cold air return vent in basement
Currently finishing my basement and need to add a cold air return vent(s) before dry wall goes up but not sure what size. The basement is about 1100 sq. ft/ and has 5 4x12 supply registers. There is only one return air vent for the main level of the house and it is very large and centrally located. It handles that area well but no return vents were added to the basement when built. I had the furnace (dual fuel, high Eff. unit)services a couple of weeks ago and the tech suggested I had just one cental cold air return but did not suggest a size. The basement is well insulated and never drops below 55 degrees during the winter even with the supply vents closed and heats well when open but because I am dividing the area into 5 rooms (3 large rooms, one small and a bath) will just one work or do I need more and what size? The funance is centrally located so adding more than one should not be a problem but I do not want to effect the balance supply/return that now exist. The house is 1972 sq. ft. finished on the upper two levels and has a unit for each level. I do not need to a/c the basement in the summer since it stays around 70 degrees, I usually close the supply vents during the summer and run a large dehumidifier which keeps every thing dry and comfortable. Also it is easier to place the return vent up high then close to the floor except in the room next the furnace and there I can run a return vent about 6 inches thru an existing interior wall, which is better or will one down and two up work? Ceiling is 9 ft. high and all the supple vents are located in the ceiling. The tech said it was an easy job I could handle but what size return and what size duct work?