I was a bit surprised to see this question, since this should be readily available information elsewhere, so I did a short search to see if there is a problem finding out the answer.  It turned out there is, in that there are a lot of incorrect or incomplete answers out there.  The best answer I came across was this:"All air conditioners try to freeze up, that is the goal of the refrigerant... to chill.It is the design of the system that prevents this freezing up from occurring.There are three basic reasons for air conditioner freeze ups:1) The two most common causes of insufficient air flow are dirty filters and under sized ducts. A dirty filter limits air flow which in turn allows the cooling coil’s temperature to drop below freezing. Replacing the filter, monthly, usually alleviates this problem. Under sized ducts will also cause the system to freezeBecause it also limits the amount of air flowing over the cooling coil. This results in lowering the coil temperature and allowing the humidity in the air to collect on the coil and freeze.2)Refrigerant charge.

The refrigerant system teeters on a delicate balance. If the system is not charged correctly, the system will not function properly. This malfunction may result in the poor cooling output and/or freezing up. Only a qualified, certified service technician can properly test and charge a refrigerant system.3) Outdoor temperature. ac unit cover indoorIf night time temperatures are too low, the system will not operate properly, and freeze up may occur. window ac unit horizontalThe outdoor unit does not function well in temperatures below 60 degrees F. ac and heat units for saleA special “ambient temperature” control must be installed to allow the unit to run properly in cool temperatures, or you may open your windows and save energy."  (, Air Conditioning Freeze Up .)

Too many of the answers given on the web assume it is low charge, when it most likely is poor air flow due to an obstruction, which can be the filter, the coil itself being dirty, blocked vents, closed or dampered vents, or incorrectly set fan speed.  It can also be a symptom of too much refrigerant charge.  If the first thing your A/C serviceman does is hook up a bottle of refrigerant to your system, get a new serviceman.  The amount of charge should stay constant if the unit was installed correctly, so the most likely cause is a dirty filter, and hooking up hoses and gauges will not fix that and will typically lead to some incidental degree of refrigerant loss, especially when done carelessly, compounding the problem. Keep your filters and ductwork and coils clean and you will eliminate most problems with freeze-ups.How do you fix an air conditioner that keeps freezing or icing up? To fix an air conditioner that continually freezes or ices up, check things like the air filter, refrigerant level, blower motor, evaporator coil and ductwork.

If the cause is not immediately clear, the homeowner should contact a local heating, ventilating and air conditioning, or HVAC, professional. An air conditioning unit freezes up because the evaporator coil is being cooled more than it should be. Doing a few quick checks can help determine what the root cause is, and define an easy fix.Check the air filterA clogged air filter causes the system's temperature to drop because there is not enough air flow through the conditioner. Replace the filter every 2 to 3 months.Check the refrigerant levelNormally, Freon levels stay the same. If there is a leak, however, the refrigerant level will drop. If the pressure drops too low, it causes the unit to ice up.Check the blower motorIf the unit is running but the indoor system is not, that indicates the motor blower may not be working correctly. This lack of airflow may cause the system to freeze.Check the evaporator coilLack of airflow can also be caused by a dirty evaporator coil. It causes a temperature drop and icing up of the system.

Check the duct workThis involves going into the attic and inspecting the duct work for leaks, bends or disconnected sections. A blockage in the duct decreases airflow, resulting in freezing. A split air conditioner system is one in which the evaporator coils and air handler are physically separated from the condenser coils. These are also known... To replace an air conditioner evaporator coil, disconnect the unit, examine it for damage, cut the tubing to remove the coil, and then insert the new coil ... Where do you find used furnaces for sale? Where can you buy a thermostat for radiant heat? What items come in a Frost King heat cable kit? What causes fluctuation in AmeriGas propane prices for consumers? What is resistance heating wire? It is never normal to see ice during the summer anywhere on the inside or outside of a heat pump or a central air conditioner. This includes the indoor unit, outdoor unit and interconnecting line-set. It is possible to ice-up the indoor coil however, if the air conditioner is running in very cold weather or if the thermostat is turned down extremely low.

We recommend never turning the thermostat below 70 degrees when running your air conditioning. This could potentially cause your indoor coil to frost or even freeze up, and/or possibly cause your ductwork to sweat, which could lead to water or mold damage in your walls or ceilings. If air conditioning is needed during winter months, such as for restaurants or businesses, then a "Low ambient kit" is required, and can be installed by a service technician. If you see ice anywhere on a heat pump or air conditioner during the summer, then there is most likely a problem and you should turn it off immediately. Bad indoor fan motor- not running/running slow Loose, worn, or broken fan belt Bad indoor fan relay Extremely dirty blower wheel Extremely dirty or damaged indoor coil Clogged or blocked air filter Supply and/or Return vents blocked or closed Running air conditioner with windows open Setting thermostat too low One other thing to keep in mind: If you have a central humidifier, make sure it is shut off and if it has a damper - close it for the summer.