I have just fitted a brand new Worcester 24 Si combi boiler, and the CH seems to work fine, but the DHW seems to have a 'fixed' flow rate of about 7.7 L/min, whether the output temp chosen is 40c or 60c, and whether the incoming supply temp is 15c or 20c (haven't had cold enough weather to test it at say 5c yet); in other words, if the incoming water temp is 15c and i request DHW at 60c, it flows at 7.7 L/min, and if the incoming water temp is say 20c and i request DHW at only 40c, it flows at............you guessed it 7.7 L/min !!...so to recap, whether the boiler has to raise the temp by 45c or 20c, it always seems to flow at the same rate (7.7 L/min).
by the way, this is the first time i've fitted central heating, so i'm still learning and would appreciate all the help i can get many thanks to anyone who can help.