Air conditioner's air handler freezes at night

I live in a garden home with a split-system central air conditioner. During the summer we keep the temperature at 74F but cool it to 66F during the night. From time to time during the night the air handler will turn into a solid block of ice, blocking airflow and causing the temperature to rise. What causes this and what can I do to prevent it while remaining cool at night?
Thanks for your question! The primary cause of an air handler freezing up is because there is a low refrigerant charge in the air conditioning system. Scott Meenen of G%26amp;S Mechanical Services explains it briefly at http://toad.net/~jsmeenen/ice.html: %26quot;This is usually caused by a lack of refrigerant in the system due to a chronic leak. The reason that the coils form ice is that when the system is short on charge part of the coil runs very cold and ice starts to grow. Once the ice starts to grow it is in insulator and keeps on growing until the coil and the refrigerant lines are one block of ice. If you keep running the system while it is frozen, you run the risk of damaging your compressor. The best thing to do is to turn off the compressor and just run the fan to circulate room-temperature air over the coils until the ice melts and falls off. Switch your thermostat from AUTO/COOL to FAN to manually run the fan with the compressor turned off. The melting ice will need a place to drain, make sure that your evaporator coil has the proper drainage setup in place. There is usually a hose or tube that runs from the air handler to a floor drain or evaporation pan. If your system does not have this, be prepared to soak up the water as it comes off the coils. Since you%26#39;ve seen this before and haven%26#39;t experienced a wet floor, you%26#39;re probably in good shape. Refrigerant loss from an air-conditioning system is obviously indicative of a leak somewhere, refilling the refrigerant every time your system is low will not solve the long-term problem. You will need to consult a repairman and determine the real source of the coolant leak. There are other, secondary causes that can be causing your air handler to freeze up, such as an airflow blockage (obstruction in the air ducts, dirty air filter, or dirty evaporator coil). You may want to examine the entire system and make sure everything is clean and dust-free. Running the system when it is too cold outside may also cause ice in your system. How cold does it get at night where you live? Your system may also have the wrong size evaporator coil or wrong size compressor for the system, but only a qualified repairman can know for sure. Hope that helps, and stay cool! Sources: %26quot;What to do if your air conditioner freezes up%26quot; http://toad.net/~jsmeenen/ice.html WARMAIR.NET - Air Conditioner Troubleshooting http://www.warmair.net/html/troubleshooting.htm Carrier (Thailand), Frequently Asked Questions http://www.carrier.co.th/html/faq.html Search Terms Used: air handler freezes condenser freezes +%26quot;air conditioner%26quot; -%26quot;refrigerator%26quot;