If you are running off your existing ducting for heat and air, make sure you put in a return duct also, and you probably want to put a carbon filter on that or your whole house will smell dank. Either that or you could probably use a passive exhaust, with filter do to positive pressure from the duct blowing in heat and air. If you do it taht way I think I would use a cool tube for the light ducted on both ends with a fan exhausting that right into the attic. You may have to damper the duct for heat/air depending on how warm it gets in there since heat rises and most likely your thermostat is on a lower floorand will regulate the temps based on that room, not the grow room temp.