Hydronic System Design

• Lower pressure than household water pressure is industry standard in hydronic systems. 15 psi. Rationale: it appears the main reason is that the lower the pressure, the less risk of bursting PEX pipes at high temperature.
• Thermostatic mixing valve - while normally used in showers so people don't get burned, the specific reason for using it in a hydronic system around the heater element is not clear. Why preheat the water going back into the stove? Why reduce outgoing water temperature? Is raising inlet temp important to reduce soot buildup on heating pipe or jacket in a wood stove? Is it to reduce overpressure conditions in the system? The case for buffering hot water at the exit of a pipe-based heater is clear: it prevents overheated water from entering PEX pipes. But if there is a buffer tank at exit - is a thermostatic mixing valve necessary?