What exactly does the heater lever do? I mean that one that points to a
blue circle on the left and a red circle on the right. The manual simply
says 'Lever B - heat output'.

I have tried it at both extremes and it does not seem to make a difference.
There is always full heat (which comes out when I let the air flow with
Lever C).

For example, is the lever meant to control a valve restricting coolant flow
to the front heat exchanger?

Lionel Tun
84 Caravelle GL
lionel@ark.prestel.co.uk