I plan to install a HRV system to save on labor costs and have a local dealer checkout and balance the system afterwards. My install questions, what is the maximum length I can have going to an exterior wall from the HRV unit using 6" duct? Also, what is the reason for not installing the fresh-air and stale-air duct on the vertical drop down of the return air plenum? My return air plenum T's just above the furnace going to separate ends of the house. Haven't decided yet on manufacture but leaning towards a Bryant unit. I have total area of 4000sq ft (includes basement) so with my calculations I need a unit that will handle at least 186 cfm.