Variable Air Volume (VAV Terminal), Dump Box
This device is an addition to an air ducted system to provide additional control of the temperature and air flow in a particular zone or area. A VAV terminal is very simply a motorized damper box that operates based upon a signal from a wall thermostat. This wall thermostat only controls the damper in the VAV terminal and not the operation of the heating / cooling system. The VAV terminal will have a minimum position that allows approx. 10% of the normal air flow rating to pass through for ventilation. This will also stop air noise or whistle that would occur if the damper closed all the way. The VAV terminal will have a sensor that is measuring the temperature of the incoming air. If the incoming air temperature is hot and the wall thermostat is calling for cooling the damper will stay closed to the minimum position and prevent overheating the space. Should the incoming air be hot and the space thermostat call for heating, the damper would open allowing heat into the space until the room thermostat is satisfied. The room thermostat will then let the damper close to minimum. The same operation can be inferred for cooling in the space.
If every outlet on this system was controlled by a VAV terminal and many of the terminals were in the closed position, the remaining open terminals would have to pass all the air that the unit is producing. This would be extremely noisy and effectively blast air into a space at a very uncomfortable rate. The more VAV terminals that close, would result in more and more air trying to squeeze out of the remaining dampers that are open. To control this problem in the ductwork there would be a sensor that senses the buildup of pressure and would open a damper between the supply duct and return air, effectively bypassing the VAV terminals. This bypass damper will modulate open and closed to maintain a comfortable pressure in the system. There are other methods of controlling the pressure in the ducted system, but this method and explanation should suffice for this example. VAV terminals are used for zone temperature control in larger buildings where a roof unit may be set to deliver one constant temperature year round. With this method each zone can have cooling or ventilation air at any time by adjusting the zone thermostat. If a zone requires heating the VAV will close and an electrical interlock can be energized to turn on baseboard electric or hot water heating in the zone. These systems are relatively simple and the only thing that would complicate service is having to move furniture to access a VAV terminal in the ceiling space.
A dump box is another form of zone control that could be termed a "poor mans' VAV system" A dump box operates in much the same manner as a VAV terminal, sensing incoming air temperature and controlling the damper with a wall thermostat. A dump box can also be interlocked with auxiliary heating like the VAV. The difference is that the dump box does not need a pressure sensor as the dump box opens a damper to relieve the air flow to the ceiling plenum when the air is closed to the space. The air is "dumped" into the ceiling space and recirculated back to the roof unit via the return air plenum. Dump boxes can be added to any existing system and would normally be installed in a single office space or two where additional temperature control is required. In the case where a dump box is added to an existing system, the roof unit will be controlled by a thermostat in another area covered by the same duct system.