Sorry answer a 'bit late' in coming but I had the same problem. Not being a plumber or boiler specialist, I sripped mine down anyway! (well, saved me a fortune).
Please note: I did NOT disconnect the electricity or gas, merely ISOLATED them at switches / valves.
DO NOT MESS WITH EITHER - THEY ARE NOT FRIENDLY AND COULD BE FATAL.
IMPORTANT:
Please read the WHOLE answer before starting.
Here goes: Inside the boiler, once the front cover is off, you can see the two 3 way groups, see parts breakdown in SERVICE manual for details. One group, on the left, has the radiator flow out and also the hot water out attached, with one pipe coming in on the top from the main heat exchanger. This 'group' also has other bits attached including a microswitch fitted. Basically when the hot tap is run the pressure changes in the pipe and makes this group change over to H.W output and at the same time a pin moves outwards and pushes a microswitch. This starts the pump and fires the burner to heat the water.
Having isolated the water, gas and electric and drained the water from the system, remove this group. Refer to SERVICE manual for this. You will need to unscrew a very large nut on the right-hand side of this group (It clearly has a s/steel 'PIN' poking out of it). The problem was a hard crusty residue built up on the stainless steel center pin which actuates the microswitch. Mine was coming out just far enough to push the switch but not enough to fully switch the output to the hot taps. The hot water was being pumped into the radiator closed loop.
Wire brush the residue off, and refit all. Refill the system and try.
Not saying this will be the cause of your fault, but it fixed mine.
While you're in there, remove the temperature sensor and any other 'unscrewable' part of the group and clean any debris / residue from it all before refitting. Might as well while you're there!
Any questions about this particular boiler, email me at axel@supanet.com and I will try to help.
Clive, February 2007
