Central Heating system blowing pumps!

[D Simmonds]

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

[Set Square]

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.

[D Simmonds]

Thanks for the above. I measured the pump with it out of circuit, and get a zero resistance. There is definately a short somewhere, as even with the pump disconnected from the circuit i am still blowing the 3 amp fuse. I am going to try and chase the short down, but none of the wiring has changed from the original setup as far as i'm aware - of course i'm only human! luckily i was away for the weekend so got heat and hot water!!!!

The cylinder stat has resistance as i'd expect, - but i'm not sure how (which wires) to check for the mid position valve .... - i presume they are capable of shorting out as well. I shall see if i can discover more .....

Dave

[Set Square]

Any chance you can post a link to an installation manual - or wiring diagram - for your new boiler. I'd like to try to work out just how the pump is supposed to connect - particularly since you've done it differently!

Do you mean there is zero resistance across the pump itself, or across the terminals where it connects but with it disconnected? With what did you measure this resistance?

Have you tried connecting the pump directly to the mains? What was the result?

[D Simmonds]

I measured the pump across the terminals - so suspect its the capacitor thats blown perhaps? I was thinking that replacing the capacitor (if its cheap) might fix the pump .... - thoughts?

Anyways - i found the cause of the short one of the cables in the wiring had nicked insulation, AND by coincidence the sleeving on the earth wire adjacent to it also had a nick - and hence the short occured - i'm mystified why it ran and then blew rather than just blowing - however its all solved now. For the wiring i used std Y plan, however when looking at it again it also had the HW off feed to the programmer missing!!!!!!!, - and had been that way for years .....

Thanks for your help

Dave

[raden]

In message , D Simmonds writes

It depends whether it's the capacitor which has gone, doesn't it. It's a motor start capacitor, so the usual problem with a dead pump capacitor is just that the pump won't start

It depends what the cable was connected to. If its to e.g. the fan, it would blow when the fan was powered up and not when the boiler was on without a call for heat

[Set Square]

I suspected it was in the wiring rather than the pump. Glad you've fixed it.

Not good! However, it will still work as long as the HW is on at the programmer whenever the CH is on, and provided the cylinder stat has change-over contacts which are wired correctly.