The only satisfactory cure is to convert from a Y-Plan to an S-Plan by
throwing away the 3-port valve and replacing it with two 2-port valves.
This
removes a major single point of failure from the system. It's not too
difficult to do, provided you can get at the pipe runs immediately after
the
3-port valve, inserting a 2-port valve in each leg, and replacing the
3-port
valve with an ordinary tee. If your system is vented, with the vent pipe
connected near the HW cylinder, the HW zone valve needs to be inserted
close
to the cylinder, *after* the vent pipe connection in order to ensure that
there is always a clear path from boiler to vent, with no valves in the
way.
If your boiler has a pump over-run stat, you'll also need a by-pass
circuit - to give the water somewhere to go when the pump is running with
both valves closed. An automatic by-pass valve between flow and return,
connected after the pump but before the zone valves, is best.

--
Cheers,
Set Square
______
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Thanks for your suggestion. I remember considering S-Plan when I originally
installed the system about 20 years ago. At that time I decided on Y-Plan
due to it being cheaper and easier to install.
Now, I realise that the extra cost could be worthwhile and I like the way
that they should go either full on or off, as some of the problems I had
with Y-plan were due to the valve sometimes sticking in the wrong position
etc.
I can modify the pipework as the vent pipe is fitted next to the boiler and
I can add a by-pass valve without problem.

Thanks,
Dave