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I'm trying to install a nest thermostat from a stand LN1234 programmer, and I know to move it over I need to move L to L, N to N, 3 to 6 and 4 to 3, then add a bridge from L to 2 and 2 to 5, but I'm confused by the wiring of my current programmer. The live wires (red) seem to go to the N terminal and vice versa and where I would expect a bridge from live to 3, its bridging N to 3... Am I still OK just moving the wires as I said or do I need to do some more investigating to see how the wires go back to the boiler?

Pic for info

I can't find the original programmer, however in the main we used three core and earth to wire in the past, which would have the colours red, yellow, blue and bare and there was no brown, black, grey and bare so you can't expect the colours to match modern use, it could be line is blue.

There are a number plans, C, W, Y, S and variations of them, the basic idea is the thermostat controls the motorised valve if there is one, which in turn controls the boiler which is either in parallel with pump or controls the pump.

Wires in walls are identified as being there because there is a thermostat or other device, you could simply turn the thermostat to max, or short out the contacts, (not the neutral) or remove at wiring centre.

With my own house the wiring was a mess, there should have been three wires between the programmer and boiler but there were only two, because it should be wired one way does not mean it is, I did my house in the summer, I have one small mod that needs doing, I will wait until summer before I do it.

Do the research yes, but I would not do the job until summer.

Sorry when first looked picture did not come up, since all new colours it does seem some one got it wrong and has swapped line and neutral.

Number 4 black wire seems to be the only output from the old box, the number 3 is clearly linked to line.

I will get the unit is a thermostat? With no control for domestic hot water?

I thought LN1234 was the make of programmer, I now realise you were listing the terminals. There is no convention to how the terminals are arranged, however in the main 1 and 2 are N/C contacts for DHW and CH and we only use the 3 and 4 contacts which are the N/O contacts for DHW and CH so it seems yours has be wired so DHW is always on, and only CH is controlled the black wire.

I would guess there is a grey wire hidden some where maybe not used?

The brown and black wires on terminal 3 and 4 likely go to a wiring centre, and from the wiring centre the black will go to the wall thermostat and brown to tank thermostat and the returns from both will likely go to a brown to the motorised valves one for CH the other for DHW.

The motorised valves will likely have a micro switch in side them which has orange com, grey N/O and white N/C often no white wire as not used with most systems, this normally turns the boiler on.

This is rather a basic description as there are many variations.