Nest controls my Idea Logic 15 but yesterday, when I selected Hot Water, my water was heated but it refues to shut down displaying 'C'

A few days ago my central heating system seems to have developed a problem.

My boiler is an Ideal Logic 15. It appears that the Nest thermostat is working fine but somehow the boiler thinks there is a constant demand to supply heating / hot water as it displays a constant 'C'. Usually the heating / boiler activates only when the flame icon is on as the thermostat is at a temp below where sought. This all works as intended.

A few days ago, I switched on my Nest to heat the Hot Water for 30 minutes which all worked fine & I had hot water However, when it came to automatically swtitch off, the boiler acts as though it is still on & displays a Status C, even thought the blueight is off but I my household Gas & Electricity Consumption has doubled; it's as though the boiler is doing something. My MagnaFlow Hot Water Tank upstairs is now constantly humming as though it is always on.

For now, I let the Nest Thermostat get the house to its desired temperature & I then switch off the Boiler, but I understand that this is only a temporary fix. The Nest Thermostat controls all work fine, its just that the boiler never seems to switch off although it doesn't seem to be heating anything (i.e. no blue light)

Hope someone can help please, I’ve tried resetting everything and powered on/off so I know its a fault with something, such as one of the valves or perhaps a pump.

Any ideas? Thank you

I would guess a micro switch stuck on in the motorised valve, had the same myself, took the head off the valve, gave it a sharp tap, and replaced, and it has worked ever since OK.

Do you have any idea whether it would be the return valve or the cold water valve? How do I know which one it is?

Do I just take it apart & see the results, then repeat for both motorized valves? This way I can hopefully isolate which valce is causing the problem?

As above with those symptoms it's micro switch on heating zone valve.
It will be valve that stays hot !
If you have a multimeter and are competent to work with mains voltage check in junction box.
If wired correctly the grey to valve will be permanent live the orange is switch wire.
Check if live still there with nest switched off.

I got a new micro switch for my motorised valve, and have never fitted it, it has not (touch wood) gone wrong again. I removed the head, gave it a sharp rap on the cupboard floor, refitted, and it has continued to work for well over a year.

OK, that is not an approved method, but it worked. At 70+ I am not really agile enough to work in the cupboard where the motorised valves are, I had hoped the service guy would turn up, and I would have got him to change the head, but he didn't turn up, so crossing fingers it does not fail again.

Apploogies, as I have had a hospital emergency in the family, so my boiler issue has taken a back seat for a few days.

Well, one of the Micro Switches was realy warm & by manually turning it off, I could control the boiler & thus the gas / electric consumpton; could this be the issue? However I still wasn't convinced that it was the micro switch, as it seemed to work at times, so I did some more investigative work.

It transpires that my property has a secondary heating circuit, controlled by a DanFoss micro-switch & this was in turn subject to a second thermostat, one that I have never used before & is not controlled by the Nest; I had to search for this. The battery on this second thermostat was really really low, so I had to replace them. Low & behold, over he course of a couple of days, everything started to go back to normal & my boiler / hot water was being controlled as before.

So even though my upstairs thermostat never does anything, as my upstairs is always warm, an expired battery has caused this weird control issue with my boiler; I'll know next time to check my secondary theromstat.

Whilst I am still a little wary, as I just don't understand why my heating / hot water should react in the way it did to a 'failed thermostat' due to battery falure but everything apears to be back to normal.

I had to work out how my heating worked and slowly make a diagram. This one is actually not correct, I found I needed relays to stop a back feed, but the only way is to draw out what you have.