In an earlier contribution to this discussion,
Tony Bryer wrote:

In article , Set Square wrote:
All the ones I have installed have had this. IIRC the older MV's
needed DHW on and DHW off feeds


Most 3-port mid-position valves still do!

I've always avoided them like the plague - reading the FAQ
explanation of how they work just makes my brain hurt. With 2 x
2-port valves it's always obvious what's happening.

I agree with you and - given a clean sheet - would do the same, since they
*are* a little complex and they *do* represent a significant single point of
failure in heating systems. However, I inherited a Y-plan system when I
bought this house and it works well enough for me not to want the hassle of
re-jigging everything. I've had to replace the actuator a couple of times,
but I've got quite good at recognising the symptoms when it goes wrong - and
(hopefully) at advising others about the delightful things when they enquire
in this NG.
--
Cheers,
Set Square
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