You'll have to bear with me on this one, as I'm new to this whole DIY game.

We've just got our first house, and before moving in we had a stud wall put in upstairs, and a combi boiler fitted in the loft. I'd like to put a socket in the loft - it's not a conversion, I just want to plug a computer network router in up there. I figured the easiest way is to cut into the power to the boiler, which itself gets power from the upstairs socket ring main. There's one cable going into the back of the boiler, from a switched fuse box. That's obviously the power to the boiler. However, there are two cables going into the fuse box. I can't work out which one goes where, because they both disappear out of the loft and down into the new stud wall. Having taken the front off the fuse box and had a look, it seems that one of the cables has three cores and the other has those three plus a fourth grey one. My question is - which one do I cut into to get power to the new socket? I'm guessing the one with four cores is the one which goes to the controller on the wall in the kitchen? Are three of the cores for power to the controller, and one a control wire? And if so, would I be better cutting into the other cable, or doesn't it matter? Sorry for all the questions, but I'm learning loads as I go along and I'm keen to understand all of this as well as just getting an answer about which cable to get power from. :)

Where does the 4 core go, is it connected in the fused switch.

A pic may be an idea.

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Sorry, very basic diagram attached. Both the 3-core and the 4-core come out of the stud wall, through the floor of the loft and into the fused switch. There's then a single cable from there into the back of the boiler.

I need to know what the twin and earth has in common with the 4core.

How many cores go to the boiler.

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Ok, reading this back there was a complete lack of information in my first post! I've attached a diagram of the wiring. I'm pretty sure that in terms of getting power for a socket, I need to cut into the right-hand of the two lower cables? However, I'm still intrigued as to how the controller in the kitchen tells the boiler to turn the heating on and off.

True

You could take a point from the back of the switch, keeping in mind the boiler supply should be fuse 3-5amp.

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If you take the feed from the L out and N out terminals the router will be protected by the fuse in the box so, if the fuse goes due to a boiler fault, your router will go down. If you take the router supply from the L in and N in terminals you avoid this but you should then provide the router with its own fused spur.

To answer you question about how it works. The boiler and the downstairs controller are supplied a permanent electric supply from L out and N out. However this does not turn the boiler on. This only happens when there is a voltage on the grey/black wires, which is known as a "switched live". When the controller downstairs wants to turn the boiler on, a switch will close in the controller connecting the live (brown) to the grey, thus sending 240v to the boiler.

Excellent, thanks very much. The follow-up questions come in three parts :)

1) Is it legitimate / sensible to just make another hole in the side of the switched fuse box and run a cable from the existing terminals out to a (fused) spur and socket? So I've got three cores into each of the L and N out terminals? The spur can then be fused at 3 amp with a socket at the end.

2) Presumably somewhere in the boiler, the power goes to lots of different places? i.e. the switched live from the controller doesn't turn all power to the boiler on and off, otherwise the hot water and LED display would turn on and off along with the heating.

3) Is the 240V on the switched live wire from the controller to the boiler actually part of the circuit required for the boiler to run the central heating, i.e. it's running 240V the entire time the CH is turned on? Or does a brief voltage on this wire just act as a signal to 'flick' a switch within the boiler to turn the heating on (and then off again)?

Apologies if the above are silly questions; all this is new to me but very interesting!

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The permanent supply to the boiler will run the display and power the circuit board etc


There is 240v on the switched live all the time the boiler is alight. Remove the 240v and boiler will go out.

Ah yes, obviously confused myself while writing that post. Taking a feed from the L and N IN, to make a fused spur. Brilliant.


Thanks!