Vaillant boiler wiring

[shaunbh]

My boiler is a Vaillant Ecotec Plus.
Programmer, 3 port valve, room and cylinder stats are all Honeywell. Y Plan wired.
The boiler flex is fed from a fused spur through a cord outlet and it is behind here that the wires from the airing cupboard are connected to the programmer (3 wires, all labelled and connected to the correct terminals in the programmer).
The other wires are L N E. I have 1 wire (red) connected to the switched live of the boiler and then a mystery (black) wire.
My problem is that I have moved my back box up by a couple of inches. All of the wires that were connected when I disassembled have all gone back to the right place. The black wire wasn't connected when I took things apart, but it may just have broken when I took the cord outlet off the wall.
So my question is simply, should this black wire be connected to the switched input of the boiler. Testing it through, I haven't actually found where this wire comes from - its not in the terminal box in the airing cupboard. I assume it is from the room stat, although I do need to confirm this. It is live when the room stat and the programmer are calling for heat.
This may seem like an obvious question, but I don't want to common this with the red until I am sure incase it shorts out and kills something in the system.
Thanks in advance.

 

[Agile]

Never assume anything!

 

[ChrisR]

Google for Honeywell Y Plan diagrams - easy to find.
You'll see that it's likely to be a wire from the cylinder stat.
But double check, heating wiring is often not right, even though it might have seemed to be working correctly before.

 

[shaunbh]

I've got plenty of Y Plan Diagrams, but none that I've found show the wiring to the boiler to any more detail than a simple L N E, where as I am looking at 2 lives - permanent and switched. Should I have 2 wires to the switched terminal - one to fire the boiler for CH and one for DHW

I don't think my black is off the cylinder stat because blue from the stat is connected to a black wire (and another blue) on term 2 of my wiring centre - so neutral but not assuming this to be the case. My black downstairs goes live when CH calls for heat and is 0v when off or DHW only

 

[Dean744PiN5]

The only wires that should be linked with the boiler switch live is m/valve orange and call (1) from cylinder stat, if that black wire is for room stat calling it should be linked with white wire of m/valve.
It does sound like its the room stat from your description but the easy way to tests isolate electrics go to room stat look at color wires on common and call have stat calling , go to junction box and test for continuity across the cable that has those 2 colors from the same cable, then turn stat down and test again should be no continuity.

 

[Dean744PiN5]

You should still follow the y plan as well the live it shows for the boiler is switch live.