Nest Thermostat to Replace Horstmann 425 Coronet Controller

[FurryPotato]

Hi

We currently don't have a thermostat installed with out combi boiler, just the controller switch.
This is for heating only.
I'm looking to replace that controller with a Nest Thermostat and need to double check wiring.

I have attached wiring diagrams for hortsmann, nest & nest wiring for 230v combi.
Below is a photo of the Horstmann Controller as it is currently. Mains cable comes in from the left, and then 2 cables go to the boiler to the right. One of the cables going to the boiler is main power & earth, the other only has one wire connected (To position 5, CH ON)

So if I'm correct

, I connect the 2 mains cables to their respective L, N & Earth connectors in the Nest.
I connect the L also to Nest 2(common) , and then the final wire( was Horstmann 5) to nest 3 (call for heat)

 

[stem]

You have posted a diagram of a Horstmann Diadem/Tiara, but your controller is a Coronet. [There are no pins for connection to backplate terminals 1,2 & 3]

As you don't have an existing room thermostat it's just a matter of moving all of the wires from the Coronet to the terminals to those at Nest which have exactly the same functions. Assuming that the Horstmann link goes between L and 5 as shown in the diagram above (looks like it does in your photo, but I can't be 100% sure) you are not quite right when you say:

and then the final wire (was Horstmann 5) to nest 3 (call for heat)

.....because 5 would just be the live link wire. It should be:

Coronet E = Nest E (Earth)
Coronet N = Nest N (Neutral)
Coronet L = Nest L (Live)
Coronet 4 = Nest 3 (Heating on / call for heat)

Add a link between Nest L to Nest 2

The Coronet terminals 4 & 5 will work if the wires are transposed. So if the existing link is between L and 4, instead of L and 5 then your description of where to put the "final wire" connection is correct.

 

[FurryPotato]

Perfect reply, thanks.
No idea why I posted the wrong diagram, I'd previously found the correct one !

All very clear now. Finally we can have a dammed thermostat instead of just on/off (Which due to laziness ends up meaning the house gets too hot or too cold constantly, wasting energy and moneyin the process)