In an earlier contribution to this discussion,
spamlet wrote:

A cautionary tale for those investigating probs with their central
heating:
A couple of years ago I finally tracked down a problem that had had
generations of visiting plumbers busy, in frenzied and expensive,
radiator bleeding / pump replacing exercises, all to no effect, at a
friend's house. On careful analysis it turned out to be the motorised
valve actuator (Danfoss) that was the culprit all along.

SNIP

You can replace a Danfoss actuator for 30-odd quid - it just isn't worth
trying to fix them.

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!
--
Cheers,
Roger
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