Morris Dovey wrote:

My current aim is to lower that 140F to 120F for a 6'-tall panel
without major cost increase. The Madison County installation won't
quite make it but I'm getting closer with every design iteration.
Interestingly, I'm not sure that achieving that goal will necessarily
produce a more efficient /system/.

How about a hybrid system? Would not adding, say, a low-cost
(operation) axial/muffin fan with a rheostat allow you to tweak the
internal temperature?

In your scheme of things that'd be cheating, I know, but... I'm just
thinking aloud here. Even from a testing/design aspect you'd at least
be able to see if the 120F would be more efficient, no?