Nest 3rd generation UK Install Terminals 4, 5 and 6 are used to connect cables that switch your domestic hot water on or off. Hi have my new nest 3rd gen installed heating is working great for radiators. Its the Hot water i'm stuck on. any help appreciated.
From the following documents it seems to me that 2 & 3 (common and call-for-heat) are used to provide power to the zone valve (which presumably then provides power to pump and boiler as in a conventional system)
This assumes you installed the Nest-display so that it repurposed the old 240V thermostat wiring for 12V power from the Nest Heat-Link unit (T1,T2).
You should check this with the installation manual supplied by Nest or using the online configuration tools on the Nest web-site.