Worcester greenstar heatslave external 25/32

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Hello,
looking for help/ideas with the above oil boiler. It is switching itself on and running and blowing the boiler stat and sometimes the water high limit stat. There is no demand set on the controller. There is no in house stat, purely run by timer. Electrician has tested demand isn`t coming from controller (due to fault). I can run heating by switching on at controller and putting in the stats at the boiler but when heating demand is shut off it immediately switches to water and blows one or both stats. (basically we are not freezing thankfully but not able to get hot water.).
We (engineer) suspected frost stats might be causing the boiler to cut on and overrun but even with them disconnected it does the same (temperature was well above freezing). The frost stats are marked in the wiring diagram as optional (presumably if the unit is ex/internal) so we have been trying to find out if one of the other thermostats in the boiler/flue/water store works as an inbuilt frost stat and has the power to switch the boiler on??
Thanks for any help. Cheers David
PS Worcester technical help did mention an inbuilt frost function separate from the two marked frost stats but couldn`t isolate this for us when on the call, the engineer is going to speak to them again when he is round again in a couple of days...
 
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Not worked on these but...
I'd assume if the hot water store is cold it should fire up to bring it up to temp regardless of the heating demand.
I'd be looking in the pump terminal box for water ingress and if ok running it off a fly lead (and 3A fuse) to test it.
The diverter valve (especially the Drayton) would be suspect too...check for water etc.
 
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Thanks, not sure if I made clear, its cutting on when Heat and Water no demand on controller so water store should be able to sit cold.
Diverter valve etc all been checked too..
 
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Frost stat in boiler seen a few same on Grant's to be fair.
Agree, should be the only thing able to fire up the boiler if no demand but even with the frost stats disconnected it comes to life...hence trying to find out if another stat also acts as a frost protection..
Thanks
David
 
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1?2 hour with a multimeter should be able to find the source of power supply.
 
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1?2 hour with a multimeter should be able to find the source of power supply.
It should, heating engineer bringing a spark with him when he`s back again. Just trying to get a head start if the answer leapt out at anyone.
Thanks, David.
 
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He is hardly an engineer if he cannot use a multimeter

Check the flow switch is not sticking !
 
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He is hardly an engineer if he cannot use a multimeter.
He can, (even I can on a good day) but works alongside an electrician for new installs etc and both are popping in.
Whatever is the cause isn`t immediately apparent.
As above was hoping someone else recognised the behaviour and had sorted in the past.
Cheers
David.
 

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