Nest v3 - heating not switching off

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ChrisWms

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Can't find an identical problem, so here goes.

Nest v3 installed in the summer, by my local gas engineer. Don't want to bother him again as he took pity on us and charged us an unbelievably low price to sort out many issues left by a previous cowboy.

Heating and hot water are on a Y plan system, fed by a Worcester 35CDi. Hot water and heating come on according to schedule, and manual hot water boost and Nest room stat calls for heat as they should.

However, when the room stat is up to temperature, boiler continues to run, and radiators still get fed. Clicking of the relay in the heatlink when the temperature is reached, but still reads 10v at heatlink terminal?

If you then turn hot water to boost, and off again, the boiler shuts off and all is well again until the next schedule or movement of the temperature dial to call for heat.

Valve head? Incorrect wiring? Faulty heatlink?

Thanks for any help.
 
You are right, it is an unusual fault. The boiler should be controlled from two places, an internal switch inside the motorised valve which operates when only the central heating is required, and the cylinder thermostat whenever the hot water is required.

I don't think the problem is the Heat link. If the Heat link was not switching off, it would keep the heating on, but then I can't see how it would 'unstick' when you "turn hot water to boost, and off again". [Heating and Hot Water are separate circuits as wired to the Heat link]

I also don't think it's the motorised valve. When 'heating only' is required the valve winds fully across to operate the aforesaid internal switch. However, this switch gets its power from the Heat link, so even if the valve or the internal switch were sticking, the switch wouldn't be getting any power when the Heat link turns the heating off, so that wouldn't keep the boiler going either.

So that brings me to the wiring. Initially as @ianmcd I thought about the HW off connection which comes from the programmer and the hot water cylinder thermostat. I tried to introduce mistakes, including transposing the wires to create all sorts of errors, but can't come up with one that produces the symptoms you describe.

At the end of the day, I think it will have to be a matter of checking that the wiring between each of the components is correct and goes where it should.

Capture.JPG

[In the above diagram 'Hot water tank' = Hot water cylinder thermostat, and the boiler will probably also have a permanent Live]

The motorised valve and its wiring can be tested with a multimeter if you are competent to do so. If you are, here's a table of the results you should find.

upload_2021-12-14_10-5-25.jpeg


Heating 'on' = Nest set to heating 'on' and thermostat calling for heat
Hot water 'on' = Nest set to hot water 'on' and hot water cylinder thermostat calling for heat.

With regard to the 10v. That could be being fed back from other parts of the system. The motorised valve which has power on even when the hot water and central heating are 'off' has various resisters in circuit which can cause odd voltages. There have however, been instances with certain boilers being triggered by lower voltages but they are usually in the region of 70v plus. Fitting a capacitor usually sorts it, there's an example here.
 
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