Fan Powered VAV
Parallel Fan Powered VAV
The design is block air and block cooling capacity. This system is composed of a central, variable volume fan located on the cold deck that supplies cool air to each zone’s fan-powered VAV box. The return side of the fan powered box is said to be on the runaround deck since it draws plenum air from above the room served by the fan-powered box. The cooling supply fan is variable volume and will modulate in proportion to the sum of the cold deck airflows.
System Simulation
For non-mixing situations, the sequence of events is as follows. When the room drift temperature has risen above the room cooling thermostat, the cold deck side of the room mixing box is opened. The resulting room airflow is proportional to the room's design cooling load. When the room drift temperature drops below the room heating thermostat, only the return air side of the room mixing box opens and the unit fan is activated. The heating coil is activated within the fan-powered unit if the runaround air is unable to maintain the space above the heating thermostat setpoint. The unit fan is on for the portion of the hour it takes to bring up or maintain the room at the heating thermostat setpoint. When the room drift temperature lies within the dead band region, both the return air and the cold deck sides of the room mixing box are closed and no supply air is admitted to the space.
Mixing situations can occur in a room mixing box only when a reheat minimum airflow has been specified. A minimum stop defines the minimum amount of cold deck airflow that must be delivered to each space. As long as the room drift temperature stays above the room heating thermostat, no mixing takes place and the mixing box supplies air only from the cold deck. If, however, the room drift temperature falls below the heating thermostat settings, the runaround deck also opens such that the mixture of the warm and cool airflows controls the room temperature to the heating thermostat. Thus, as long as the room drift temperature remains in the dead band region, the room cold deck airflow will equal the room reheat minimum setting.
Note that shutting off mechanical cooling does not shut off the cooling supply fan. If the fan schedule reads 1% or greater, the cooling supply fan will modulate accordingly, supplying untreated return/outside air to the cold deck side of the mixing boxes that are open.
System Options
1. The value of minimum outside air nominally follows the outside air schedule. The amount of outside air introduced into the ROA deck may be greater than the ventilation minimum if an outside air economizer or nighttime purge is activated this hour.
In addition, the amount of outside air brought into a particular room is a function of how much cold deck air is needed by the room. If, for example, the drift temperature is below the cooling thermostat, the cold deck is closed and no outside air can be admitted (unless a reheat minimum has been specified). When the drift temperature is above the cooling thermostat, a proportional amount of outside air is admitted into the space.
2. If the main cooling fan is scheduled at zero percent for a particular hour, no ventilation airflow can be delivered to any of the rooms.
3. Supply air temperature reset can be defined using the temperature control in the controls tab of the system properties.
4. A reheat minimum will determine the minimum amount of cold deck air that is delivered into the space each hour.
Application Notes
● While supply air temperature reset control of the cold deck can save mechanical cooling energy, the room humidity may be higher and the central VAV fan may consume more energy.
● Humidity compensated controls can be implemented by using the humidity controller on the controls tab of the system properties to modify the default settings. By using the option supply air reset per maximum, for example, whenever a room relative humidity is greater than the design room relative humidity, the room's VAV dampers will continue to open until the room relative humidity is lowered to the design room relative humidity.
Alternatively, by specifying the option supply air reset per multizone maximum avg, whenever the return air relative humidity is greater than the design system relative humidity (which is the average of the design room relative humidities for the zones served by the system), supply air reset controls are deactivated and the main cooling coil leaving air temperature is depressed to its design value for that hour.
Series Fan-Powered VAV
The design is block air and block cooling capacity. The system is composed of a central VAV fan located on the cold deck which supplies cold air to series fan-powered (FP) boxes located in each room. Air from the central VAV fan is mixed with plenum air at the FP unit. The FP unit delivers air to the space at a constant volume. If heat is required in the space, a reheat coil located at the terminus of the FP box will activate to meet the extra heating load.
System Simulation
A constant volume of air is delivered to each room as long as both the main cooling and main heating fans are scheduled on. Once the drift temperature is known for all rooms this hour, the program calculates the supply air temperature and the proportions of cold and runaround deck air required by each room. When the room drift temperature has risen above the room cooling thermostat, cool air from the cold deck side of the FP unit opens and mixes with warm air from the runaround deck side of the FP unit such that the mixture temperature is sufficient to meet the cooling load. When the room drift temperature drops below the room heating thermostat, the reheat coil in the FP unit is energized, heating the supply air to a temperature which will maintain the space at the heating thermostat temperature. When the drift temperature is in the dead band, only untreated plenum air is admitted to the space.
System Options
1.The value of minimum room outside air airflow nominally follows the outside air schedule. The amount of outside air brought into the cold deck side of the room mixing box may be greater than the minimum if an outside air economizer or nighttime purge is activated this hour.
The quantity of outside air brought into the cold deck side of the mixing box may be less than the minimum if the cooling load for an individual room calls for cooling airflow less than the minimum ventilation airflow. In either the heating-only mode or in the deadband temperature range, no outside air can be delivered to the room (unless a reheat minimum has been input).