Why does my AC System keep Freezing Up?


An outdoor AC condenser with a frozen copper refrigerant line.

Hey Houston! Is your air conditioner frozen, freezing up, and not operating? Or has it ever frozen before? Or, perhaps you're just wondering why air condtioners freeze up and want to know why, whatever the reason may be, today we're going to go over why air conditioners freeze up, what you can do to prevent you're AC system from freezing up, and what happens if you keep running the air conditioner if it happens to be frozen.

So if you're air conditioner is frozen, make sure you shut it off as soon as you can to prevent any further damage to any other components in your AC system. When an AC system is freezing up, this is definitely a sign that there is a problem, so let's dive into what's happening and why this problem can occur and what we can do to prevent it.

Ok, my unit is freezing up, I shut it off, why is it doing this again?

The reason air conditioners freeze all boils down to 2 main problems:

  •    Lack of Air Flow   
  •    Lack of Refrigerant   

So.. What exactly do you mean when you say "Lack of Air Flow" or "Lack of Refrigerant"?

1) Lack of Air Flow means the system is freezing up because it is not receiving the required amount of air (or heat in that air, rather) it needs to keep it from freezing up.
2) Lack of Refrigerant is referring a system freezing up because it is low on Freon, A.K.A "Refrigerant".

In order to understand why an AC system freezes, we need to first understand what an AC system does, and that means we have to start thinking in terms of Heat - AC systems pump the invisible yet uncomfortable heat out of our homes by absorbing the heat out of our home's air, and pumping that same heat outside and blowing that same heat outside of our homes via the outdoor unit. If you've ever hovered your hand over the top of your outdoor unit while it's running and felt the air blowing out of the top of the unit, you may have noticed the air is very warm. Well, why is that air warm? - You may have thought. All the hot air you feel blowing out of your outdoor AC unit when you hover your hand above the outdoor unit happens to be the heat that used to be in your home - That is to say, your air conditioning system is just sucking the invisible heat out of your home, and blowing it outside, which is how it makes your home feel colder inside. It may be hard to imagine because you can't see it happening, but once you start thinking in terms of Heat and even better, removing heat you'll start to understand exactly what your AC system does and how it works.

So since your air conditioner is removing heat out of your home (when it's working properly), it first has to have a heat load inside of your home to remove in order for it to actually remove that heat load from your home, right? So, when a system is freezing up, there simply isn't enough heat being absorbed out of your home's air to keep the system thawed, and thus it freezes.

A frozen evaporator coil in an AC system. For example, say we want a piece of ice to melt, we have to add heat to the ice in order for it to melt, right? If we don't heat up the ice, then the ice doesn't melt. When you turn your air conditioning system on, it begins to make the freon inside of the system very cold, so, the freon inside your air conditioner has to be kept in a constant "melting" state, other wise as the system runs it gets so cold that it beings to freeze.

So imagine if you will, that your air conditioner has to constantly be kept in this constant "melting" state other wise it freezes - If there is no heat available in your home for your air conditioner to remove, or it's having trouble removing the heat from your home, the entire system will stop "melting" and begin to freeze. The freezing typically starts inside your home, within your system's indoor evaporator coil and as the AC system continues to run it forms ice, the ice grows and eventually works it's way all the way out to the outdoor unit.

Ahh, I get it. My AC system gets cold in order to suck heat out of my home because hot goes to cold, right? So what exactly should I be looking for that is causing my system to freeze?

Dirty Air Filters

A dirty AC air filter.

Dirty air filters happen to be the most common reason why an AC system freezes up. We often forget to change our air filters and since all the air in our home (along with all the dust within the air) passes through these air filters, they get dirty. Once the filter gets dirty enough, it becomes clogged, which puts the AC system in a state of lacking air flow.

With a dirty air filter, the AC system can no longer pull enough air through the filter to keep it in it's "melting" state, and when it cannot pull enough air through the filter, it cannot absorb heat out of the air, and in turn the system freezes.

Remember how I said earlier that the air conditioner's freon has to be kept in a constant "melting" state by comparing it to an ice cube? If we took a blow torch to an ice cube, the ice would melt rather quickly, but if we stuck a wall between our blow torch and the ice cube, the heat from our torch has no way of reaching the ice to melt it, and thus the ice remains frozen. The same thing happens with a dirty air filter - The air in our home contains heat (the blow torch), when we block the flow of air with a dirty air filter (the wall) from getting into the AC system, the AC system can no longer absorb the heat out of our home (the melting ice cube), and the system freezes up (doesn't melt).

