In an earlier contribution to this discussion,
logized wrote:

I am looking at replacing my Danfoss 3 way motorised valve as part of
a central heating re-fit.
I have changed the actuator a few times over the years, so I am now
looking to find a type that is known to be very reliable.
So far, I cannot find much independent info to help me decide on a
make. Drayton seem to make good TRVs, so are their motorised valves
as good? or maybe Myson or Honeywell, their specifications look good
also. Any ideas.

Dave

3-port mid-position valves have an inherent weakness in that they rely on
microswitches in the actuator to control both the valve and external parts
of the overall system - inluding switching boiler and pump on in CH-only
mode. It only needs a bit of wear on the cams which operate the
microswitches for the whole system to fail. Some makes may be marginally
better than others - but there ain't a lot in it.

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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