"Set Square" wrote in message ...
In an earlier contribution to this discussion,
Ed wrote:

Where to begin? The symptoms: CH doesn't always start in the morning,
it seems that the colder the house the less likely it is to start, but
that may just be our perception..! Secondarily it seems to be ignoring
the DHW tank stat.
The system is a very dull indirect setup with a Baxi back boiler at
the front, a Tower / Grasslin Mid pos'n valve (replaced two years ago)
controlled by a Tower roomstat (with acc), an old Tower tankstat and a
Honeywell digital 2 channel programmer, the square type. All has been
fine until a few weeks ago.
The timer is a 2nd hand one fitted 3years ago (give by a mate whose
system I updated) and seems fine, it replaced an old Towerchron
mechanical (DHW only or CH+DHW, no CH only) timer with pins (for which
previous student tenants had lost almost all the pins,how?). The 3 way
replaced what you woud have expected to be a 3 port diverter due to
the old type timer it had been installed with when new, but it was a
mid pos'n one and after much learning I got it all wired and it worked
fine for about a year (until a small crack in the baseplate allowed it
to distort enough under the spring pressure that the cam sometimes
missed the second microswitch, depending on whether heat from the
pipework had made it expand a little more. I only mention this in case
someone finds it useful in the future.)
I have been all through the wiring and am sure that both stats and the
timer are working fine. Having fiddled with the stats to study every
possible permutation of inputs to the 3 way (not just in isolation but
also trying every input that can follow from each possible previous
set of inputs.) The only consistent weirdness I can find is:
With BOTH stats enabled by the timer and Both calling for heat, the 3
way moves to the centre AB pos'n as it should. If you then reduce the
setting on the room stat (to simulate the room coming up to temp) so
that the room stat is now satisfied I would expect the 3way to spring
return to the DHW ony pos'n, but instead it stays in the AB permiting
heat to flow to both rads and DHW. This can't be right, can it? The
only way to get it to spring return from there is to turn DHW stat to
Satisfied, or to remove the mains power from the system. You can
disconnect ALL the wires in the 3way and it still stays in the AB
pos'n. Aha! thought I, that'll be the flow from the pump holding it
open, disconnected the pump, no change, gave-up. However it doesn't
explain either of my symptoms, tank too hot and CH not starting.
I have a feeling but have been unable to confirm yet that the Ch
starts on mornings when the DHW stat AND the CH stat are BOTH calling
for heat when the timer powers the system up at 6.30. If the DHW isn't
calling for heat, then the CH takes a lie-in too regardless of what
it's own stat has to say. As I say this is a hunch so far but I'll be
provoking it at the weekend when I have time, otherwise it's over to
you lot, help!


I can't pinpoint the exact problem - but I'm pretty sure that it's something
to do with your mid-position valve and/or its actuator. This device - and
its horrible little microswitches - plays a key role in the control logic
for a Y-plan system - and your symptoms are typical of what happens when all
is not well in that department.

The first thing to check is that the spring return takes the valve to the HW
only position when you remove *all* power from the system. This means
turning it off at the FCU which powers the whole caboodle - not just at the
programmer and stats.

If it doesn't return, there is either something wrong with the spring return
mechanism in the actuator, or the valve itself is seized. With many of these
things (but not all) the actuator can be removed from the valve with no
danger of water spillage. If yours is like this, remove the actuator and
check that the spindle of the valve turns fairly freely. You should be able
to turn it with finger and thumb - or at any rate with a pair of pliers,
exerting only a small force. If it is seized, you'll need to replace the
valve - which requires draining the system. If the valve is free, the
actuator is duff. With many of these valves, it is possible to replace just
the actuator - or even to rebuild it with new motor or microswitches if you
are good at soldering and like small fiddly things!

If the spring return does work when you remove all the power, but the CH
still doesn't work, the most likely cause is still a duff actuator.

[I know you replaced the 3-port valve a couple of years ago - but some of
them don't last any longer than that!].

The spring return is smooth and fast, no problems there. As I write
the system is behaving fine so it's a little hard to do diagnosis.
When I did the wiring checks I did find one wire in the connection
unit that could have been tighter but really didn't seem loose enough
to be a troublemaker. I've given it a little tweak anyway so now I
just have to wait for the next cold morning. Still baffelled by the
way it sits on AB with ALL wires disconnected, what is holding it
there? If you pull the power at the FCU it spring returns sharply...
Watch this space...