Install nest thermostat on main eco elite 28 boiler via s plan junction.

[Awake106]

Hi, I was looking to upgrade my thermostat and followed a tutorial on wiring nest via s plan junction box. The only issue is that the final cable from the tutorial was a 4-core from the boiler and not a 5-core like I have.
L,N,E PLUS grey cable for switch and black 230v out.

Does the 230v directly power the heatlink via the junction box rather than being powered by the mains input.

 

[CountryFan]

Yes just take the heatlink supply from the wiring center (what you’re calling a junction box). The black can then go to common, rather than adding a link., switch live back to the boiler.

 

[RandomGrinch]

Does the 230v directly power the heatlink via the junction box rather than being powered by the mains input.

The boiler instructions state "...230V MUST be taken from terminal 2 to the controls...".
The live output from terminal 2 is protected by the boiler fuse - if this fuse were to blow, the boiler and controls would both cease to be powered.
This is a safety precaution to ensure an engineer working on one part of a dead circuit can't come into contact with another part of the circuit that's live.

This may be sensible for a single thermostat, but when you add in a wiring centre, multiple zone valves, cylinder stats etc, a more sensible approach is to have the FCU, that provides the power to the wiring centre, as the single point of isolation.
Or put simply, ignore the black wire. Connect it to a spare terminal in the wiring centre, or cut it off at both ends.
Power the Nest Heat link from the wiring centre and use the grey switched live.

The black can then go to common, rather than adding a link.

Taking the black to the common terminals of the heatlink doesn't really make sense - there will be a separate cable (or 2) to the heat link from the wiring centre. There's no need for the black wire as well.

 

[ericmark]

If S plan, then why do you need to go into the boiler? The S plan works the motorised valves, and in turn the motorised valves work the boiler, unless using some sort of e-bus control (Open Therm for example).

With Nest Gen 3 it is volt free and there is access to the com, so it does not matter if feed to com or normally open, in fact it has to be feed to normally open and normally closed and feed out to boiler from com when using a C plan.

The main idea is as @RandomGrinch says that if the boiler's internal fuse ruptures then it is not bypassed by some external feed, and also that all can be supplied from an external power supply if required, if there is a grid failure for example. However, Nest Gen 3 does have the option for the thermostat to be supplied from a USB supply, which rather messes up that idea, mine is fed from the heat link, so should I get a grid failure I am still supplying all central heating either from pairs of AA batteries or the main solar panel back up batteries.