New water heater

Bought a new Addax on-demand gas water heater today.  Should be plenty good enough to power a couple of showers.  Went to buy the largest 14l/min (1.9kg/hr), but ended up with the 11l/min kW model as the flue diameter was too big on the large one.  Will fit a changeover kit too, as the gas flow is huge flat out. (1.6kg/hr)