If your central air conditioner is freezing up then I am here to go over the reasons that this may be happening. Some of the causes you may be able to fix yourself, while other causes will require you to call for service. I am going to list the possible causes that I have personally ran into, in order, from most likely to least likely. This is all based upon my personal experience from working on central air conditioning units.
Low Refrigerant Charge
Central air conditioning systems require specific temperatures to transfer heat properly. If the system is low on refrigerant then the unit could freeze up. There is no set operating pressure for R-22 or 410a to where a unit will freeze. Factors such as humidity and air flow come into play.
From my experience though if everything else is okay, and you have a reading around 50-55psi on R22 or 90-100psi on 410a, the unit will freeze up, or build frost. Think saturation temp on the suction line here, if it’s around 32 degrees F, this is freezing. This is a problem you should not try and fix yourself unless you are EPA certified. Call a trained technician out to evaluate the refrigerant charge to fix your air conditioner freezing up.
Keep in mind, that if your refrigerant charge is low then you more than likely have a refrigerant leak, which is only going to cost you money in the long run. This is especially true if you have an older central air conditioning system using R-22. Locate the leak and fix it, then charge the unit per manufacturer specs.
Dirty and Clogged Filters cause Air Conditioners to Freeze up
Check the air flow. Each household will vary on how often a filter should be changed. I’ve found that the more people that live in the home, along with pets, the faster air filters can get clogged. It is best to check them monthly until you get an idea of how often you should change your air filter.
If your air filter is clogged, that means your evaporator coils are probably dirty as well. If you run your central home air conditioner without any filter installed at all, especially for long periods of time, then this is another reason why your central air conditioner may be freezing up.
There have been some rare instances that I’ve see where the evaporator coil gets smashed in. Although sometimes the fins can be ‘straightened’, sometimes the coil will just have to be replaced, such as the one pictured below.
Not enough Heat Load
I am currently living in Colorado. I work at an apartment complex, and I have to deal with several air conditioners freezing up. Many times the tenant will turn the a/c down to 68 degrees when the outside temperature is below that. On air cooled systems this is going to cause the air conditioner to freeze up. Central air conditioners are for removing humidity, not for creating ice boxes or freezers.
Indoor Blower Motor Not Running
If you have a indoor blower motor that is not functioning, and the compressor is pumping refrigerant, your unit is going to freeze up. When I used to do hvac service professionally, it was usually a bad capacitor which would cause this. Or just a crap motor, such as many variable speed indoor blower motors.
Refrigerant restrictions/metering device
If there is a refrigerant restriction inside of the air handler, this may be a reason why your air conditioner is freezing up. Although in my experience this seems to cause air conditioners to freeze up the least compared to the other reasons stated. Some other weird situations may be that the refrigerant line set too long, or sized improperly. Kinked suction lines, can also cause central air conditioners to freeze up. This is something I have seen in high-rise apartments and condos.
What do I do If My Air Conditioner Freezes Up?
Turn your compressor off. This can be done by setting the thermostat to the ‘off’ position. Then set the ‘fan’ to the ‘on’ position, if your indoor blower motor is running and not the cause of course.
By setting the fan to the ‘on’ position it will help thaw out the frozen evaporator coil. Depending on the location of the air handler, this is also going to help not make a mess of getting water every where.
By doing this, as a homeowner, you will save yourself money, and your technician time, in troubleshooting why your air conditioner is frozen up. When there is a thick block of ice on your air handler coil, there is not much your hvac technician can do, until the air flow is corrected.
Depending on how long the unit was running at low pressure for, it may take an hour or two, to completely thaw out. I’ve seen some pretty extreme cases where a unit was running with low pressure and freezing up for months. Obviously this took longer to thaw out.
After a couple of hours check all the symptoms stated above. I would start with the filter, and also make sure that there is nothing blocking the return air grill as well.