Yeh agree mate... I guess that's where touchy feely takes over
However Baxi are the only manufactures I'm aware of to take up this strange way of 'pre-heat' .... ! Most others pre-heat the phex and not a 'storage' in the expansion!! You'll always be caught out mate!
I've most likely mentioned it before but as an apprentice and ever since I've been taught and learned how most component parts work individually and work in with how the appliance itself functions ... ! I don't know how folk are taught these days but for me it's been an asset..!