The idea of an AC compressor cover is wrong because condensation, not weather, damages the unit. It should not have a winter cover for central air compressors.
The store is in a dilemma regarding central AC covers. Customers want to cover their AC compressors for the winter and we do not want to sell them.
First, I want to clarify what unit is being discussed. The topic is the central air compressor unit, not a window air conditioner. The compressor is on a pad next to the house, connected to the furnace with copper and electrical lines.
It is designed to be waterproof, not moisture proof.
Condensation forms inside the compressor when it is enclosed with an airtight cover. Corrosion begins when water droplets form inside the cabinet, the fan, and the relay. Do not cover the unit unless your compressor manual or the manufacture recommends doing so. Let it breath, good air in, moist air out. After all, it is designed to be in the weather all year.
Our suggestion is to put a flat cover over the top of the unit and leave the sides open. If you do have a cover, do not pull it all the way down to the base of the unit. Tie or tape it so the bottom four inches or more are exposed. If you have any questions or doubts, call your service man and follow their instructions.
We might not sell many covers, but we hope to get you back in the spring for coil cleaner and hose nozzles. The better air flows through the radiator (the coils) the more efficient the unit will perform.
Window air conditioners, on the other hand, should be covered to prevent drafts into the house. If the window air conditioner cannot be covered or sealed from the outside, seal the unit on the inside. An inside cover can be fabricated with a reusable storm window tracking and two-mil plastic sheeting.
Come spring, remove the plastic sheeting and leave the track on the wall for next winter. The tracks are reusable for many years to come. One advantage to this system is the plastic is inside the house, tight as a drum, while the wind and weather beat on the window and air conditioner outside.