Also cleaned dhw heat exchanger.but also cld be main heat exchanger getting too hot? Secondary heat exchanger wasn't that bad.obviously trips out on overheat stat
Problem with dhw on junior 24i.lights up runs for a while get hot water but then stops running when gets too hot. Changed dhw thermister last year and seemed ok. Goes hot and cold tested all components all seem fine. Could it be intermittent fault on thermister as doesn't seem to regulate