I do not have Vents to cool or heat my basement, what is the easy way to do it? where to put return or supply vent. I read somewhere that just to put a return in existing return duct. I have existing supply line cut for humdifier was wondering if I can just put flexible duct with the matching hole in dry wall . Will that work? Please advise
The "easy" way is rarely the proper way and it could seriously upset the proper performance of your entire heating system. Ideally you want to do a heat loss / heat gain calculation on at least the room to which you will be adding the ductwork to in order to properly size the ducts.
The proper place to connect the supply duct is at the furnace discharge plenum or trunk duct. Be sure to install a balancing damper at the point of take off. Return air ducts should be 1-1/4 to 1-1/2 the size of the corresponding supply duct and should connect to the furnace return in a manner that allows free flow from all the returns with minimal resistance and also so that all returns are filtered before entering the blower suction chamber.