Thermostat wiring query - seem to have extra wire

[emjayess]

Hi,

I can't find any reference anywhere to anyone having the same situation as me so I've started a new topic.

I've got a British Gas installed system - BG C1 combi boiler (which I understand from my research is actually a Worcester Bosch SI 28 ) and a BG RS1 thermostat (which I understand is a Drayton RTS1).

I'm trying to replace the thermostat and I was expecting 3 core + E.....but it has 4 core + E.

There is a brown and blue wire which are live and neutral as expected, and then 2 black wires. Both black wires are connected to the same terminal, labelled 'call' - which appears to be equivalent to switch live (?)
Has anyone seen this setup before? Does anyone have any ideas as to why I have 2 wires connected to 'call'?

I've only found 1 semi-related reference through a huge amount of online searching. One site suggested that sometimes plumbers run extra wires at the time of install to provide redundancy - but I only read that in one place, and it seemed to suggest the extra wires were left unconnected unless/until needed.

I've looked through the online manuals and there is no apparent need for the extra wire. The install was originally done by BG, and was only done a few months before I bought the house - so it is unlikely there has been any DIY tampering with it because it was new.

I could just go ahead and rewire both into the new switched live terminal, but I'd prefer to try understand exactly what is going on first lest I make a huge mistake!!

Any thoughts?

Thanks in advance.

John

 

[D_Hailsham]

The only way you will find out why there are two wire connected to Switched Live (aka Call), is to find the other ends of the wires. They will probably go into a large junction box; it's then a case of finding which devices they are connected to.

When you have done that, post the info and you will be told if you need both wires.

 

[emjayess]

Many thanks for the quick reply.

I've taken the front panel off the boiler, and I have terminals labelled:

L
N
Ns
Ls
Lr

There are cables feeding in.

cable 1 - Twin & Earth that goes to a plug that powers the boiler.

Brown is connected to termianl marked 'L'
Blue is connected to terminal marked 'N'

cable 2 is the 4 core + E that appears to run direct to thermostat.

Blue is connected to 'N'
Brown is connected to 'Ls'
Both black wires are connected to 'Lr'

(nothing is connected to 'Ns')

Regards,

John

 

[emjayess]

....just to add to that, from what I understood from the Worcester 28SI manual, the thermostat controls should have been connected:

Live to Ls (so correct)
Neutral to Ns (so not correct - albeit is still connected to a neutral terminal)
Switched live to Lr - so correct, but why 2 wires?!

I gotthe 28 SI manual from here http://manuals.arated.com/index.php?d=Boilers/Worcester+Bosch - other websites have said my C1 is a rebadged one of these and it certainly *looks* like the same boiler as my C1 in every aspect I can see

 

[D_Hailsham]

Terminals N and Ns are linked together internally, so it doesn't matter which one is used for the connection to the thermostat's N terminal.

The only reason I can think of for connecting both black wires is that it's safer - no possibility of a loose wire coming into contact with some metal work and blowing a fuse.

Leave it as it is - just connect both black wires to the switched live of your new thermostat.

 

[emjayess]

Thanks, that's what I'll do.

Still find it strange BG have done this.

 

[D_Hailsham]

Thanks, that's what I'll do.

Still find it strange BG have done this.

Probably that was the only cable the chap had on the van at the time.