VAV Reheat
VAV RH (30% Min Flow Default)
Note, there are three versions of this system:
● VAV RH (30% Min Flow Default) (DX): Variable Volume Reheat (30% Min Flow Default) (Direct Expansion)
● VAV RH (30% Min Flow Default) (CW): Variable Volume Reheat (30% Min Flow Default) (Chilled Water)
● VAV RH (30% Min Flow Default) (CW)(HWRH): Variable Volume Reheat (30% Min Flow Default) (Chilled Water)
The only difference between these two systems is the Direct Expansion version uses a direct expansion cooling coil whereas the Chilled Water version uses a chilled water cooling coil. The third option uses a chilled water coil for the primary cooling coil and a hot water coil for the reheat coil.
The design is block cooling capacity and block air. This system is composed of a central VAV fan located on the cold deck that supplies conditioned air to each room's VAV box. The individual rooms may see a variable air quantity depending upon the cooling load and the minimum room airflow setting. A reheat coil in each VAV box is available to reheat the conditioned air if necessary at part load conditions. Return air is drawn from a common return air plenum where loads to return air are picked up from all rooms and brought back to the ROA deck for mixing with outside air.
System Simulation
For the case when no minimum stops exist, the following events will occur. When the drift temperature rises above this hour's cooling thermostat set point, the VAV box opens and delivers a proportionate quantity of supply air to the space, i.e., only enough cool supply air is added to bring down and maintain the room at this hour's cooling thermostat setpoint. When the drift temperature falls below this hour's heating thermostat setpoint, the VAV box is fully closed and neither heating nor cooling is possible by the main system since no supply airflow is available. As long as the room drift temperature is below the cooling thermostat setpoint this hour, the VAV box is fully closed. While the drift temperature is within the deadband region, there is no air movement and absolutely no mechanical heating or cooling can occur in the room by the main system.
For the case where minimum stops exist, the sequence of events is as follows. The VAV box is initially fixed at the reheat minimum airflow setting. Introduction of this cool air will drive the room temperature downward if the space cooling load is not great enough. The reheat coil will be energized only if the room temperature starts dropping below this hour's heating thermostat set point. If the admittance of this minimum quantity of conditioned air does not bring the room temperature below the cooling thermostat during times of high sensible load, the VAV box is opened past the minimum stop position until there is adequate cooling airflow to satisfy the cooling load.
Note that shutting off mechanical cooling does not shut off the VAV supply fan; if the fan schedule reads 1% or greater the supply fan will modulate accordingly, supplying untreated return/outside air to the terminal boxes that are open. However, if the supply fan is scheduled off for a particular hour, no mechanical cooling or mechanical heating is possible.
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.
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.
2. The cooling supply fan (entered as the main cooling fan) is available whenever the schedule reads 1% or greater. Its energy consumption will be proportional to the system cooling load for that hour. The supply fan may not be duty cycled during the cooling mode.
3. Specifying fan cycling on the availability manager tab of the system properties will allow the supply fan to cycle with the heating load for rooms which have minimum reheat airflow scheduled during unoccupied hours. Fan cycling can only apply to rooms that have a design reheat minimum airflow > 0 and their reheat minimum schedule reading > 0% for the hour(s) that the people schedule reads 5% or less.
4. Supply air temperature reset can be defined using the temperature control in the controls tab of the system properties.
5. Reheat Minimum. A reheat minimum will determine the minimum airflow into the space each hour. A reheat coil is assumed to be located at the terminus of each room's VAV box. A default value of 30% of the design room cooling airflow is assumed.
Application Notes
1. Warning: hours in which the reheat minimum is scheduled at 0%, no heating can take place.
2. While supply air reset control will save cooling energy, the room relative humidity will be higher and the VAV fan will consume more energy.
3. 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.
VAV w/ Baseboard Heating
The design is block tons and block air. This system is composed of a variable volume fan located on the cold deck that supplies conditioned air to each room's VAV box. The individual rooms see a variable air quantity proportional to the cooling load. Return air is drawn from a common return air plenum where loads to return air are picked up from all rooms and brought back to the ROA deck for mixing with outside air.
System Simulation
For the case when no minimum stops exist, the following events will occur. When the drift temperature rises above this hour's cooling thermostat set point, the VAV box opens and delivers a proportionate quantity of supply air to the space, i.e., only enough cool supply air is added to bring down and maintain the room at this hour's cooling thermostat setpoint. When the drift temperature falls below this hour's heating thermostat setpoint, the heating load is handled by a radiation heating unit that responds to the room thermostat, i.e., enough heat is added to the space to bring up and maintain the room at the heating thermostat set point. As long as the room drift temperature is below the cooling thermostat setpoint this hour, the VAV box is fully closed. While the drift temperature is within the dead band region, there is no air movement and absolutely no heating or cooling can take place within the room.
For the case where reheat minimums exist, the sequence of events is as follows. The VAV box is initially fixed at the reheat minimum airflow setting. Introduction of this cool air may drive the room temperature downward if the cooling load is low enough. The radiation heating unit will be energized only if the room temperature starts dropping below this hour's heating thermostat setpoint. If the admittance of this minimum quantity of conditioned air does not bring the room temperature below the cooling thermostat during times of high sensible load, the VAV box is opened past the minimum stop position until there is adequate cooling airflow to satisfy the load.
Note that shutting off mechanical cooling does not shut off the VAV supply fan; if the fan schedule reads 1% or greater the supply fan will modulate accordingly, supplying untreated return/outside air to the terminal boxes that are open. However, if the supply fan is scheduled off for a particular hour, no mechanical cooling is possible.
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.
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.
2. The cooling supply fan (entered as the main cooling fan) is available whenever the schedule reads 1% or greater. Its energy consumption will be proportional to the system cooling load for that hour. The supply fan may not be duty cycled during the cooling mode.
3. Specifying fan cycling on the availability manager tab of the system properties will allow the supply fan to cycle with the heating load for rooms which have minimum reheat airflow scheduled during unoccupied hours. Fan cycling can only apply to rooms that have a design reheat minimum airflow > 0 and their reheat minimum schedule reading > 0% for the hour(s) that the people schedule reads 5% or less.
4. Supply air temperature reset can be defined using the temperature control in the controls tab of the system properties.
5. Reheat Minimum. A reheat minimum will determine the minimum airflow into the space each hour. A reheat coil is assumed to be located at the terminus of each room's VAV box. A default value of 30% of the design room cooling airflow is assumed.
Application Notes
● While supply air reset control will save cooling energy, the room relative humidity will be higher and the VAV fan will consume more energy.
● The shutoff VAV system is almost identical to the Variable Air Volume Reheat (VRH) system except that VAV assumes radiation heating (with no reheat minimum) while VRH uses a reheat coil (with a 30% reheat minimum) for heating.
● 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.