In an earlier contribution to this discussion,
D Simmonds wrote:

Hi All,

I have a newly installed Worcester Bosch CBi boiler (- a conventional,
not combi). This was installed to replace an aging myson appollo
fanfare, into the existing system. Both systems have pump over-run so
I think the wiring ought to be identical ....

Once installed it fired up without any porblems, however did not shut
off as expected with the room stat. I left it running so that at least
there was some hot water and heat in the house!

After the hot water cylinder had warmed up I completed the other
tests, e.g. lock-out etc, all of which were fine

I started the boiler up again, running it in the normal way, and after
a while it shut off as expected - however the cause was not the room
stat etc - the fuse in the mains isolator switch had blown. Replaced
the fuse and same again. I investigated for the short circuit and
found the resitance across the pump was 0 - not good!. I checked the
wiring and it all seems to be correct for "Y-Plan" style wiring. -
also bear in mind that the wiring hadn't change, just the boiler
connections. I had an old pump lieing around which i fitted, and again
the circuit (fuse) blew and the pump again - without the system even
starting up.

Their is one variance from the worcester-bosch instructions and my
wiring, in the the pump neutral goes to the common neutral in the
wiring centre (which just a a connector box - not a special one),
rather than to the boiler, but i don't see that this should make any
difference, and i want to avoid recabling! If that neutral connection
is vital i was thinking a link between boiler mains neutral and pump
neutral aty the boiler would suffice

To test the system, i disoccnect each component (mid-position valve,
cylinder state, room stat, pump) and connected each in turn to check
the fuse didn't blow, and it is ok until the pump is connected

Any help would be greatly appreciated

Thanks in advance

Dave

When you found the resistance across the pump to be zero, was that with the
pump in circuit - or with it disconnected? If the former, my guess is that
you've somehow managed to wire a short circuit by misconnecting something.
If you disconnect the pump and put a 13A plug on the end of its lead (with a
3A fuse) and plug it into a normal socket, what happens? I suspect that it
will work, and that you haven't 'blown' it at all!

Come back with some answers, and we'll try to work out what's happening.
--
Cheers,
Set Square
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