I just started my first HVAC job a few months ago at the STL Zoo; a bunch of different equipment, Residential Split Systems, RTU's, Chillers, ect. Anyway, went to PM the penguin building, a very critical building, obviously. Two 180 ton York Chillers feeding Temptrol(Spelling?) AHU's. On the each of the AHU's there are gauges showing pressure drop across the filters. Today they were showing about .4 inches of water column. At what value would you change the filters? I opened the doors, and they didn't look too bad, they were dated 7/29, so it hasn't even been a month since they were changed. Thanks for the help, Adam.