Greenstar 24i coming on at random times

[simon2win]

I have a Greenstar Junior 24i which is 6 years old.
Really great up to now, but recently started to come on at random times.
Recently undergone it's service and all well. PRV replaced a little while ago.
Boiler recently started to fire up and come on in the middle of the night when it was supposed to be off according to MT10 mech timer (lugs out)
MT10 replaced for new and still doing it (4 o'clock this morning)
Radiators still warm even though boiler off..
Thermostat will respond to fire up boiler if cranked round to higher temp..

Please help as I fear this might be internal circuitry and need whole boiler replacement..

[albatross]

Probably normal operation. Boiler has integral frost protection which is most likely to kick in on a cold night.

[simon2win]

Thanks Albatross but I don't think its that. Not that cold here in London and now at 00.53 the boiler timer says it should be off but the rads are piping hot. Boiler humming away nicely like its ON.. My bills are gonna be ridiculous if this keeps up!

[albatross]

try turning eco button on so it glows green. if eco is off the boiler will preheat the hot water and if the diverter
valve is passing this will leak to the radiators.

[simon2win]

Cheers will give that a try. Update from previously...
Just switched boiler off and flicked mech timer to off, then restarted boiler and it didn't fire up. Then flicked mech timer to 'timed' and boiler stayed OFF.. interesting but what could that mean? Don't really wanna do that every night.

[albatross]

it could be as i described above and if so the timer would have no effect either way. if timer is off and eco is off boiler will fire regularly to preheat hot water.

[simon2win]

Update... Boiler sitting quietly OFF so I go to wash hands at bathroom hot tap. Boiler fires up to heat water then stays ON. I've turned Eco light on now so will check in the morning.

[simon2win]

Eco button pressed and green light on last night before going to bed and mech timer set to 'timed'....
Woke up this morning and mech clock was showing correct time but boiler stone cold. Switched Eco button off and it fired up. Then shut off again after short time. Only fired up when hot water drawn. Boiler currently sitting with lights on but no heating. Hot water draws on request fine..

[nickwheat]

Sounds to me like the diverter valve.

[simon2win]

Thanks nickwheat, I'm interested in why you think it might be diverter valve? They seem to get the blame for a lot of things when boilers go wrong. Can you elaborate? cheers