I have been having trouble maintaining the proper operating pressure and keeping air out of my single zone hydronic heating system in my single level home. I have a Weil-Mcclain gas boiler that I installed 5 years ago. Last year I replaced the air vent and expansion tank that was leaking. I also relocated the tank to match the manufacturers instalation instructions. Should the expansion tank and water make-up valve be installed on the suction side (return line)of the system? How should the tank be orientated? (up, down or sideways) It seems that after I get the system charged at 10PSI and turn the pump on it pulls additional water through the PRV. So what I do is charge the system to 10PSI with the pump running, remove all air and close the makeup water supply. It seems to work fine for a few days but after a week or so the pressure creeps up to 20PSI and air gets into the system. I have inspected the entire system and have found no leaks. Any suggestions would be appreciated as I am stumped.