So check that air filter, if it's dirty, change it, allow the system to thaw until you don't see anymore ice, and then turn it back on and you should be good to go. Try to change your air filter regularly, every month if you can remember, because the dirtier the air filter gets, the harder the system struggles to remove heat from your home, and the longer the system runs to remove heat from your home.

Hmm.. My air filters are clean, and I change them religiously, I don't think this is my problem.. What else should I look for that could be causing my air conditioner to freeze?

Crushed or Broken Ducts

A crushed duct in an HVAC ducting system.

Another common reason why air conditioners freeze up is due to crushed, collapsed, or broken ducts. Just like a dirty air filter creates a restriction in the air flow, which doesn't allow the system to remove heat from your home, a crushed duct does the exact same thing, and you get the exact same symptoms, because the system is once again in a state of lacking air flow.

In order to discover these ducting issues, you'll need to crawl around your attic crawlspace with a flash light and examine the system's duct work. You'll be looking for anything sitting on the ducts, or any type of object that is crushing the ducts. Many times we get in the attic to store things and unknowngly set a box ontop of a duct which crushes it and causes the system to enter a state of lacking air flow. Other times, the cable guy or other repair guy accidentally steps on the duct, in turn crushing it.

Old ducts can also collapse internally due to old age, and they often rip apart or the duct tape that holds the ducts onto the AC system gets old and falls apart causing the entire duct to fall off of the AC system or the vents that rest above a home's ceiling. You'll want to crawl around and examine all of the ducts, if you notice any ducts that are crushed, do your best to return them to their original shape. If you notice any ducts that are torn, repair them as best as you can with duct tape by sealing the air leaks. If you notice any ducts that have come loose or fallen off the AC system, do your best to reattach these ducts back to their original location. Sometimes ducts are also suspended from the ceiling with straps, and over time these ducts begin to sag around the straps that support them from the ceiling, these ducts also need to be returned to their original shape by loosening the strap and eliminating the sag or adding more straps to evenly distribute the ducts weight. For more examples of crushed, collapsed, broken, torn, crimped and fallen ducts and what they look like so you can easily know what to look for, check out our Prioritizing AC Ductwork and Air Flow section of this site.

Well that's good to know, but all my ducts are in a row and my system is still freezing. What's next?

A malfunctioning Indoor Blower Motor

An HVAC system indoor blower motor.

Is your indoor blower motor running at all? Try this - With the AC system turned on, go up to one of the vents, can you feel any air being blown out of the vents? If no air is being blown out of the vents, you may want to pay the furnace a visit and see if you can hear the fan spinning. If the fan doesn't come on when the AC system turns on, then the AC system cannot move any air throughout your home and therefore it cannot pull any air into it's componenets and is suffering from a state of lacking air flow.

Once again you get the same symptoms you see with a clogged air filter or a crushed duct because the same exact thing is happening like you see with a clogged filter or a crushed or broken duct. No air is passing through the system, therefore no heat is being removed from the air, and once again the system begins to freeze up. Remember: We have to have warm air passing through the HVAC system to keep it from freezing, and when this doesn't happen, the system freezes.

So if your blower motor isn't running, it's always best to call an AC company out to determine why unless you're familiar with the sequence of operation and how to diagnose and repair HVAC systems. It could be a number of electrical issues that are causing the motor not to run, or only run periodically, or it could even be the outside unit not shutting off properly. Whatever the case, at this point diagnosing the problem is something you need to do and the cause of the problem can be anything from a broken wire, a malfunctioning thermostat, a malfunctioning circuit board or electronic sequencer, a stuck contactor, a failing blower motor or even a broken blower motor. The problem should be pin-pointed and repairs should be made to whatever componenet is malfunctioning in the system that is keeping the blower motor for operating correctly.

A dirty blower motor wheel. If your indoor blower motor is running, you may want to inspect the wheel that the motor turns inside of the furnace, especially with older furnaces that are more likely to be dirty. The blower motor is attached to a wheel inside of the furnace, and it's just like a ceiling fan - The motor spins the wheel and as it does air is moved throughout the vents and in and out of the HVAC system and your home. If the wheel gets so dirty like the one shown, even if the blower motor is spinning the wheel a dirty wheel simply doesn't allow the wheel to move enough air through the AC system, and once again, the system is in a state of lacking air flow. So even if warm air is moving across the HVAC system, it may not be enough of it, and this is similar to a crushed duct scenario where very little air is being allowed into the system, so very little heat is being taken out of your home. In order to fix this, the wheel will have to be removed, cleaned if possible, or replaced entirely.

