I think that when your ac button is on you need to have your heating controller set to cool & and it's better to have the recirculate button on aswell so it's only pumping cold air around car instead of drawing in air and having to cool it down all the time. as far as I'm aware the ac doesn't work if you have any windows open. (This info might be completely wrong, I'm only going on what I was told). My ac is very poor in my 2007 GT TDI 140. I was planning on popping into kwikfit and seeing about getting it re-gassed, this could be the problem.