Every winter we receive inquiries about refrigerant lines (in particular, the liquid line) either “sweating” or “frosting” inside the house when the weather gets very cold. People think that the air conditioner is “running”, yet when they look outside, it is off.
This phenomenon is caused by the migration of refrigerant in the lines, boiling off and causing the lines to sweat or frost.
In most cases, this is caused by the crank case heater on the compressor driving the refrigerant out of the oil and causing it to migrate into the lines. The crank case heater is doing what it is designed to do and normally does not present the sweating or frosting issue in milder weather. However in very cold weather, there is more refrigerant in crank case of the compressor because refrigerant will always go to the coldest part of a system. The crank case heater drives the added refrigerant out of the crank case into the lines and the lines act as an evaporator coil and begin to sweat or frost because there is no “air” blowing over the line.
The other possible cause is the little bit of heat generated by the use of “single pole” contactors keeping a winding powered and the windings “warm” the oil causing the same issue.
The simplest remedy to this phenomenon is to turn off the power to the outdoor unit, disabling the crank case heater or disconnecting power to the un-switched winding and stopping the migration of the refrigerant into the lines. This will stop almost every instance of lines sweating or frosting inside the home.
If you do turn off the breaker, REMEMBER that it should be turned back on at least 24 hours prior to turning on the air conditioner in the spring.
York Central Tech Talk 
Thanks for this information. This was the only information I found about the liquid line sweating in the winter. I just experienced this as we had a temp change from 65 degrees Fahrenheit yesterday to 15 degrees overnight. If it continues to sweat, I’ll turning off the power to the outside unit.