Install of nest 3rd gen.

30 Dec 2018
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this is what we have at the moment.

in the utility room we have
  • a ferrolli boiler
  • a siemens rwb27 controller

up stairs
  • there is a seperate on off switch for an immersion which provides hot water only

but when the siemens is switched on, it heats the house and also heats the water
the switch upstairs is for water only

we do not appear to have a thermostat, our heating is manual, and turned on/off by us or by a timer system.

i have attempted to install the nest, i have made the error of thinking it would be simpler than it is..

As you can see, there is a L & N
the L is looped through to 2, to enable 240v system
there is a second wire connected to the 2 ( aside from the loop from L )
there are 2 wires in the 4 position, one is brown and one is black

my understanding is that the 2 position is call for heat off
and the 4 is call for heat on .

if you look at the nest diagrams, they use different terms

can someone help me with the following.

the wiring for the nest
can someone explain the ch off ch on thing.
is call for heat off - an open system ( an open circuit)
is call for heat on - a closed circuit, thus engaging the boiler

the two wires in the 4 position, is one of them a controlling wire for the immersion system, would it be possible to wire this into a different position in the nest, so that we could control our hot water from the app
if so, would it be a case of identifying the cicuit for the boiler i.e ch on ch off , and then the remaining wire must be for the immersion.

thank you.
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