9 Oct 2022
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United Kingdom
Hi all,

Apologies another nest thermostat query re installation. Reading the other pages has been helpful and think I'm nearly there.

Current set up: from mains fuse box 3 core cable to isolation switch - then a Live and Neutral come off and go to control panel (currently centaurplus c27), a further live and neutral go to boiler via 5 core care, and lastly an earth goes to boiler and further earth to control panel.

On the control panel: L, N as coming off isolation switch, a neutral wire coming off and going to thermostat, grey wire in number 3 from boiler (call for hot water), brown wire from thermostat into 4 (call for central heating).


Wall mounted thermostat - Thermostat position 1 brown wire - call for heat into the control panel position 4, Blue Neutral coming off control panel to thermostat position 4, and lastly direct connection to boiler via a black wire in the boiler 5 core which is joined to a current carrying yellow/green wire located in thermostat position 3.

Using the nest wiring diagram:
- Control Panel Neutral Live get changed over to the nest Neutral and Live
- Jumper wires from Live to position 2/5 in the Nest
- Grey wire in position 3 in the control panel to Nest 6 (call for hot water)
-Then Nest T1/T2 position wired directly to Nest thermostat

My question really is about the call for central heating which will eventually go into Nest position 3. So current set up black wire directly connected to boiler comes to join in picture 2 behind control panel from which a yellow/green current carrying wire goes to position 3 in the thermostat, with position 1 in the thermostat then joining control panel position 4 the call for central heating. Can the yellow/green current carrying wire be connected directly to position 3 in nest as thermostat will no longer be needed?

Any help would be greatly appreciated
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Green/yellow should only be used as an “Earth“ or better known as a cpc. I don’t believe the black wire will go directly to the boiler, more a wiring center and from there.
Hi Chris,

Thanks so much for the reply. All wiring has been like that since it was installed by so called 'professional' when boiler put in. If the black wire is connected to the call for heat on the nest heatlink will it work?
I am having a problem working out what you have. There are two common systems.
One you have motorised valves with micro switches inside, and the controller works the micro switches, and the micro switches control the boiler (and pump if required)

The other is all motorised valves, hot water storage, pumps, and heat exchangers are inside the boiler, and the controller connects direct to boiler.

Gas and oil work different, with oil often they can't modulate, so they have a water store inside the boiler shell, and the boiler turns on/off to keep a reservoir of water warm. The Gas boiler can often modulate, (turn down output) and DHW can be truly direct, although often the option to use a reservoir. In some gas boiler the wall thermostat can turn the boiler up/down rather than on/off, which allows the boiler to run more efficiently.

Not worked on solid fuel domestic, the ones I work on actually boil water, and heat the carriages with steam.

Nest in UK has three versions Gen 2, Gen 3, and e, the latter does not do the DHW, the thermostat can work either on/off or modulating if the boiler has OpenTherm, but is rather poor as it will not connect to TRV heads. But unlike Drayton and Honeywell the OpenTherm on Gen 3 and e is built in, not an added module. And the Nest e is about the only one where the base is battery powered.

So boiler type is important, and also where not a combi if running Plan C, S, W, Y etc. There is not a single answer which covers all, and also if control is extra low voltage (normally 24 volt) or low voltage (230 volt) as at the moment we can only guess.
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Hi apologies I should have included that in the intial info.

This one: "The other is all motorised valves, hot water storage, pumps, and heat exchangers are inside the boiler, and the controller connects direct to boiler."

Oil run combi boiler is the boiler.

Nest generation 3 would be the thermostat/heatlink of choice. Controls 230 volt.
Hope that this is more helpful.
I am new to oil, as far as I have been lead to believe, the oil combi boiler is just a big box where the all the bits are collected together, normally there is two connections in the boiler one for CH and one for DHW, so 5 or 6 core cable from heat link into boiler.

But although we know it's a oil combi, only way I could say how to connect is to hunt internet for the installation instructions and work from there.
Yes from my understanding the oil combi has 2 connection one for CH and one for DHW, which would be the 5 core cable currently from boiler to control programmer. I know the DHW links into nest position position 6. The CH wire at the moment goes from boiler via a black wire to block connector behind control panel to thermostat then back to control programmer. My understanding is that by wiring the black wire connecting into the boiler for CH to nest position 3 would work as nest taking over the defunct wall thermostat.

I will search for the wiring diagrams online

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