It seems your boiler has the option of three types of control 230 volt off/on, 24 volt off/on and ebus, I don't know if the ebus option is compatible with opentherm?
So Nest e can also work with 230 volt off/on, 24 volt off/on and opentherm so it can be used with your boiler in at least 2 ways possibly three.
So the Line and Neutral supply to the old Drayton will no longer be required as the Nest e it seems is totally battery operated, not sure I like that idea, but it does make fitting easy.
As to if you want to connect simple on/off which is straight forward or use the ebus is not such an easy selection, I am not sure if your boiler is opentherm or not but it does show the ebus connection so it may be.
The problem with a modern boiler is the return water needs to be cool enough for the flue gases to condense out the water, switching off/on to control can stop this working, so better to turn boiler up/down or modulate the boiler than switching off/on. But I don't really know how much better.
Nest gen 3 has the advantage of no batteries required, and in my case it also controls domestic hot water, but as far as I know both Nest e and Nest gen 3 will connect to energenie TRV heads and for you only gain would be no batteries to change.
Biggest problem I found was finding UK instructions as USA is completely different, not sure I like how it works with the TRV heads, but not run a winter yet, so will know better next year, but the great thing about Nest for me was the options given for wiring, I have Nest Gen 3 and both DHW and CH have volt free contacts so I could wire how I wanted, and it only needed two wires main house to heat link where boiler is which suited me.
However although wiring was easy, at least for me, I found the set-up rather daunting, it has so many functions, like the anti legionnaires (only with DHW so not used with Nest e) it took some time to set it up. With a phone don't think you need the Nest software, the Energenie does it all, with a PC you have to use Nest as the Nest thermostat does not show when controlling with a web page.
I would wait to see if any other answers, but I see no reason why you can't use Nest e, only question is wired to switch off/on or wired to modulate?
Thank you for all the info, that's really helpful.
Researching it, I don't believe the boiler is opentherm compatible.
You mention wiring to modulate the heating which does sound preferable to on/off (although I also don't know if it makes a real life difference) - would that option be via ebus and therefore not an option for me?
Out of interest, my issue with the Nest guided setup was that it did not provide a route for power to the boiler, as this previously went via the Drayton SCR. I tried to wire up the Nest E connected with the No and Com wires and to route the power wires through the old Drayton box as before as a temporary test. This seemed to lead to the boiler being permanently on which confused me as the diagram implies the power is on a separate circuit to the circuit instructing the boiler to turn on/off. I now wonder if this was the 'comfort mode' that keeps some water hot in the boiler ready... or have I misinterpreted the circuits and system?
In the find on page I typed in ebus and it showed me a connector strip.
It shows 11 connections ebus + and - , 7,8,9 are 24 volt, LNE feed in and 3,4,5 230 volt thermostat. So it seems you can use 2 or 3 methods to connect it up, it shows a link between 3 and 4 so I would expect for 230 volt thermostats that link is removed.
I will guess 5 is the neutral however the instructions don't seem that good. As I read down the pages it does seem to confirm that, and says 20 volt not to be used in UK again a little confusing, as to why.
It says a VRT 360 connects to the ebus it is a 46 page manual sorry not reading it for you.
However reading the boiler manual I see no reason why Nest e NO and Com should not connect to 3 and 4 in the boiler and work. There are only two wires plus earth in cable which I don't think is used, so it should be simple.
Or 1 on Drayton to Com on Nest e and 3 on Drayton to NO on Nest e, and just terminate L and N somewhere safe. I would consider a single socket box with a blanking plate and some 5 amp terminal block to neatly make connections, or even a conduit box if the Nest would then cover the box.
Having neither got Nest e, or Drayton, or your boiler I can only read instructions, but it does seem it will work.
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