Fixed identical problem on same boiler about 6 month ago. Fault caused by wear in the diverter valve which will have to be replaced. When heating in off position diverter valve remains at rest in DHW position but owing to wear on seating. hot water passes through to heating circuit. Parts for Protherm not available at all stockists but Glow-worm, who import the boilers will infom you where to try.
Barnie
April 2007
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