My brother just purchased a Goodman CPLT36 CU and AER36 AHU HP system, still new\never used in the box from a person. (built circa 2004) Bottom line it is a 14 SEER Variable Speed AHU split system with a few system bells and whistles. It is being installed in the Down Draft position in a family members modular home. He called and asked me if there was a external drain pan available, like on most other AHU installations. I was stumped. I suggested a model SS2 condensate overflow shut off switch threaded into the secondary drain outlet. I understand his concern. If there is ever a internal condensate leak past the internal drain pan, the water may flow over the fan motor/thermals/electronic's, etc before settling in the supply duct work. To be honest, I never have installed a counter flow AHU.

Any ideas, or is the condensate switch the only option?

Also, for grins, what does the "R" stand for in AER36 AHU? If it stands for "R"esidential, I'll kick myself in the behind.