i sometimes use hot water from the tap, but i wouldn't do if i had copper pipes, my main water supply comes into the house at the kitchen, then there is 1 12" length of copper from the pvc to the boiler and 12" from the boiler to the flexi attaching to the sink, so if i run the tap for a minute or so first it must only be in contact with copper for a matter of milliseconds! i never bring the temp up too high though and as T1 said there is not a perfectly stable temperature anywhere apart from the sea and maybe in a couple of the biggest lakes so its natural to have slight temperature drops.
i still looking forward to pictures of the doors and tank surround, sad eh! ha