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!