Biasi M90F.24S CH working but no DHW

[fishinuk]

Any advice with this would be very gratefully received. We have a Garda (Biasi) M90F.24S combi boiler, which is no longer providing domestic hot water. The Central Heating works fine, but the boiler goes off as soon as we open a hot water tap and stays off for as long as we run the hot tap (all hot taps the same result). As soon as we close the tap, the heating fires up again and continues to work normally until we next open a hot water tap. If heating is off when a tap is opened there is still no hot water - boiler starts up but makes no attempt to light - ie. no spark. After about 20 seconds of running the boiler goes quiet. Please advise likely cause and whether likely to be a costly repair. Thank you very much. Ian

 

[AllyBongo]

Needs checked out with a multimeter mate

 

[fishinuk]

Many thanks, I should have mentioned that the problem is intermittent, I would say that DHW fails 90% of the time, usually working best when hot taps have not been run for a while, so water is cold. Does this help at all? Thanks Ian

 

[oldplumber]

could be a micro switch failure on the hw diverter valve or the hw thermister.ntc/ptc (whatever is fitted) that has failed, so test for its resistance, so a couple of starters for ten. so call in an rgi to carry out tests and replace the faulty parts.

 

[fishinuk]

Many thanks, for replies - much appreciated. Would anyone know an average/reasonable callout/hourly labour rate for a registered fitter?