I wonder that larger boilers, 60KW or more, might be better served with dual pumps. A couple small, off the shelf pumps like the Grundfos 15-58 could move around 15 gpm with a low pressure drop boiler and piping circuit. Once you put a 3 way thermostatic you add that pressure drop, and if it isn't a 100% bypass shutoff valve, you always bypass some flow. With dual pumps you could move 100% of the output and use a variable speed function for the return protection function. When the pump speed ramps down, so does the power consumption. With a 3 way thermostatic, the pump runs 100% speed and always works against the pressure drop of the valve so the pump output is lower. The higher the floe rate, the higher the pressure drop. It's a parasitic device, really.