Mark, if a master mixing valve does not close off tight at the hot inlet port, you need those 2 "balancing" valves to minimize the amount of hot water going back to the storage. Many master (ASSE 1017) thermostatic mixing valves do not close off 100% (Caleffi's 521 and 5231 Series, for example). Some do, for example our 520 Series and our 6000 Series LEGIOMIX electronic mixing valve; they close off tight to prevent creep.
Mark, if a master mixing valve does not close off tight at the hot inlet port, you need those 2 "balancing" valves to minimize the amount of hot water going back to the storage. Many master (ASSE 1017) thermostatic mixing valves do not close off 100% (Caleffi's 521 and 5231 Series, for example). Some do, for example our 520 Series and our 6000 Series LEGIOMIX electronic mixing valve; they close off tight to prevent creep.