Lots of different boilers have a HW pre heat function but most can be disabled, not sure if you can or how on an Isar as I dont touch them they are possibly the worst boiler I have ever come across, lots of lads a lot more experienced with this boiler on here will come on and advise you better
Come, come Ian. I was of the same opinion a while ago. Use to swerve working on it, even fried a board (and paid for replacement too may I add) when got one of the plugs mixed up. Then a light came on in my head, why not get to know the beast better. Was in Landan, on the way back went to Hull for a day course at their factor. Very informative course, lecturer did not mince his words. Came away feeling this boiler is nice dog with a bad name. Now I look forward to repair calls for Isars.
On my patch there are a few estates with this boiler (it now being the builders choice)