biggurs

Hi my centre vent is only blowing hot air intermittently, mainly when revving the car. The idrive setting for my centre vent is set at hot. So I'm thinking there may be air pocket in the coolant so I want to bleed it but can't seem to locate the bleed screw.
2004 e60 530d.
Another peculiar thing is that the passenger side centre vent is blowing cold and drivers side hot (but only when I rev) but the passenger side is always cold. Blocked pipe on passenger side? Is the passenger side centre vent independent of drivers side center vent? ie if there is a fault with centre vent will it affect both or can it only effect one centre vent? Can someone help please.
I topped up coolant last night as it was a bit low but couldn't find bleedscrew. But same prob again this morning