A mixing valve without 'brains' will not properly perform.
I like to think of the mixing controls and piping around them as a “mixing assembly” that forms a bridge between the higher temperature boiler circuit and lower temperature distribution circuit as shown in Figure 1. All heat that makes it from the boiler to the distribution circuit has to cross this bridge. With proper control, the “Btu traffic” across the bridge can range from zero to full design load rate.