much refrigerant, which can only be recharged by a licensed HVAC technician (at least where we are in Santa Clarita), or it is also possible that you have a problem with your blower fan or refrigerant lines; neither of which you will want to mess with yourself. At this point, I would give your local HVAC contractor a call (and if it is time to buy a new air conditioner, make sure that you take a look at this article first: The Best Time of Year to Buy an Air Conditioner. It is unlikely that everyone who is reading this is in All Systems Mechanical’s service area of Santa Clarita or Greater Los Angeles, so I’ve included an article to help you choose a reputable air conditioning contractor in your local area: How to Choose a Contractor.

Don’t Overpay for Your New Air Conditioner

If you’ve performed the steps above to no avail, then it may, unfortunately, be time to think about getting a new air conditioner. Make sure to check with your local HVAC contractor first, but I wouldn’t talk to anybody without first getting the facts...


Excerpt Page 3 of 12 from the HVAC Design & Consultation Program Report

Final Thoughts on an Air Conditioner Freezing Up

We often get asked the question, “why is my air conditioner freezing up?” Hopefully this article has answered your question and provided you with some useful information about what can cause your air conditioner to freeze up, as well as how you can fix it. A frozen air conditioner not only does’t work properly, but it can damage your unit. Think about what ice does to roads over time – it expands and damages them. It splits them up, and can do the same thing to your air conditioner. This won’t fix itself, so turn it off, follow the above steps, and you will have a healthy air conditioner that runs for years to come. For more information on frozen air conditioners, and other air conditioning problems that you can troubleshoot yourself, visit the ASM Air Conditioning Blog. If you found this article helpful, All Systems Mechanical services all of Southern California, from Santa Barbara and to Northern Orange County. Click below to see if you’re in our service area!


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Timothy Kautz
About the author
Tim K.

The University of Virginia - 2005 / The Wharton School of Finance - 2016 / U.S. Naval Aviator 2005-2015. At All Systems Mechanical air conditioning and heating, we believe that the experience our clients have is every bit as important as the products they receive. Simply put, our results speak for themselves, and we'd be happy to help. If you're in the market for a new AC or furnace, make sure that you get a fair price! Try our online calculator; click the tab on the top of this page for more information.

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