I'm assuming this is an Isar?
The most common fault for this boiler to be stuck on domestic (d) would be water movement, look for a leak on the HW supply including dripping taps.
Try turning off the cold water feed to the boiler, if this cures the problem, you have a leak somewhere.
If however it dosen't, you probably have a faulty HW thermistor.
Carefree, December 2010
