Sounds live the divertor valve passing to me...........when you heating on this wont matter so much as the system is already hot so will not affect your hot water as much. You can replace the Gaivanni valve or cheper stil get a valve kit and repair it. Its easy enough to do but best will a corgi plumber me thinks.
Easy was to prove this is to wait for the sytem to go clap cold (heatin off0 turn a hot tap on for about 10 mins and see if any of the rad start to get warm.
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