Hot Water Boiler pressure

steamfitter

A 3 story building has three condos, one on each floor with its own HW heating system. Each boiler has its PRV set at 18 psi. Each system has its own diaphragm tank charged to 18 psi. The question I have is although the 1st floor condo should only need 12 psi to properly operate, how much energy is wasted by having it operate at the higher pressure?

steamfitter

Thanks!