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Both - thanks very much for your help pleased to say all now operating correctly. I will buy you a virtual pint needs a fair bit of tidy up and link off the old thermostat in the circuit box - will post in a few days once I’ve sorted. thanks again

Well blow me down this seems to have worked... loose wire seems to be the missing neutral and needed to join the neutral going into the nest... Just going to test it a few times and will report back.

So I’ve checked all the connections again this morning and I can’t find anything obvious. Running out of ideas. Is there a way to test other components of the system - eg the hot tank thermostat? The valve might be working but is missing a neutral supply - any possibility other than bad...

Morning - the boiler firing was a coincidence- it was running some sort of start up program (picture attached). However that’s not to say I don’t have a loose connection so will go round and check everything again this morning.

This is odd - having re assembled the boiler the nest now seems to be operating the boiler as expected - ie I put the nest on manual and the boiler fires as you would expect. I will test some more and report back later... it could just be doing a “reboot” cycle and coincidence

Ok attached. Mains supply goes into a 3amp socket in the wall next to the boiler. The LR, LS and NR go into the wall, with the LR and NR forming live and neutral in the controller and the LS joining the red wire up to the circuit box.

I will need to open up the boiler later to check but attached is the diagram in the manual which I’m basing my understanding on

The neutral at the nest comes out of the boiler (with live, earth and a grey wire I guess is live return. This last one goes to a red wire in the circuit box). The light blue wire in the old room thermostat box is live but in a terminal block isolated off. There is another cable joining the...

Ok will do. Not the answer to missing neutral supply though... any idea where I might start looking or what could be wrong?

Assuming I’ve done it correctly for now. Measuring it with the HW/CH on/off doesn’t seem to make a difference. Doesn’t sound safe to have a live wire knocking about in the junction box but that aside could it be the issue? The only thing it could reasonably connect to in there is the grey wire...

Could it be this one from behind the controller...? Picture attached - pointing at it with black probe EDIT: testing neutral from the nest to this one is 240v, earth to this one is also 240v

240V

White (from the Valve) to N is nil

At the nest, all are 240v.

The white wire is terminal 5. You said terminal 4... so I did terminal 4 - see picture 1? I guess you meant N+4, then N+5, then N+6? If I knew what I was doing I wouldn't be asking chap. Give me 2 mins and I will update.

Both CH and HW on Neutral and 4 - nil 5 and 6 - nil appreciate pictures aren’t great terrible angle let me know if need better ones ... many thanks for your patience!

Doh! I blame the crap lighting in that cupboard! You are right on both counts. Testing white to earth 240V... more importantly... testing white wire to neutral with HW or CH on gives 0V in both cases.

[Ok so with the nest on manual (both hot water and CH on) I’ve got a reading of nil across those poles (see picture). With just CH and no HW same result. Pretty sure I’m on the right wires from the valve (black cable entering wiring from the bottom in this picture -as awkward as everything in...

Sorry to be clear - the CH doesn't work when I reinstall the Altech. So my sense is it's something else. I will check the valve as well as the Nest when I've sorted my multimeter and report back. Many thanks I also forgot (foolishly) there is a separate loop for the under-floor heating in the...

I’m quite sure I’ve damaged something yes. The short which blew the fuse was across the L/N connections to the nest controller (I think I had stripped the wires too far back - it’s amazingly fiddly) and I was initially very concerned I had damaged it but since it powered on and communicated with...