In article ,
"Roger Mills" writes:
As others have said, the only *real* solution is to convert to an S-Plan
system, with two 2-port valves - which is what I've just done.

The problem with the 3-port valve is that it's too clever by half - and has
multiple modes of failure, many of which are not that easy to diagnose. In
particular, getting the thing to sit at the mid position without hunting
really challenges the laws of physics!

There's no hunting in the mid position unless something's broken
or wasn't wired properly. The motor's rotor is locked in mid
position by feeding it DC (or half-wave rectified AC), which is
generated in the head by a diode in the circuit.

I am the first point of call for a number of heating systems
around my family. Most have a mid position valve and none have
failed. One's about 5 years old, and the others all at least
10 years old. They are a mixture of makes.

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]