A LITTLE MORE DETAIL - SORRY !!
i have ravenheat RSF 84 E - HEATING OPERATES FINE ! WHEN HEATING IS ON AND I TURN HOT WATER TAPS ON I BRIEFLY GET HOT WATER THEN MAIN BURNER CUTS-OUT , WATER IS LUKE WARM FOR 2 MINS ( APPROX ),THEN MAIN BURNER FIRES UP AGAIN THEN SHUTS OFF AGAIN. WHEN HEATING IS TURNED OFF, I OPEN HOT WATER TAPS , I RECEIVE HOT WATER ( TEMPORARILY) THEN BURNER CUTS -OUT AGAIN !
P.S I HAVE NO INTENTION OF TRYING TO FIX THE FAULT PERSONALLY - I JUST WANT AN UNDERSTANDING OF THE PROBLEM SO AS WHEN AN ENGINEER ARRIVES WITH SLEDGEHAMMER AND STARTS SPEAKING SPANISH ,QUOTES ME £200 THEN PROCEEDS TO HIKE THE BILL UP TO £350 (LIKE THE LAST GUY) ... JUST DONT LIKE GETTING HABITUALLY SPANKED BY A SWEATY-PLUMBER WITH A SLIPPERY SMILE- NOTHING PERSONAL, IM JUST A HUMBLE JOINER WANTING A FAIR SERVICE :) THANKS

rob, April 2008

its the diverter valve diaphram thats gone

pee, May 2008

Or the dhw thermister may be faulty

Also check the water filter on the mains water connection to the boiler, this quiet often blocks with crap and needs a good cleanout.

T, April 2008

sounds like the domestic water heat exchanger could be blocked

ck, April 2008