On Thursday, June 5, 2014 1:31:15 PM UTC-4, Scott Lurndal wrote:
Perhaps I wasn't clear enough. There was no ductwork, the entire

basement was the "duct". There was just an opening through the

floor at the base of the wall cavity, and a vent high on the wall.



The air handler unit had the filter and just pulled air from the

basement.

I've never seen that in residential but it was common in commercial at one time. Often outside air and return air mixed in the same space and was drawn into the air handler. It was impossible to really control outside air ventilation but the systems worked.

Your setup is not that different from a plenum return, albeit a large plenum.