If you are referring to the outside unit freezing up, the issue is in the defrost cycle. The defrost cycle works something like this: The unit reverses the flow of freon causing the inside coil to blow cold air into the conditioned space and the outside coil is now working like an a/c unit, it gets warm to melt the frost/ice. The inside unit electric heat strips come on so you do not notice the cold air being blown in. The fan on the outside unit stops running to allow the coil to get good and warm. Now the unit should go into the defrost cycle every 90 minutes or so. It should not terminate the cycle untill the coil is clear of frost. There is an "Klixon" type termostat that measures the temperature of the outside coil. This is most likely the problem. It is prematurely terminating the defrost cycle. I'd start there first.
Hope this helps, Let me know if I can be of any further help.
Answers & Comments
If you are referring to the outside unit freezing up, the issue is in the defrost cycle. The defrost cycle works something like this: The unit reverses the flow of freon causing the inside coil to blow cold air into the conditioned space and the outside coil is now working like an a/c unit, it gets warm to melt the frost/ice. The inside unit electric heat strips come on so you do not notice the cold air being blown in. The fan on the outside unit stops running to allow the coil to get good and warm.
Now the unit should go into the defrost cycle every 90 minutes or so. It should not terminate the cycle untill the coil is clear of frost. There is an "Klixon" type termostat that measures the temperature of the outside coil. This is most likely the problem. It is prematurely terminating the defrost cycle. I'd start there first.
Hope this helps, Let me know if I can be of any further help.