The HVAC system discussed here is a variable-air-volume (VAV) system with an economizer. The economizer allows outdoor air to be used for cooling when conditions permit. The system shown below is a normal HVAC operating mode.
This mode includes the return gate, the exhaust gate and the outdoor air damper modulating to adjust the amount of outside air supplied to the building. The VAV system has VAV fans for supply and return that are used to adjust the flow rates of the HVAC system as needed. The system serves several HVAC zones, and each zone has a terminal box that controls the amount of air supplied to that space.
The mode of operation used for an HVAC system depends on the kind of zoned smoke control and on the arrangement of HVAC zones in the building.
Separate HVAC Systems for Each Floor:
For buildings that have separate HVAC systems for each floor, zoned smoke control can consist of putting the HVAC systems in the pressurization mode or the smoke exhaust mode as appropriate. HVAC systems serving other floors either may be shut down or allowed to operate in the normal node.
For an HVAC system in the pressurization mode:
In this mode, the return damper and exhaust damper are closed, the outdoor damper is opened, return fan is shut down and supply fan is set to a flow rate determined during balancing of the smoke control system.
For an HVAC system in the exhaust mode:
In this mode, the return damper and outdoor damper are closed, the exhaust damper is opened, the supply fan is shut down, and return fan is set to flow rate determined during balancing of flow rate system.
For pressurization and smoke exhaust mode of operation:
In this mode, the return damper is closed, the outdoor air damper and exhaust damper are opened, the supply fan and the return fan are set to flow rates during balancing of a smoke control system. To minimize any smoke from flowing past and return air and damper to supply air, the return damper needs to be very tight.