My Nest system has developed a strange fault after many years of trouble free use (not touched the wiring centre etc). It has started heating the hot water when it is not scheduled to do so, and there is no boost selected. It comes on randomly, and seems to heat indefinitely (generating far too much hot water), and in fact the only way I can stop it is to cut the power to the whole system. After power-cycling the whole system it appears to behaviour normally, but then after a few days the hot water comes on again. It is usually on for hours until I spot it, and power-cycle it again.
The Nest thermostat show a hot water drop icon on the screen, but the schedule and boost are not active.
At first I thought it was a setting issue on the thermostat, but I can't see anything wrong. I've listed all the things I've tried below at the bottom.
I wondered if it could be a non-Nest fault, eg with the wiring or thermostat that was forcing the system to call for heat. I wondered if the hot water drop icon on the screen would only show if the Nest was calling for heat, or even if it had detected that something else was calling for heat (eg. some weird fault with the cylinder thermostat). I asked Google AI and ChatGPT the following question, but one said "yes" and the other said "no", so that wasn't very helpful: "On a 3rd gen Nest thermostat, if input 6 in the heat link is driven live by an external source, even though the Nest is not calling for heat itself, will the Nest still show the hot water drop icon on its screen?"
Things I have tried (note, I have an S-Plan system):
- Factory resetting the Nest.
- Using a completely different Nest thermostat and heat link for the hot water.
- Fitting a new motorised valve.
None of the above made any difference.
The Nest is on s/w 6.4-8 and the heat link is 2.1.2-1.
FYI. I have two Nests and two heat links in my house. Only one controls the hot water, but I have one thermostat downstairs and one upstairs. They connect wirelessly to the heat links.
Any ideas? It's driving me mad, costing me more gas than needed, and probably not doing the boiler and hot water cylinder any good.
Any helpful would be hugely appreciated, and thank you in advance.
