Gas and Oil Home Heating Furnaces - Furnace air duct?

We hired a contractor to build a house for us approx 6 years ago. This year I decided to finish the basement and had previously wondered about an insulated air duct which brings fresh air from outside into our furnace room. Our furnace is an oil fired hot water boiler unit. About a month ago a friend of mine suggested that this fresh air duct was not necessary and only served to leak cold air into our heated basement and should be removed. I've been cautious and left it in place but now I wonder if it's better to either remove it or even tie it off in case there is an air supply problem down the road. There must be a fresh air return already built into the furnace venting system. Does anyone know if there is any danger in removing this air duct?

Jeff

It's for combustion air and more than likely you need it. If you do anything to close it off fix it so it can be opened in the heating season.