Google Nest wiring, C plan?

LewisM · Aug 23, 2023

Hello, I hope you are well, I have fitted a few Nests but only to Combi, S and Y plan systems just been looking a job where I’m doing the kitchen, looks like the heating system is a C plan?

There’s 4 cables at the programmer, power in two 3 cores up to the wiring centre then a flex to the boiler

The wiring centre feeds a cylinder star, two position valve and a circulating pump and a 3 core to the stat, I’m guessing wiring is the same as a s plan just need to feed both the boiler and and circulating pump when call for heating ?

Thanks Lewis

oscar21 · Aug 25, 2023

A C plan system is the old gravity system is it not? If so why is there a motorised valve and cylinder stat then? Also when you say two position valve do you mean a 3 port, it sounds more like a fully pumped system to me with an external pump.

some boilers have a terminal inside them that powers an external pump, its needed for the pump to run on. A few pictures would help.

LewisM · Sep 4, 2023

oscar21 said:
A C plan system is the old gravity system is it not? If so why is there a motorised valve and cylinder stat then? Also when you say two position valve do you mean a 3 port, it sounds more like a fully pumped system to me with an external pump.

some boilers have a terminal inside them that powers an external pump, its needed for the pump to run on. A few pictures would help.

No it’s definitely a two port think it’s known as a mid position? it’s in-line with only an in and out

LewisM · Sep 4, 2023

Well after more research I was definitely wrong, so this is the nest and current setup

I think that I need to remove 2+3 from the wiring centre and use these later for 12v supply for thermostat, put a wire from terminal 3 on the nest to terminal 4 on the wiring centre and then 4 on nest to 8 and centre and 6 on nest to 10 on centre and the rest is pretty easy linking our communes etc

davesparks · Sep 4, 2023

oscar21 said:
A C plan system is the old gravity system is it not? If so why is there a motorised valve and cylinder stat then? Also when you say two position valve do you mean a 3 port, it sounds more like a fully pumped system to me with an external pump.

some boilers have a terminal inside them that powers an external pump, its needed for the pump to run on. A few pictures would help.

C plan is a modification of a gravity system where a 2 port valve is added to the primary flow pipe to the hot water cylinder operated by a cylinder stat. This is used to prevent overheating of the hot water and operates as a hot water priority system.

LewisM · Sep 4, 2023

davesparks said:
C plan is a modification of a gravity system where a 2 port valve is added to the primary flow pipe to the hot water cylinder operated by a cylinder stat. This is used to prevent overheating of the hot water and operates as a hot water priority system.

That makes perfect sense

davesparks · Sep 5, 2023

LewisM said:
No it’s definitely a two port think it’s known as a mid position? it’s in-line with only an in and out

It can be mid position if it's 2 port.
A 2 port valve can only have 2 positions, open and closed.
A 3 port valve has an A or B position and a middle position which is both A and B

LewisM · Sep 5, 2023

Well I’ve gone back to plan

davesparks said:
It can be mid position if it's 2 port.
A 2 port valve can only have 2 positions, open and closed.
A 3 port valve has an A or B position and a middle position which is both A and B

Yeah I think I was getting my terminology all wrong, anyways I was totally overthinking it and just went back to basics of drawing out what controlled what and nest has been installed and working fine now