how can i fix ideal combi he 35?

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