AC blowing hot air from centre vents, what to check?

This car model for example:
2001 BMW 330ci

A/C blowing hot air from centre vents. (The knob is set to the blue dots). Cooler air comes out of the side vents but not cold. If I turn on the feet vents they are hot.

Possible Reasons:
Low refrigerant from a leak or faulty water valve

What to check:
Quote: It was nice when I first started the recharge then added some more while the engine was warming then I just got hot. I’m guessing its the hcv.

Advice to do: Meaning, you probably would have been fine if you had taken it to a shop in the first place but now you are hosed. Your only option is to leak-test, evacuate and recharge the system. With professional equipment. Forget the HCV until you’ve charged the A/C properly. Oh, and stop making guesses on a system that can burn you for thousands of dollars when you screw it up. Take it to someone who has the proper knowledge and tools to service it.

Successful Case: Take to shop for repair:
I have warm air on passenger side and a little cooler on drivers side. I’m taking in to the shop for recharge. What they do is evacuate the refrigerant and using the scales install the exact amount of refrigerant. Then they look at the gauges and if there is a rapid drop in level happens then you have a problem – there is a leak somewhere. The repair is usually expensive. If there is no drop then all is good. There is probably a small leak somewhere but you can probably get 3-4 seasons with icy cold A/C. That’s the way a mechanic explained to me.

Now, those DIY freon kits are not recommended. If you overfill the system your compressor can lock – not a good thing. Happened to my dad actually. It’s better to pay $120 (that’s what they quoted me) and have it done right.