Connecting A to B will mean the central heating will run, Hive can replace some but not all programmers, this instructions page 10 http://www.free-instruction-manuals.com ... 286225.pdf shows the connections, where Hive controls domestic hot water 2b duel channel there is no common connection, so it can not be used volt free. Also Hive does not have a option to connect to the bus on boilers designed for that system.
As a result there is a list of boilers which can't use Hive. Nest looks at first glance similar however it does have volt free contacts and the OT1 and OT2 connections are designed to be used with OpenTherm enabled boilers.
Nest also uses IFTTT so can follow electronic TRV heads or reverse. Although neither are a patch on EvoHome.
Any cables in the wall which are live need to be in safe zones which means you can't remove thermostat even if disconnected.
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