I see.. Well all of these things you described seem to be in good working order, is there anything else that could be causing my system to freeze up like it is?

A Dirty Evaporator Coil

A dirty evaporator coil in an HVAC system. The system's evaporator coil may be dirty itself which is preventing the system from absorbing the heat out of your home and keeping it, once again, in a state of lacking air flow.

The entire reason we use air filters is to keep the system's evaporator coil from getting dirty. All the dust and debris in the air in our homes gets trapped in the filter as the AC system runs, and that means it doesn't get sucked into the evaporator coil and cause the coil itself to become dirty, which causes it to freeze up. Air filters are much easier to buy and change than evaporator coils, so that's why it's important to keep your air filters regularly changed. Sometimes however, ducts and chases that make up our ducting system are not completely sealed, or the seals break, which allow dust and debris to enter into the duct work beyond the air filter, in turn bypassing the air filter entirely and getting pulled directly into the evaporator coil and causing it to get clogged.

Just like with a clogged air filter, air cannot pass over the dirty evaporator coil, and therefore no heat can be pulled out of your home and the system begins to freeze up. The evaporator coil will have to be inspected, which could be a task on it's own. Sometimes inspection doors are installed in AC systems for this entire purpose of visually inspecting the front of the evaporator coil. Other times, holes can be cut in the ductwork to examine the front of the evaporator coil. A skilled HVAC technician may also use a tool called a manometer and test an evaporator coil's static air pressure to determine if the coil is clogged and if air is passing through it, other times the coil has to be pulled out and examined for debris.

If the coil is clogged with dirt or if it looks dirty, it will have to be cleaned before any air can pass through it and allow the coil to once again remove heat from your home. This usually involves uninstalling the evaporator coil, taking it outside, cleaning it with a special foaming cleaning solution, and washing away all the dirt that is trapped in the evaporator coil's fins. It can be an expensive repair that is usually avoided if we change our air filters. I have been to homes where people didn't know they were supposed to be using air filters because noone has ever told them and now their coil is dirty and frozen - Every air conditioning system uses air filters, so if you ever find yourself wondering why your AC system doesn't have air filters, then you just haven't located where they go yet, and it's vital you find out before it's too late and you have to pay an AC company to come out and clean your evaporator coil.

Ok. Well my coil is clean, my filters are good, no ductwork issues, and my blower motor and it's wheel are just fine. What else is going on?

Lack of Refrigerant

An HVAC manifold showing no pressure in an HVAC system. The next step definitely involves using an AC pressure manifold to determine if the HVAC system is low on refrigerant. A system that is low on refrigerant gets too cold, and the heat within the air simply isn't warm enough to keep the system from freezing. It may sound bizzare, but pressure and temperature have a direct relationship with eachother and you may remember this fact from science class. As pressure increases, temperature also increases, and AC systems work by regulating the pressure of the refrigerant inside of them at a constant ratio. When there isn't enough refrigerant inside the AC system, this ratio still exists, but it causes the freon to get too cold and causes the system to freeze up due to lack of refrigerant.

If you suspect your HVAC system may be low on freon, or you have had freon put in your HVAC system before, there is most likely a freon leak. Air conditioning systems are never supposed to leak freon, and like a tire with a leak, the leak will continue to cause freon to leak out of the system until the hole is repaired. You'll want to have a good understanding of how air conditioning systems work, as well as the pressure and temperature relationship in order to discover why the AC system is freezing up due to a lack of refrigerant. There could be a leak, or there could be a crimp or a clog in the copper line that the refrigerant in your AC system is pumped through, which is causing it to freeze up.

The leak or restriction in the refrigerant system will have to be pin pointed. Leaks are found using various methods, some leaks can be spotted visually at first, by locating an oily residue and using soap bubbles to locate the leak. Since oil is mixed inside of your AC systems refrigerant, wherever a leak is present, there is usually an oil stain, and a spray bottle with soap bubbles is sometimes sufficient enough to locate the leak by spraying the suspected area with the soap, and if bubbles form, a leak is definitely present. Other times special leak detectors are used, which are devices that will make a loud noise when they "sniff" freon, and by using these leak detectors a HVAC technician can easily pin point the location of a freon leak. Other times, for hard to find leaks, a special dye can be injected into the HVAC system's refrigerant lines, the dye will begin to leak out of the hole in the refrigerant line, and a special UV flashlight and sun glasses is used to locate the florescent glow of the dye leaking through the holes of the HVAC system.

