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Here we go:

I thanked you this afternoon, but after had a re-think, I stand by my original saying, ie, the grey and brown of Honeywell 4043 on S plan can joint together to the switched live.

For most two port valves like Honeywell 4043 etc, the grey can also be joint with brown to a switched live.

Unlikely by PCB

Y plan. Switched live to the boiler, and pump live connected to the boiler pump terminal not to the switched live.

Don't get it wrong, but this back plate is Hive single channel one. Edited: sorry but thought OP referred to Nest, lol.

After 2012 Vaillant started using RT not 3 and 4.

It is a rebadged Drayton digistat. http://www.gennard.net/blog/2010/01/manual-for-british-gas-rc-plus-thermostat/

> 3. If the HW is on and in demand and the CH is on but “satisfied” (thermostat set low) then the HW refuses to work when going into demand or going into satisfied. At this stage, we need to measure the signal live from LP722's 3, to cylinder stat's input, then stat's output, finally to the HW...

In your case, Nest controls the heating only. Nest 2 comes from boiler live, and Nest 3 backs to boiler RT, also remove the 24v link.

Which ecoFTI? combi, system or heat only?

Really?

Buy two then, keep one for spares. Yes, it is very easy to replace.

What is the boiler and what is the fault?

This is a system boiler. Likely you will have a filling loop in the airing cupboard where the hot water cylinder is.

Couldn't fully see the wiring inside the junction box, not 100% sure through.

Ok

I will also add a link between between the red (BLACK?) and yellow in the Wiring Centre. The yellow is in a terminal block with the Heating Valve and the red cable (BLACK?) is in a terminal block with the Programmer.

Remove the both.

The existing pair of wires from the junction box to the old room stat, will be used for T1 and T2, not to be connected to any 1 to 6 of the Nest. The write wire from the motorised valve should linked to new Nest 3.