Hi everyone, just to preface I had my radiant heating system designed my the Radiant Design Institute (Fred Seton).

Fred told me that the Phoenix PH100-55 would be able to supply both the radiant and domestic. From what I have been reading I want this system to run in closed system. Which I believe means that the domestic hot water supply is separate from the radiant. What I can't figure out is how it possible for that function correctly? I look at the water heater and it has a inlet and outlet. Best guess is the water stays separate because of the heat exchanger?

Could someone explain the concept to me?