New C/H room thermostat

[BS3036]

The OP only mentioned borrowing a neutral from elsewhere.

What he said was...

From my point of view powering it from the lighting has the advantage that

 

[Harry Bloomfield]

What he said was...

I interpreted it as him having a live, but lacking a neutral - that had a possible implication of him borrowing the neutral from another circuit, which is a really bad idea on several fronts. As someone suggested above, the best solution is a stat which doesn't need a neutral.

 

[BS3036]

I interpreted it as him having a live, but lacking a neutral - that had a possible implication of him borrowing the neutral from another circuit, which is a really bad idea on several fronts.

What he said in full (which doesn't really allow your interpretation).

From my point of view powering it from the lighting has the advantage that if the C/H is isolated for service the stat wont lose its settings

As someone suggested above, the best solution is a stat which doesn't need a neutral.

Yes, the second power source is not ideal as it may confuse a future electrician.

I just thought that we should respond to the actual suggestion, rather than everyone piling in and going on about borrowed neutrals.

 

[Taylortwocities]

Yes, the second power source is not ideal as it may confuse a future electrician.

Or worse, confuse a future plumber. An electrician should understand the risk because he(she) will carry out safe isolation procedures.

 

[ericmark]

When trying to sort out my central heating step one was test and record?

Although a three core and earth was in the utility room, and seemed to go to the flat where the boiler is, only 2 cores had continuity and also colours changed, so seems likely some where there is a junction where either one core never connected or it has become disconnected.

So I looked at how I could control CH + DHW with two wires (plus earth) and if possible keep the thermostat running without the need to replace batteries.

I also already had Energenie TRV heads which claimed would work with Nest.

So using Nest Gen 3 allows two wires working with very low current, and extra low voltage, to maintain the thermostat back up batteries, and send information down same two wires.

Not saying Nest Gen 3 was best option, but I selected a thermostat which would do the job required. Should the cable joint not be good, there is no undue strain on it, and should it fail I have option of using USB, I have designed the new system to allow for lack of wires between utility room and flat, also found cable utility room to hall so thermostat mounted in hall.

Yes I made errors, but not due to lack of research, it was due to incorrect information being published.

I balanced the cost of Nest Gen 3 to cost of running new wires, and Nest Gen 3 won.

I am sure there are other thermostats which can also be hard wired, I stopped looking once I found a way to do what I wanted. Having used wireless in past did not want to use it again.

 

[mr_chaela]

Thanks for all the advice and info. From my point of view borrowing a neutral just seems a messy way of doing things and as has been suggested powering the stat (with a volt free switch) from the lighting circuit is no more dangerous than the original battery operated stat (again with a volt free switch), when the system is isolated so is the circuit to the volt free switch.
However, being a bit pernickety at doing things right, I spent the day cutting holes in ceilings (luckily I am decorating at the moment) and have managed to get a supply from the load side of the isolator switch to the stat, so all is good and I can keep the new stat. Tomorrow I'll patch the holes and carry on decorating.