BMW - 325i :: Air Conditioning Blowing Hot Air

Last summer, my air conditioning was blowing hot air. I took it to the mechanic where they added refrigerant. It worked. Last week, it was blowing coolish warm air on the driver side and warm air on the passenger side. I took it to the same mechanic where they added refrigerant. Now, it blows very cold air on the driver’s side and cool air on the passenger’s side. I inquired about a leak, but they said that they didn’t see one, but I could take it back in a couple weeks where they could run a dye to determine whether there really is a leak. They also mentioned that I might need to bring it every year to have refrigerant added.

I read a few articles and have concerns that they would just add refrigerant, (especially a second time) without determining the root cause of the problem. I’m also wary that they would suggest bringing it back every year and at a cost of $110, especially when I read that an unrepaired leak could have an adverse impact on other related components. Prior to reading this I was thinking of adding coolant myself every year for closer to $35. I’m also wondering why the driver’s side is blowing colder air than the passenger’s side and whether there could be a larger issue than just a leak.