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Nest thermostat with hot water function not working

Postby Sarahm89 » Wed Jun 09, 2021 6:32 pm

Hi all,

I got a nest thermostat installed on December and I've recently realised its not working right, the hot water function doesn't work. I have to have the heating on also, which now that it's summer isn't really great! I decided to take a look at the wiring and knew straight away something was wrong! I'd rather not pay someone again to fix it, is there any advice people can give to help me fix it? I have basic wiring knowledge!
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Postby Mr White » Thu Jun 10, 2021 6:52 am

Without knowing which wire goes where and more importantly what is at the other end of each wire, it is not possible to say how to correct it.
I would however suggest you get the original installer back as it is only 7 months old and has never worked properly since the date it was installed.
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Postby ericmark » Thu Jun 10, 2021 8:22 am

I agree with Mr White.

It clearly has some connections to both central heating (CH) and domestic hot water (DHW) so only fault seen is the amount of copper showing which will not stop it working.

The thermostat can be wired for many options, from opentherm with a combi boiler to the Plans, C, S, W and Y the S and Y Plan diagrams are in the book that comes with the thermostat, the C plan is not shown.

The Y and W plan look the same, in the main Y where there is a three port valve which has a centre option, with W no centre option.

The S plan uses two x two port valves.

S, W, and Y plans have a tank thermostat, and the DHW is pumped.

Some times the C plan has a valve, there are around three versions of the C plan, the basic idea is DHW is thermo syphon so does not need a pump, and CH needs a pump, however often upper rooms will still heat up slightly even with no pump running.

I fitted Nest Gen 3 with C Plan as I could limit the DHW to 1/2 hour a day, in summer better if there is a tank thermostat.

However as standard the Nest has anti legionnaires software which will fire up the boiler far more than I wanted. It had to be disabled in the software. I have had other issues where it tries to learn, that is now also turned off.
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