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&S Mechanical Services explains it briefly at
http://toad.net/~jsmeenen/ice.html:

"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've seen this before and haven't experienced a wet floor,
you'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:

"What to do if your air conditioner freezes up"
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 +"air conditioner" -"refrigerator"

This is a common problem with an air conditioner or dehumidifier. If
ice is forming that means that the temperature in the coils has to be
at or below freezing. Rather than being short of gas your system is
probably doing what it should but because of a high relative humidity
ice is forming. Many systems have sensors that adjust the system set
point so that the compressor operation is controlled to prevent icing.

I suggest finding out if your system has such a control system and if
it's operating correctly

Though I beleive that your question has most probably been answered ,
The most probable reason for an air conditioner to operate normally at
higher temps, but freeze at lower temps is that the system is over
charged ( how ever a lack of refrigerant can cause symptom) and
excluding air flow problems --such as a door not being on the blower
compartment or pluged coil---- The correct method for chargeing to be
used should be the superheat method-- if your serviceman doesn't know
what super heat is you need to get some one else. If an AC system is
charged to cool more than (a suggested) 20 degree split(the difference
of air coming into system, and air going out) at high temperatures,
then it will freeze at lower temps. in regards to darren, No Way----
If a system is properly charged-- also  humidty is removed interiorly
by condensation, thus the hose to the drain, humidity only lowers
efficiency--- most home ac systems don't have an econimizer--- Never
the less your problem is most likely related to charge, or airflow- if
it is not airflow you are most probably overcharged-----check with a
professional HVAC technition and remember super heat. thanks E-mail me
gtrdddy@aol.com.

P>S> Koz answer is in reference to Ice on the condensor---which is the
outdoor coil---- In my answer it is in regards to the inside-- a good
first looking spot. tks. call a service tech