Installing nest 3 on combi boiler with unvented indirect cylinder.

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The pictures have attempted descriptions if you click on them
I could be wrong, but not sure if you have heating on and hot water on mixed up at Nest? What did you remove?
Thanks! I thought that at first too, so I switched the blue and brown wires on the nest (3 and 6) but then the radiators didn't turn on. Perhaps my setting of the nest wasn't accurate at the time.

Can I ask why you think this?
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Yes, because if they were the same colour in the old programmer then they should be the same in the Nest, then you need to sort the black wire out from old thermostat wall should work. This black wire is your on signal.
Sorry but you shouldnt be touching the wiring to an unvented cylinder unless you are qualified to do so, it has additional safety features that need to be wired correctly, there will be an overheat thermostat on the cylinder try re-setting it and see if the HW comes on
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