The temperature of the hot water output on my Worcester Bosch 240 boiler has recently dropped to tepid at best, even after running HW taps for several minutes. I had a similar problem several years ago and, as many of the forums suggested, it was the diverter valve. As the sypmtoms were identical to last time I bought and replaced the diaphragm in the diverter valve but this time it has made no difference at all. The CH output is still fine, rads just as hot as usual.
Could it be the HW Heat Exchanger? Has anyone ever had to change/clean out one of these on a Worcester Bosch 240 before? or is there some HW temp control system that could have failed?
With the whole system cold, try running the hot tap and check the central heating flow pipe, if the flow gets hot you need to replace the diverter - the diaphragm is only part of the diverter - which is only used to push the pin out to activate the microswitch (tells the boiler you want hot water).
I ran the hot taps for well over 10 mins this morning CH Flow did not even get warm, however, for the first time in days I got quite hot water coming out the tap. I let everything cool down again then ran hot tap again. After almost another 10 mins started to get relatively hot water again.
The tap I ran is no more than 3 metres from the boiler so obviously a problem taking this long to get hot. Certainly doesn't look like its the diverter valve so must be one of the other probs you mentioned.
Any tips on how I can test which is most likely cause?
But if you check the flame picture when the heating is up to temperature the flames will be quite small, when you first put the heating on or the hot tap is running the flames should be noticeably larger.
If when the hot tap is running the flames stay small (low rate) then it's a modulation/gas pressure setting problem.
Is the 240 the one with the black DHW heat-ex ? bottom of boiler ?
Alot of worcester parts are now becoming obsolete. How old is the boiler ?
Checked this out several times but the flame picture didn't seem to differentiate at any point between CH and HW demand. The flame picture even remained constant after the heating had been on for a couple of hours possibly suggesting an under specced boiler.
Having also spoken to a couple of other people about this its seems most likely to be, as you've suggested, gas flow related, so I think I will get someone in to test it out.