That is what I originally thought but the problem is when set to a higher total output the boiler does not modulate down well enough when only a few circuits are open.John will probably be along with the data but I see no real reason for doing this.
Can’t see any big savings to be made. The hot water cylinders need x amount of energy to heat, the house has heat losses and needs y amount of energy to heat.
Hot water priority/range rating aren’t going to change the energy needed.