nest gen 3 learning thermostat install


Novice Member
hi there, recently bought a nest learning thermostat. went to wire up the heatlink to the boiler, but no clue on what wire is what. the combi boiler junction box is a danfoss wb12 junction box and the existing room thermostat is a danfoss rmt230. would anyone be able to help with what wire is what and where they go on the heatlink from the boiler and the heating/hot water valves


Well-known Member
Is it a combi-boiler or a system boiler with hot water cylinder? You do not normally have any hot water control if it’s a combi-boiler.

First step is to identify where all sources of power are isolated to ensure any wiring is completely “dead” before working on it, and you should really use an approach contact rlyester to prove that is the case before continuing.

You would normally trace the cables from each device and label all cables t the junction box to clearly identify what everything does (just because there is an industry-standard heating junction box does not mean the installer followed any of the standardised wiring diagrams supplied with your Nest stat). Once you know which cables are which you can then map them to your installation drawing and proceed from there.

No one can simply look at the junction box and say with any certainty how it has been wired, so tracing and labelling is an essential first step.

If any of this does not make sense then I’d suggest looking for a Nest Pro to sort it out for you - get it wrong and at best the heating and/or hot water will not work, or you could damage the controls or the boiler, and at worst you could damage yourself if you’ve not got experience if safely working on electrics.

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