My wife and I recently purchased a new home. The airhandler unit, which is located in the garage, continues to have condensation on the unit.

The Lennox rep. and the local dealer were at our house to try to figure out the problem. It was determined that we had an air flow issue. They said that the plenum on the supply side was creating turbulance with the blower, thus creating the condensation. The suppply side plenum came out about two feet, and then 90 degrees up into the attic. So now Lennox has replaced the airhandler(even exchange for a new one, which was under warranty) and also corrected the supply plenum by extending it out 4 ft, at a 45 degree angle.

Once this was completed, about 6 hours later, the new airhandler began to condensate again. We are now being told that our return air ducts are not big enough. It appears that everyone is just guessing at how to fix this.
Isn't there measurements that need to be taken to determine the correct air flow? static pressure, manual D, etc?

Does anyone have any ideas? Any suggestions would be greatly appreciated.