Why are hoods with combination sashes offered as open bypass by special request only?
The face area for a hood with a combination sash is based on the opening through the horizontal siding panels. It has roughly one half the face area of the same size hood with a vertical sash. Therefore, the hood with a combination sash requires a smaller (restricted) bypass even if it is to be used for constant volume operation. In fact, having an open bypass provides no operational benefits and increases the exhaust requirement for the hood as air must be drawn through the bypass into the hood. The only time an open bypass is required is if the exhaust volume is based on the sash raised vertically rather the opening through he horizontal panels.
