Viessmann 222f, its as big as a fridge though, it comes in 3 flavours 19, 26, and 35 kw (the 26kw gives 30kw to hot water though) there is a small cylinder built into it, it will give you 20 litres a minute, while its giving you 20 litres, the combi is replacing the hot water to the cylinder at 14 lites per min, this gives the 100litre cylinder the hot water capacity of a 250litre cylinder, when this runs out the boiler reverts back to being a normal combi giving 14+ litres per minute indefinately (depending on size of combi), then when you turn off the tap the cylinder is replenished back ready to start again in under 20 mins. Not cheap though, but neither is a 550. I think Alpha do a similiar system CD50? Vaillant 937, not quite the same capacity as the Viessmann though but a bit cheaper I think.