There is Nest Gen 3 and Nest e, which are you using? And it can connect as on/off or with OpenTherm, again which are you using.
Nest Gen 3 EU version is not that good, the US version can have remote sensors, but not UK version. But it can like only other switch, be used in parrellel with other switches.
OK, it is a pair of change over contacts, one for DHW and one for CH, the latter can connect to the boiler, or to a motorised valve, most thermostats need two or more wires, so adding a Nest e is easy, adding Nest Gen 3 may not be so straight forward.
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