That is why I said to go through the filter then the radiator. if you took the biggest filter they have at the hardware store and then found a truck radiator or something as large as the filter then made a new housing between the cold air return and the furnace you wouldn't lose much if any efficientcy. You cound then go to an underground tank (Bladerunner style) using the heat for your house in the winter. You could probably bypass the radiator in the summer so as not to hurt the A/C efficientcy.
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