i have what i believe is a y plan fully pumped system with a baxi bermuda back boiler. all controls ie programmer, room stat, cylinder stat and diverter valve are honeywell. programmer fairly new but others old, metal covers on cylinder stat and diverter valve. problem is this-if i switch controls to heating only it takes about one hour for valve to switch over to heating ie it appears as if it is satisfying dhw 1st even though i am not calling for water. if i turn down cylinder stat it immediately switches to heating. cylinder stat is set at 120. any ideas?