Faulty cylinder stat?

[Oilyragg]

Will do Stem ironically it hasn't done it for a weeks until last night but I was to tired to start running round with the multi meter.

 

[penguin16]

Our cyclinder stat has a permanent live. It automatically heats the water up to 60°c once a week regardless of any input from my nest. Also, our frost stat is connected to our HW so when it drops below 10°c in the garage (where the boiler is), the HW will be heated.

Last week ours was coming on throughout the night (when it's been much cooler outside) but nothing this week (while warmer) so it may be related to temperature somewhere.

 

[Oilyragg]

A heating engineer came out last week to service the boiler and I told him about the problem and he was kind enough to have a look but he admitted he had never seen a nest system before but it seemed to him that the yellow wires were the wrong way round at the heat link. I swapped 4 and 6 over but when I turned the hot water on it went off and when I turned it off it came on. I checked the wires from the heat link to the tank stat/zone valve and it seems I was right to start with, 6 should go to Common and 4 to 1 (tank stat) and Gray wire to zone valve. So I swapped them back but what I did find was the wires in the tank stat were the wrong way round as well. 1 and 2 had been swapped around but then the system worked as it should. With the stat wires the correct way round again the hot water went off when turn on and vice versa? I fitted a new tank stat which I found in the garage while I was fiddling but kept wires 1 and 2 the wrong way round so the system would work correctly

 

[stem]

I swapped 4 and 6 over but when I turned the hot water on it went off and when I turned it off it came on.

That is exactly what will happen, you are moving the live switched wire from the 'call for heat' position to the 'satisfied' position, so it will work in reverse. If it was that simple we would have spotted it much earlier on. Put it back as it was

 

[stem]

Our cyclinder stat has a permanent live. It automatically heats the water up to 60°c once a week regardless of any input from my nest.

That's done to heat up the system to kill off any Legionella bacteria. Normally it's done with solar heating systems, where there is the possibility that during the winter the temperature might not get high enough to do so. I have never seen it on a boiler only installation.

Also, our frost stat is connected to our HW so when it drops below 10°c in the garage (where the boiler is), the HW will be heated. Last week ours was coming on throughout the night (when it's been much cooler outside) but nothing this week (while warmer) so it may be related to temperature somewhere.

The OP says he doesn't have a frost stat, but if he did, frost stats usually either just fire up the boiler, or the heating. Not the hot water.