I understand that if I apply pressure to water in a closed system, the boiling point increases. So at atmospheric pressure, it boils at 100 degrees C and at 100 PSI, this number increases to 164 degrees C. In my experiment, I'm using the sun to heat the water as it runs through a solar water heater on the roof. What I don't understand is why if I have the system at atmospheric pressure, it gets hotter than is I pressurize the circuit at 100 PSI. For example, at atmospheric pressure, I can get 65 degrees from my collector, but if I prime the circuit to mains pressure of 100 PSI, I struggle to get 47 degrees C. Why ? Can someone please explain this and provide a reference for me to research this more.