124 Heating System - What so the bits do?

[Haig Tyler]

I have a 230TE with problematic heating.  It seems very difficult to set a mid-heat; it's either full cold or roasting hot.  There also seems to be some delay in it going from one extreme to the other.  There is still control to each side (ie I can have it full cold left and full hot right, and vice versa).

To help me solve this problem, I'd really like to know what all the bits are that I've seen in the Haynes manual and in related thread, eg:

-  What's a duovalve?
-  What are the temperature dials connected to - and are they manual or electric or vacuum?
-  What does the electric circulation motor do?
-  If it has left, right and interior temp sensors, can I assume that it's all thermostatically controlled?

All other bits seem to work fine (blower motor, vent selection, etc).

My inclination (or hope!) is that it's simply a faulty temp sensor, but if anyone out there can help advise, I'd be really grateful.

Thanks