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So after reading the many other threads about problems with the nest, i cant find any issues which match mine.
I'm pretty sure i have a Y plan system and that i have wired the Nest correctly, after wiring it in the heating and hot water all come on but the heating will not switch off regardless of what i do with the nest thermostat, the only way i can switch it off is buy switching off the main power to the heatlink.
My system consists of the following
A boiler in the kitchen with the old mechanical programmer next to it
A water tank upstairs with one valve, a pump, a cylinder thermostat and a room stat wired into the wiring center which is in the cupboard with the water tank
I have left my old wired thermostat in place and switched it fully on.
I've attached some photos, I feel like iv'e come as far as i can with my limited knowledge of these things and would really appreciate help from someone, I accept that any damage/wrong wiring is completely my responsibility
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With that simple swap the original thermostat is still in circuit so if you turn it down central heating should turn off. Normally there is a wiring center near the motorised valve, and the valve wire colour code reasonable standard so by following the valve brown and seeing what it connects to you can often work out which cable goes to the thermostat so you can remove it.
Nest has a few advantages over standard thermostats, the main being the way it controls the boiler output, using opentherm it will modulate (turn down flame height) the boiler rather than switch on/off although up to now I have talked about on/off only, the second is it can talk to the TRV using the follow command. However unless using the opentherm option then it is only really there to switch off boiler when all rooms are satisfied.