Any time a water heater is used for both heating and domestic water the domestic water must have a anti-scald valve installed to protect people from getting burned.
This whole house mixing valve allows you to run the combo heater at any temperature to heat your house and allows you to adjust the domestic hot water to a safe operating temperature, it also increases the amount of hot water available.
This tank does not require dielectric unions. Do not use steel or galvanized pipe. Use copper only. Do not sweat fittings while on mixing valve or with rubber gaskets installed. Mixing valve needs to be installed below top of tank.