I HAVE RUN INTO THIS PROBLEM MYSELF. I INSTALLED 13 13SEER RHEEM SYSTEMS THIS YEAR. IF YOU DO NOT HAVE AT LEAST 24 INCHES OF STRAIGHT DUCT COMING OUT OF THE AIR HANDLER, IF IT IS A RHEEM AIR HANDLER, IT WILL GO OFF ON HIGH PRESSURE. IT IS NOT GETTING ENOUGH AIR FLOW. THIS CAME FROM THE RHEEM TECH REP IN OUR AREA. THE ONLY THING YOU CAN DO IS BLEED OFF UNTIL YOU GET ABOUT 250PSI ON HIGH SIDE IN THE HEAT MODE. OTHERWISE YOU MUST ADD SUPPLY DUCT AT THE AIR HANDLER. BEEN THERE, DONE IT.