Nest thermostat 3rd gen wiring question

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Hello, just trying to see if someone here can help me with the wiring of a 3rd gen nest thermostat.

We've just moved into a house, where the boiler is controlled by a Grasslin Towerchron QE2 (Fig.2) programmer to control the central heating and hot water (unvented cylinder). Unfortunately there is no thermostat in the house. We have got a 3rd gen next thermostat and I am hoping to change the programmer with the heatlink and place the nest thermostat in the living room on the stand.

Upon looking at the wiring behind the programmer (Fig. 1) and the corresponding wiring diagram (Fig. 4), I am a bit confused at how to wire this to the nest heatlink. L & N are self explanatory and 1, 2, 3 and 4 on the programmer means HW off, CH off, HW on and CH on respectively. So should I connect 1 and 3 (HW off/on programmer) to 5 and 6 (HW off/on heatlink) and 2 and 4 (CH off/on programmer) to 2 and 3 (heatlink)?

I also noticed that there are two wires connected to both L and N in the old programmer. Any idea where the extra L&N are going?

Any help is very much appreciated.
 

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The additional L and N could be daisy chained through the programmer and FCU (fused connection unit). It’s unusual to have all terminals occupied. Do you know if you have momo valves installed? If unsure, post a photo of the cylinder cupboard
 
Thanks. The cylinder and related bits are in the garage and here are some photos.
 

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As @Stuckinarut says, it's unusual for all the terminals to be occupied, but the Nest can replicate the Grasslin nonetheless.
If all those wires are used, is another question!
(I think it may just have been a place to park the spare wires from the triple and earth cables)

The wiring terminals are quite small within the Heat link, so I would suggest the following...

Connect one of the two red wires in L of the Grasslin, to L on the Nest.
Connect the other red wire in L, to terminal 5.
Add 2 short link wires between terminals L, 2 and 5 on the Nest.
Connect the black and blue wires in N of the Grasslin, to N on the Nest.
Yellow wire in terminal 2, to Nest terminal 1.
Yellow wire in terminal 4, to Nest terminal 3.
Red wire in terminal 1, to Nest terminal 4.
Blue wire in terminal 3, to Nest terminal 6.
Earth wire, to Nest earth terminal.

That should look something like...

nest.png
 
I am hoping to change the programmer with the heatlink and place the nest thermostat in the living room on the stand.
Will be interested to hear how it works, did consider that myself, but could not think of a reasonable way to get a USB supply to the thermostat, which would not fail with a power failure, the rest of central heating is backed up with a 6.4 kWh battery and solar panels.

I decided to use two thermostats in parallel, so if one fails, the other will keep the heating running. But living room has a Drayton Wiser thermostat, which comes with a stand, so thought would try it in different positions, room was cooler than wanted, then found the cause, cat sleeping on the thermostat, now wall mounted.
 

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