By-pass Box Systems
Where the main supply unit must handle a constant volume of air, by-pass boxes can provide a variable volume of air to the zones served. The bypass boxes can be used on each zone, or as you saw in Figure 7.4, a single central by-pass can be used with variable volume boxes serving each zone.
Figure 7.6 shows the use of the by-pass box on each zone. A thermostat in each zone controls the damper in the by-pass box servinag the zone. The flow of air to each box is essentially constant. The bypass box, shown on the left, is set for full flow to the zone. The box in the center is passing some air to the zone and bypassing the balance. The zone on the right is unoccupied, and the box is set to bypass the full flow. The zone thermostat controls how much of the air is directed into the zone and how much is by-passed into the return-air system. In many buildings, the return can be via the space above the dropped ceiling, the ceiling plenum, and then, via a duct, back to the return of the air-handling unit.
With the by-pass system, it is important to keep the ceiling plenum at a negative pressure, so that the excess cooling air does not leak into the zone.
The danger of keeping the ceiling at negative pressure, though, is that this can cause infiltration of outside air through the walls and roof joints, resulting in moisture and load challenges.