Return air in furnace room is a big no-no. If you have any naturaly vented appliances in the room you could definetly have a negative pressure on them. Check around the drafthood on your water heater and see if there are any signs of back venting. Look for soot around the draft hood or in extreme cases even the plastic control valve will begin to melt. If the unit is in a wide open basement, the return air will probably not present too much of a problem. Rule of thumb in our area is no return air within 10 ft. of the furnace.