Restrictions can also occur within the copper lines that the freon flows through and lines are sometimes accidentally bent or crimped by falling trees, or people who accidentally step on the lines in attic crawl spaces. If a line is crimped, the portion that is crimped will have to be cut out and replaced. Other times, pressure regulating valves known as expansion valves can begin to malfunction, close completely, or become clogged with debris which will then cause the system to freeze up. When it comes to issues with lack of refrigerant, a professional will have to address your problem due to licensing and regulation of refrigerant systems. So if you need any help in these areas, feel free to chat with us or give us a call and we'll glady take care of it for you.

Ok. So what happens if I fail to address any of these problem and what happens if the AC runs for a long time while it's frozen?

HVAC System Malfunctions from Freezing Up

A freon leak from a frozen coil. One thing you don't want to do (or get in the habit of doing) is allowing your system to freeze, then thaw, then freeze again, and thinking it's supposed to do this and you're manually supposed to shut it off. AC systems are never supposed to freeze and when they do it's definitely a sure-fire sign that a problem exists and so the problem needs to be repaired.

Every time the system freezes, you are causing damage to many of the HVAC system's main and vital (and expensive) componenets. Due to ice forming, the copper lines that the freon flows through will being to stretch and morph, and as the compressor continues to run and attempt to pump freon through the frozen line, copper lines can burst just like frozen water pipes.

Compressors are also going to suffer when a system freezes because they are trying to pump refrigerant through a frozen line. This causes the compressor to overheat because it simply cannot pump refrigerant through the frozen line. This will ultimately end up causing the compressor to get too hot, electronically short circuit, and trip breakers. The compressor is the heart of the whole AC system and the most expensive single component that makes up your HVAC system, which is why if you notice your system is freezing you need to shut it off and protect the compressor for over heating and failing.

Fan motors will go through the same process a compressor is experincing with when a system freezes up - The fan motor simply cannot push or pull any air through the frozen evaporator coil which will quickly become a wall of ice. As the motor continues to run and blow air into this wall of ice, air pressure is created and the fan struggles to compete with the air pressure it is creating by attempting to pass air through the frozen coil. The fan motor will also being to overheat, consume more and more electricity, and ultimately short circuit and have to be replaced.

A frozen AC compressor. No matter what the reason, it's always best to repair the issue that is causing your system to freeze up and many times systems are freezing up due to simple things that you can fix yourself. Many new AC systems come with sensors that allow them to detect if they are freezing, which will cause the system to shut down and show an error on the thermostat so nothing damages the AC systems componenets, so if your thermostat is showing an error, you may want to consult the thermostat manual or check online for what the error is, some newer systems can tell you exactly what is causing it to freeze. Older systems don't have these features and will continue to run until you manually shut them off and if you don't shut them off you are causing damage to some pretty expensive componenents so you want to try and monitor these older systems since they don't monitor themselves. You may get away with running the system for a while, or not, but one thing is for certain, a frozen AC system is not going to cool, is consuming electricity and not working correctly, and is shaving valuable lifespan off of the expensive motors and compressors that pump the air and refrigerant throughout your home.

If you ever notice your system is frozen, try checking out all the easy stuff first that we described at the top of this article and work your way down. Start with the air filter and the crushed ducts, and work your way through our list of symptoms until you simply cannot go any further. If you ever need any help, feel free to contact us at any time via our live chat on this website, or by calling us. We always give free support over the phone or our chat room to anyone and can often help you out and solve your problem without ever having to come out. If you're positive you need a professional to help with a refrigerant issue, or any issue you don't feel comfortable doing on your own, then we can take care of that as well.

Thanks for reading! I hope you learned a few things about your HVAC system today and hopfully if you were having a problem you solved it after reading this. Remember to allow your AC system to that before turning it back on, or it will quickly freeze up again and you will wonder if the repairs you just made actually had any effect. So let it thaw, then turn it on, and check and see if everything is ok. And replace those air filters!

Good luck out there, Houston!
Your friends at Adams Air.