I just installed a roof top full of carrier equipment. 3 condensers and (2) 7.5 ton gas packs. The 2 large gas packs had no existing return and draw in 100 percent outside air on the return. The previous units were ducted the same way and always worked well..it looked like on the previous units they had slowed the fans down but that was the only alterations made to the units. I started my 2 new units up without slowing the fans down to see how they operated and on both of them pressures ran at 40 on low side and around 190 on high side...that confused me because i figured i would be running high pressure instead of low pressure. Low side lines were freezing up and line temp was around 32 degrees. I opened the compressor compartment door to bypass some of the condesing airflow and temperatures went closer to normal at 60 on the low side. Ran the units all day and they are cooling the building great. I guess I will install 2 condenser cycling controls on both the units to conteract the low pressure. Can anyone else give me any ideas other than that and an explanation as to the pressures im getting? The way they are running I dont think I will slow the fans down if I dont have to. Outside ambient on the roof top at 90 degrees today and im in the southeast with high humidity. Supply temp running at 47 degrees.