8 Common Reasons Why Your Air Conditioner Is Freezing Up

Have you ever cracked your AC unit to the lowest setting and noticed that it doesn’t cool your home very well? Or have you noticed ice buildup on the evaporator coils and pipes? Under certain conditions, the air conditioner unit can freeze. Even on hot summer days with temperatures above 100 degrees in the shade.

Intuitively you may presume that such freezing is not a problem, AC is supposed to cool your house. But in reality, such freezing prevents it from properly functioning.

It may sound odd, but frozen AC will push warm air into your home if it pushes any at all. And there is nothing less pleasant than being greeted by a tepid air when you need a cool breeze.

How Air Conditioners Make Cold Air

The most common type of air conditioner available on the market is the vapor-compression refrigeration system. It is based on the effects of phase changes of the coolant.

All matter exists, under normal conditions, in one of the three basic states of matter or phases, solid, liquid, and gas. With the change of external and internal conditions, such as temperature, pressure, and so on, the matter will change its state.

AC systems have five main parts, expansion valve, evaporator, blower fan, compressor, and condenser. As the coolant passes through the expansion valve, its pressure drops suddenly, and with it also its temperature. Then the coolant moves through the evaporator, and the blower fan pushes the warm air over it. This causes the coolant to soak up the warmth from the air and thus cool it and your home too.

Next, the coolant moves to the compressor, which compresses it and increases its temperature above its boiling point and the environment’s temperature. This so-called superheated vapor state allows coolant to be very quickly condensed and considerably cooled with air or water surrounding the condenser. The coolant exits from the condenser as a liquid and continues to cycle through the system.

How to Tell if your AC is Frozen

If you notice that the air is coming at a slower rate from the vents or that the air which blows is lukewarm no matter how low you set the thermostat, it might be that your AC has frozen. The first and most obvious sign that your air conditioner is frozen is the ice buildup on the AC unit.

In case that there is none, you should open the access panel per the manufacturer’s instructions, previously making sure that the AC is turned off and that the circuit breaker is also in the off position. Once you have it open, you can inspect the condenser, evaporator, and drain line for ice.

8 Common Reasons Why your AC is Freezing Up

There are several reasons why your air conditioner may freeze. Many parts of it are considerably colder than the environmental temperature, which causes condensation on many of its surfaces. If your AC unit is functioning properly and your whole AC system is working, it doesn’t present any problem.

But if there are certain problems, condensation will start freezing and interfere with how your AC functions.

1. Dirty Air Filters

The most common reason why AC freezes are dirty air filters. As the air passes through your AC, it gets cooled and, as such, is pushed through the ducts and into your home. It is cooled because the cooling liquid inside the evaporation coil captures heat from the moving air and turns in gas inside the coils.

But if there is not enough air movement, coils will become extremely cold and start frosting. If left to go on for too long, this frost will build up and impede the cooling.

Air filters are relatively easy to replace, and you should be doing it at the recommended intervals suggested by the manufacturer. Or even more often, depending on the condition of the filter.

2. Closed or Blocked Vents

Another cause of insufficient airflow inside your AC unit can be the cause of it freezing. Your home air conditioning system has two types of vents. Supply vents that transport cooled air from your AC to your rooms, and return vents that transport warm air from your rooms to the AC unit.

The supply vents are usually closed by a grill with a lever used to fully or partially close them and control the airflow in each room individually. If you wish to maximize the flow of cold air in some rooms, you could by accident close the supply vents, thus decreasing the airflow over evaporation coils.

This will have the same effect as a dirty air filter and lead to your AC unit’s freezing.

Another problem can be with return vents. They often do not have a lever for closing them but can be blocked by other things. Curtains, rugs, bookshelves, and other furniture can partially or fully block them.

3. Dirt on Evaporative Coils

Another thing that can disrupt the airflow effects on the evaporation coils is the buildup of dust on them. Dirt, if not cleaned from coils, can even completely choke the airflow. In case that the coil is clogged by dirt, you can notice ice buildup on the pipe exiting from it.

The air conditioners recirculate the air from your house, and all the dust particles, pet hairs, and other light debris; will find their way to it eventually. They can very easily start accumulating on the fins of the evaporative coils, and over time start to impede the airflow. Like other disruptions of the airflow, this will lead to coils going to cold and freezing.

4. Faulty Thermostat

The air conditioner functions in cycles of turning the blower fan on and off. Depending on the temperature you have set, the thermostat will turn on the fan until it is reached. Then it will shut the fan down and allow for temperature to climb few degrees before turning it back on. This operation greatly saves energy but also the parts of AC units and thus prolongs their lifespan.

For various reasons, such as with ACs with old, inefficient, or dirty coils, you may be just asking too much from your AC and have set too low a temperature. In case that your AC cannot reach the set temperature, the fan will never shut off. This can cause coils to become too cold and freeze.

