I don’t think we have to worry about air mixing problem from the 5 ton unit because you can’t even feel the air blowing under the floor thru the perforated tiles. The vendor recommended the smaller unit blow under the floor to prevent a ‘dead rat’ smell. People laughed at that but when the 5 ton unit failed he was absolutely correct. Our primary unit works well, especially when the 2nd stage kicks in (2nd compressor). We do have hot & cold isles… the cold air blows in from the ceiling unobstructed (registers removed) down in front of the server racks. Of course, there are many ways to improve this…. we have one blade server rack with no return… so that hot air is indeed mixed into the cold. Our biggest problem right now is not having redundancy at all. We spent $100,000 on new units a few years ago and now they are talking about a redundant system totalling 15 tons blowing from the ceiling. I can see that being the primary in a few short years and then we are back to not having a sufficient redundancy again. Crazy, huh? In the meantime I’m on standby like a volunteer fireman waiting for a disaster until this company wakes up and provides sufficient reduntant cooling. The key right now will probably be get ting the servers guys in here to shut down as many servers as possible. I do believe 100 degrees for an extended period spells disaster. Thanks for your input.