A few thoughts on attic ductwork.
Prior to any insulation effort, inspect all joints/access doors,etc, for air leakage. Any conditioned air LOSSES TO THE ATTIC GO ONLY TOWARDS COOLING THE this unoccupid space.
Mark all leaks, then seal as necessary to approach 0 cfm leakage prior to insulation...
I suspect the heat exchangers are a means to seperate the cooling systems primary coolant from that of the 2ndary fluids.
Without a need to maintain seperate fluid standards, the heat exhangers would be a needless capital cost and operting expense. The heat exchanger would add maintenance...
The phenomina you describe is characteristic of a forward curve centrifugal fan [ i.e. SQUIRREL CAGE}.
Typically the motor HP is selected for the fan pressure duty. So if the fan is allowed to operate at less than design Static Pressure, it will move more air, and WILL move towards overload...
There is no question that cavitation will erode any epoxy even faster than a steel, stainless steel/bronze,etc. impellar alone. Cavitation is a violent occurrence on a very small scale, that by repeated repetition will destroy almost any material I know of.
One possible easy way out may be to...
I concur with Grizzly68 on insulation with compressive strength.
Consider Stainless steel material for condensate pan replacement. It will be a major assist in reducing places for algae to form, and also will not rust through.
Gnordo
Several issues to consider:
1. The fan motor, may or may not overload, depending on the type of fan blading- backward curve centrifugal, forward curve centrifugal, propeller blade, radial, etc.
. Look at the fan curves for CFM/HP/SP, etc. for the answer.
For sure, a backward curve...