[OJFord]

I think it's: 1) might not be on-demand, so then your hot fill might not be hot at the right time; 2) if it is hot, it will almost certainly be <70C, which is typically the max cycle temp.. so you either can't have that or still need a local element to get it there; 3) even if it's hot enough, you also need cold fill, so you can mix and get the temperature(s) you need.

Frankly I think even with some assumptions about your market to ignore 1 & 2, 3 alone makes it more complex than just taking a cold fill and heating to temp within the dishwasher. I believe the very few models that do have a hot fill do it on an eco basis (you already have hot water use it here too) but the validity of that is disputed, and really the only obvious benefit is to the EPC sticker (characterising energy performance) which will appear best in class (the class of all dishwashers) just by not having a multiple kW heater, not accounting for that having been offloaded to the boiler.