Just had the waterpump replace by VW dealer. Now the A/C compressor engages, but blows hot air as soon as you turn it on, then the next time you start the car it may blow cold air from the start. When driving it will blow hot air then will blow cold air. It does this on its own as you are driving with the A/C on as well as when you speed up occassionally.
A/C Works Intermittenly on 2005 Volkswagen Passat
The dealer should have a warranty on the water pump repair and the work they performed. They may have loosened a wire behind the AC Compressor. I would have them recheck their work...