I am remodeling and adding a dedicated range vent hood. When removing the above-range microwave I found a vent opening that runs down inside the wall to the basement, and then out through the banding board (rim joist) of the house. Since the range hood I was planning to use vents towards the ceiling, I would need to make an immediate 180 degree U-turn to have air vent downwards. Does this present a problem, or should I find a vent with rear discharge and modify the opening on the wall as necessary?
You can install as you describe, however, know that your flow rate will be lower.
Bends create a lot of reduction in air flow, or alternately, more pressure is needed to maintain the same flow. Since you can not change the pressure of the fan then the bends and pipe length will have a flow reducing affect on your range vent.