Nest gen 3 - Y plan - fitted but heating will not switch off

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Nest gen 3 - Y plan - fitted but heating will not switch off

Post by md45 » Tue Feb 20, 2018 3:14 pm

Hi everyone,

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
Last edited by md45 on Tue Feb 20, 2018 4:12 pm, edited 1 time in total.
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Re: Nest gen 3 - Y plan - fitted but heating will not switch

Post by moderator2 » Tue Feb 20, 2018 3:17 pm

Better asked in CH. I'll move it.
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Re: Nest gen 3 - Y plan - fitted but heating will not switch

Post by ericmark » Thu Feb 22, 2018 11:01 am

Two independent issues, one how wired and two how programmed, so I can only really help with wiring using this PDF instruction set so page numbers match page 11 shows the Nest wiring diagram, this is very similar to a programmer wiring diagram, Image often as shown to left of diagram the programmer has less wires as not volt free, however L to L and N to N then link L - 2 - 5 and my 1 becomes nest 6 and my 3 becomes nest 4 and my 4 becomes nest 3. One easy mistake is with mine domestic hot water is first and with nest central heating is first so easy to get them wrong way around.

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.
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