I already have 2 gen 3s I was hoping to use.
There have been discussions on this page before on multi circuits for central heating. The manufacturers do not seem to use the definitions used in the regulations, they for example often call extra low voltage low voltage. and they often say the controls should have same low voltage supply (230 volt) as the boiler, which since most homes only have one supply would naturally happen, however I am sure they mean circuit even when they say supply, and for some reason not given they want all items connected with central heating to be on the same circuit.
My central heating has at least 10 circuits, as I have 9 electronic TRV heads each powered by twin AA batteries, and many central heating systems use 24 or 40 volt for control so common for a thermostat to have a mixture of extra low voltage and low voltage, so since Nest Gen 3 has volt free contacts I see no reason why it should not be supplied from a suitably overload (fuse or trip) independent from the boiler. However plumbers seem to feel this is wrong, although plumbers also use green/yellow as line supply so let him without sin cast the first stone.
But I am sure you can see the problem, people expect central heating to be isolated with a single switch or plug and socket.
It seems reading reports from LABC that the government considered TRV's to give zone control, but the central heating industry read the rules as requiring valves with the label zone valve, I have never read the law, but clearly TRV's do create zones, however you have hard wired zone valves it would seem. Where domestic hot water is heated in a cistern often the valves are found in the airing cupboard. However not unknown for plumbers to fit them under floor boards.
It may help to consider what the thermostat does. In the main there is a problem with gas boilers that use the return water to control boiler output in that unless the water is circulating then it can't control boiler output, so once boiler is switched off, there is no way to switch it back on again using return water temperature, so it needs some electrical method of turning the boiler off and on as weather or home occupation dictates.
So the idea was, fit a thermostat in a room kept cool, so warm weather will switch it off sooner, on the lower floor, as heat raises, with no alternative heating, and no outside door. Don't know about your house, but in my house no such room, so closest was the hall, it has an outside door, however using a TRV set at the same as the wall thermostat you can get fast recovery but radiator turns down before target is reached so does not switch the wall thermostat off prematurely.
I have set this up in mothers house, however I will admit adjusting lock shield, TRV and wall thermostat to work in harmony is not easy, and took a lot of tweaking before spot on.
Nest in USA has temperature sensors so it can monitor many rooms, so if any room cool the furnace (they don't call it a boiler) fires up, however the sensors have not been released in the UK. But there seems to be little point in having two independent thermostats, if the boiler is running because one thermostat is calling for heat, then may as well allow TRV's to do their job and heat rooms that need heating. Two thermostats in parallel yes, to ensure boiler runs when wind or sun causes one side of home to cool OK. But why turn off a whole section of the home? Well yes good reason, in my case the lower kitchen, bathroom, bedroom and living room are only used in summer or when we have visitors, we call it the flat, and I have a zone valve to turn off whole of flat.
However in main house the 9 programmable TRV heads select when the kitchen, dinning room, living room, hall/landing, craft room, office, bedroom one and bedroom two are heated, no time with shower/toilet, and bathroom/toilet always heated, utility room has no heating. There is simply no place for zone valves in main house, well there is, as the TRV heads are zone valves.
Ideal is for the electronic TRV head to send a signal to the wall thermostat to say switch on I need heat, for some reason which I have not worked out, Nest worked in reverse, and the wall thermostat sent (note past tense) a signal to TRV heads so all TRV's linked were set to same temperature as wall thermostat. To be frank this would never work as TRV heads set lower than wall thermostat so normally set to a lower temperature, but when google took over Nest support for the Energenie MiHome TRV heads was removed, so there is no longer any link between TRV and wall thermostat.
In other words until Nest releases the temperature sensors in the UK, their thermostat must be about the worst of the so called smart thermostats.
What we want is for the TRV heads to work better, and it seems Drayton Wiser has smart TRV heads which work out how long it takes to warm the room, and has built in algorithms to stop over shoot but allow fast recovery of the room.
Even the Energenie MiHome TRV heads could be set using IFTTT to work with geofencing, as to Nest Gen 3, early this year has a cold house due to high winds taking out the EE mast and Nest thinking I was not at home so turned the heating to Eco mode, now geofencing has been disabled.
Are you sure you want Nest? I have decided it is good enough and not worth spending out to get a proper central heating control to replace Nest Gen 3, and I hope the temperature sensors will be released in UK, if so I will get some, but knowing what I know now, would not get Nest Gen 3.
I am sure I should be able to set my Nest Mini's to control Nest Gen 3 using voice control, but Nest Mini's are like mischievous little boys, hay google turn off when wanted radio 4 extra turning off got turning off 5 switches and we were plunged into darkness, hay google turn lights back on got I don't know how to do that yet, would you trust that with your heating?
