it seems 24 volt AC at least that was written on the thermostat being removed, some UK boilers do have the option of using 24 volt some are AC and some DC not sure if USA Nest will work with DC.

Records the default button state of the corresponding category & the status of CCPA. It works only in coordination with the primary cookie. However it seems the USA Nest is designed to work air conditioning and heating, which is often by air ducts not water, so it seems it has no ability to control the DHW. Although it may be possible to get a 24 volt supply either from boiler or from a transformer and use relays, where it all falls down is getting the app for your phone that is matched to USA system and UK.

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