Another situation is if your thermostat is broken. If the thermostat is malfunctioning, it can either never turn on the blower fan, which will mean that there is no airflow over evaporator coils, and it will freeze the same as with a dirty filter. It could never shut down the fan, which can lead to the evaporator becoming too cold and freeze.

5. Using AC when it is not Needed

If the outside temperature too low, you may still be tempted to use your AC as a dehumidifier. But the air conditioners usually work very poorly when the environmental temperature is not sufficiently high. Evaporative coils must have sufficiently warm air to circulate over them to prevent their freezing, and if you run your AC when temperatures are below mid-60s Fahrenheit, there is a danger of just that.

Unless your AC unit has a dedicated mode of operation as a dehumidifier, you shouldn’t use it this way when not necessary.

6. Low Coolant Level

Another common cause of air conditioner freezing is a low level of cooling fluid in the system. This may sound counterintuitive, as more coolant should mean more thermal capacity of the system and thus higher ability to cool, while less coolant should lead to the opposite. It is very logical to presume that a low coolant level should prevent evaporative coils from becoming too cold and freezing.

Unfortunately, air conditioning doesn’t work exactly that way. If there isn’t enough refrigerant in the system, it will lead to lower pressure in it. Because of the laws of thermodynamics, this will lead to lower temperatures in the coil. If the coolant level is sufficiently low, the pressure drop may bring its temperature well below the freezing point of water at 32 degrees Fahrenheit.

If coils become this cold, they will cause moisture from the air to condense on them due to temperature differences and start freezing. And this will lead to AC freezing.

7. Faulty Blower Fan

The fan blows warm air over the evaporative coils. This way, the air gets cooled before being returned to your rooms. But this has another effect. As the coolant traps the heat from the airflow, it warms up, and it does sufficiently to prevent any condensation from frosting on coils.

In case that blower fan is not well, blowing the air over the fin stack for any reason, it will become super cold, and it will freeze after a while.

In case that you suddenly sense the smell of burning plastic, rubber, or hot metallic smell, it could indicate that the blower fan’s motor has burned. If there are some banging or rhythmically clanking noises coming from your vents, it could indicate several possible malfunctions of the fan.

Suppose it periodically switches between humming and silence, or the intensity of humming sound rhythmically changes in intensity or pitch. In that case, it could mean that the fan is very near the failure point. Of course, if the fan is completely silent, no matter how low you set the temperature, it just isn’t working.

8. Improper Size of the Ducts

If you have just replaced your AC unit, you may have installed a unit that has too high a cooling capacity for your ductworks by mistake. This is a very common mistake, as people gauge what AC unit to buy only by its cooling capacity.

If the ducts are too small, there will not be enough airflow to prevent ice buildup on evaporative coils, and the air conditioner will freeze.

How Long does it Take to Thaw a Frozen Air Conditioner?

It greatly depends on the amount of ice buildup and the environmental temperature. Unfortunately, the air conditioners often freeze when needed to provide respite from the summer days’ oppressive heat.

Still, fortunately, those are conditions that greatly speed up thawing. Depending on the amount of ice, it can take as little as an hour, but in some cases could last until the next day.

And while you are waiting on your AC to thaw, there are two things you should be keeping your eye on.

Drain Pan

The drain pan’s purpose is to catch any condensation water on the evaporation coils, and it empties into the condensate drain. Because it’s never designed to accept such a heavy flow generated by the thawing of the frozen AC, you should keep an eye on it. It can easily overflow.

In case that it is not easily accessible, you should place towels and rags around the unit to avoid any water damage.

Condensate Drain

This quite unassuming pipe serves to remove any water buildup from your AC and dump it safely into the ground where it can’t do any damage. As the air conditioner is thawing, any debris from coils and smaller pieces of ice can clog it. If it happens, it will cause the drain pan to overflow, and you should be prepared to clear the drain.

How can you Keep your Air Conditioner from Freezing Up

Keeping the AC unit from freezing is done first and foremost by following the manufacturer’s preventive maintenance recommendations.

You should check the amount and pressure of the coolant regularly checked. Once a year is enough. If your AC leaks, you shouldn’t just top it off and call it a day. You should also fix the leak itself.

Air filter you must change at the manufacturer’s intervals, which is most of the times once a month. Also, it would be best if you used only recommended replacement filters. They are guaranteed to work correctly with your make and model of AC. Any aftermarket or all-purpose filters could perform at a less than desirable level and create problems with the airflow.

Keep all vents unblocked and open. In case that you absolutely must increase the cooling of a limited number of rooms, be careful not to constrain airflow too much.

Because of the physical properties of the way that air conditioners function, certain parts of them, particularly the evaporative coils, can have ice buildup. This is called simply freezing of the AC and can cause your unit to stop functioning properly. Most of the reasons this happens are due to a lack of preventative maintenance or some smaller faults easily removed.

Thus to avoid this happening, you should follow the manufacturer’s recommendations for your particular unit to a tee.