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. 
Water – Nest 3rd generation UK Install Terminals 4, 5 and 6 are used to connect cables that switch your domestic hot water on or off
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Not sure if my system is exactly the same, but I have to run the pump and boiler to get HW and the pump, boiler & a valve to get heating. The valve is a simple looking bypass thing with just live, neutral and earth and is sprung return. Now according to NEST such a system isn't compatible. However, I've wired with the live going to 2 and 6 (that's right, not 5) on the heatlink. Along with the valve on 3 and a jumper going from 3 to 4. The boiler and the pump are then on 5. This gives me full heating and independent HW control from the NEST without any relays etc. So far it's working perfectly. I looked for ages to find an answer to this and couldn't find a satisfactory one for my application. I hope this helps someone else in the same frustrating position!
I believe you need to move the red wire at the Nest to the Rh terminal. Rh is 24v from the Heating system. The Rc terminal is 24 volts from the Cooling system. Change that around and see if your issue is fixed.
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Best Answer
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.