How would YOU approach this HA task?

[Paul Gordon <paul_gordon@xxxxxxxxxxx>]

Peeps, (sorry this is a bit long & rambling!)



I've just had a new central heating boiler fitted (actually finished
commissioning at 22:45 last night!) - The programming electronics haven't
been fitted yet, - another engineer is coming tonight to fit the
timer/programmer, and the T-stat.



Obviously this is an *ideal* opportunity to bring the heating under HA
control, - but my question is how best to implement the combination of the
"native" plus the HA control?



I'd like it to operate fully under the HA system control for "normal
operation"- i.e. I'll do all the heating on/off time programming in
the HA
system, - either by Comfort timed events, or perhaps with some CBUS WISER
code... and not, under normal circumstances, to use the onboard time
programmer within the boiler... however, I'd like to have that onboard
programmer still available to return to "traditional" (i.e. non
automated)
operation in the event that some failure occurs with the HA, - or I decide
to sell up & move out...



This part is fairly simple, I've identified the connection points on the
control board, - there are basically a single pair of terminals, -
currently
bridged with a link wire which is removed when any kind of controller is
used, so that's obviously where I'll be connecting my CBUS relay...



The questions I'm mulling over just now, are how best to arrange the wiring
connections?..



With only a single terminal pair - i.e. only 1 "input" to the
boiler via
which any external controller can call for heating demand, should I bring
that connection outside the boiler, or should I run my CBUS relay
connections into the boiler casing and double-up in the existing on-PCB
terminal block? - obviously bringing it outside to an easily accessible
terminal block would be more flexible (but the fitter may balk at that
idea?)



Secondly, should I just parallel up the onboard controller & my HA
controller? - doing so allows either to call for heating, - of course if
one
of them is calling for heating, the other can't turn it off, - but I'd be
planning to leave the onboard controller "disabled" - either by
not actually
programming any "on" times, or by programming it as normal, then
leaving it
in an off rather than a run state if possible (my old boiler timer allowed
this) - using the 2nd way the onboard timer will have a program in it ready
to run at the flick of a switch, but in normal use won't actually be
running
that time program & thus won't ever call for heat, so my HA system will
have
complete control over the heat demand...



OR, would it be better to connect up the two alternative external control
systems via a 2-way switch, so that the control could be via *either* the
internal timer, *or* my HA system, but never both. - Choice would be set by
a user-settable switch adjacent to the boiler (although I guess that could
be automated as well)...



Lastly, and perhaps my biggest quandary, is how best to implement the
T-Stat?... - AFAICT, the traditional way of doing this would be that the
onboard timer and the external room T-stat would connect in series into the
demand terminals, so that *both* devices had to be calling for heat before
the boiler would fire... given the choices highlighted above, I can only
envisage a means whereby the T-Stat would be integrated with either the
onboard timeclock OR the HA system, but not both... - perhaps I could
engineer some kind of double-pole changeover so that if I implemented the
"either-or" model described above where a manual switch is used
to select
the "active" control system, this could also switch the T-Stat
over into the
wiring for that system?

The other possibility I can envisage involves using two separate T-Stats,
but that could be confusing for an end-user, since only 1 of them would
actually have any effect on the heating demand, so then how I decide where
to position them? (so I really don't like the 2 T-Stats idea).



FWIW, the T-Stat is wireless, - the boiler gets fitted with a receiver
module, but apart from that the control wiring operates completely
normally...



So, how would *YOU* go about doing this?



TIA



Paul G.







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