If the servo is moving then the "system moves" or it should.
As one goes from hot to cold the center duct should shut-down and after the servo goes a little farther the heat appears. It may not even get to heat if the day is too hot. if the center vent doesn't close then its either not working properly or the system isn't "moving" far enough.
As to moving. Inside the servo is a clockwork mechanism,many gears working to drive the vacuum distribution valve (like a little maze on on side and all the vacuum holes on the other - as it rotates the holes are connected to different compartments in the maze), the electrical switchboard (drives the varying blower speeds cold - 5 hi down to zero then 4 hi - hot), and the the water valve allowing hot water flow. They are all geared together and they run a range that is all cold to all hot. Thisis driven by a small motor, as long as the calculation say go hot the motor is driven one direction. It stops when it thinks it has it right.
The decision to be evaluated is whether it "moved" the proper amount and whether things are chnging together (just because the servo links the vacuum doesn't mean it doesn't leak.
Bosch Master, ASE Master, L1
33 years MB technician