VAV Zone Level
VAV PTAC
The design is block air and block cooling capacity.
A separate cooling/heating heat pump is located in each zone. Each unit can draw return air either from a return air plenum above the particular room (return air assignments greater than zero) or recirculate air within the room (return air assignments are zero). The design cooling and heating supply air temperatures user input. The fans are variable volume.
System Simulation
When the room drift temperature rises above the room cooling thermostat, the cooling coil is energized (at a constant cooling supply air temperature) for a percentage of the hour that it takes to bring the room temperature down to the cooling thermostat temperature. This heat is rejected to the condenser loop. For the portion of the hour that the cooling coil is de-energized, the supply air will remain at the return/outside air dry bulb temperature (plus supply fan heat).
When the room drift point temperature drops below the room heating thermostat, the heating coil is energized (at a constant heating supply air temperature) for a percentage of the hour that it takes to bring the room temperature up to the heating thermostat temperature. The heat pump will remove heat from the condenser loop, thereby lowering its temperature. For the portion of the hour that the heating coil is de-energized, the supply air will remain at the return/outside air dry bulb temperature (plus supply fan heat).
When the room drift temperature lies within the deadband, the supply air will initially remain at the return/outside air dry bulb (plus fan heat).
System Options
1. The value of minimum ventilation airflow will be proportional to the percentage defined by the outside air schedule. The value of outside airflow may be greater than the nominal value if an economizer or nighttime purge is activated.
2. The value of outside airflow may be less than the nominal ventilation value if the supply fan (entered as the main cooling fan) has been scheduled at less than 100%; the outside air percentage for a particular hour will be multiplied by the main cooling fan utilization percent for that hour. If the supply fan has been scheduled off for a particular hour, no outside air can be delivered to the rooms.
3. The supply 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 minimum cooling airflow defaults to 30% and can be overridden by changing the VAV Minimum Cooling Airflow value on the Create Room – Airflows screen. In cooling mode, the supply air dry bulb is fixed while the supply airflow is varied to match the load. While the thermostat signal is in the deadband region (essentially, no load condition), the unit will continue to deliver minimum airflow at the SADBC setpoint. However, once the control thermostat senses that the room temperature has fallen below the heating thermostat setpoint, the cooling coil is deactivated and the heating coil becomes active. In heating mode, the supply airflow is maintained at its minimum setting and SADBH is varied to meet the space load. However, once the supply air dry bulb heating has reached its maximum (design) value, the airflow is increased to match the space load. The maximum heating mode airflow is based on the VAV Minimum Heating Airflow value defined on the Create Rooms – Airflows screen.
4. Neither the cooling or heating supply air temperature may be reset using supply air reset controls since the cooling supply air temperature is cycled at a constant temperature and the heating supply air temperature responds to the room thermostat.
Application Notes
● For PTHP systems, the unit can be oversized to include a safety factor by entering the cooling design capacity and heating design capacity (on the sizing tab of the system properties) as something larger than 100% of design cooling or heating capacity respectively.
● If the fans are to be cycled during unoccupied hours, enter the fan cycling information on the availability manager tab in the system properties.
VAV Unit Ventilator
A separate unit ventilator, including air handler and heating coil, is located in each room (on the hot deck at the room level). The design heating supply air temperature is assumed to be 95°F unless overridden by user input to the Airflow Design Temperatures screen.
System Simulation
The supply fan follows the main heating fan schedule in a time-clock fashion, bringing in outside air through the main heating coil according to the ventilation schedule.
When the room drift temperature rises above the room heating thermostat, the heating coil is de-activated, allowing the space temperature to drift upward. Since the supply air will be at the return/outside air dry bulb temperature (plus fan heat), scheduling outside air into the space will temper this effect to some degree.
When the room drift temperature drops below the room heating thermostat, the heating coil is modulated to produce a supply air dry bulb that will bring the room temperature up to the heating thermostat temperature.
System Options
1. The value of minimum ventilation airflow will be proportional to the percentage defined by the outside air schedule. The value of outside airflow may be greater than the nominal value if an economizer is activated.
The value of outside airflow may be less than the nominal ventilation value if the supply fan (entered as the main cooling fan) has been scheduled at less than 100%; the outside air percentage for a particular hour will be multiplied by the main heating fan utilization percent for that hour. If the supply fan has been scheduled off for a particular hour, no outside air can be delivered to the rooms.
2. The supply 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 minimum cooling airflow defaults to 30% and can be overridden by changing the VAV Minimum Cooling Airflow value on the Create Room – Airflows screen. In cooling mode, the supply air dry bulb is fixed while the supply airflow is varied to match the load. While the thermostat signal is in the deadband region (essentially, no load condition), the unit will continue to deliver minimum airflow at the SADBC setpoint. However, once the control thermostat senses that the room temperature has fallen below the heating thermostat setpoint, the cooling coil is deactivated and the heating coil becomes active. In heating mode, the supply airflow is maintained at its minimum setting and SADBH is varied to meet the space load. However, once the supply air dry bulb heating has reached its maximum (design) value, the airflow is increased to match the space load. The maximum heating mode airflow is based on the VAV Minimum Heating Airflow value defined on the Create Rooms – Airflows screen.
Application Notes
● If the fans are to be cycled during unoccupied hours, enter the fan cycling information on the availability manager tab in the system properties.
● Internal rooms (rooms with no walls, roofs, etc.) should be provided with adequate outside air if internal loads are scheduled; otherwise, the room temperature may go out of control.
● Since the ventilation air brought into the space will create "positive" building pressure, it is suggested that infiltration be scheduled opposite the outside air schedule, e.g., for outside air scheduled at 40% for a particular hour, the infiltration schedule should read (100-40) = 60%.
VAV Fan Coil
The design is block air and block cooling capacity.
A separate fan coil unit, including air handler and cooling and heating coils, is assumed for each zone. Central heating and cooling plants are assumed to handle the heating and cooling coil loads. Plenum return air loads are allowed although the assumption is that the return air is pulled from above the individual zones. If the fan coils are floor mounted units the lighting return air assignments should be set at zero. The design cooling and heating supply air temperatures are determined from either the Design Phase or user input and cannot be overridden by outside air reset schedules. The fans are variable volume.
System Simulation
When the zone drift temperature rises above the room cooling thermostat, the cooling coil is modulated to produce a supply air dry bulb that will bring the space down to the cooling thermostat temperature.
When the room drift temperature drops below the room heating thermostat, the heating coil is modulated to produce a supply air dry bulb that will bring the room temperature up to the heating thermostat temperature.
When the room drift temperature lies within the deadband, the supply air will be at the return/outside air dry bulb temperature (plus fan heat). If necessary, the return/outside air mixture will be heated or cooled to prevent the room temperature from going out of the deadband because of the introduction of significant quantities of cold or hot ventilation air.
System Options
1. The value of minimum ventilation airflow will be proportional to the percentage defined by the outside air schedule. The value of outside airflow may be greater than the nominal value if an economizer or nighttime purge is activated.
The value of outside airflow may be less than the nominal ventilation value if the supply fan (entered as the main cooling fan) has been scheduled at less than 100%; the outside air percentage for a particular hour will be multiplied by the main cooling fan utilization percent for that hour. If the supply fan has been scheduled off for a particular hour, no outside air can be delivered to the rooms.
2. The supply 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 minimum cooling airflow defaults to 30% and can be overridden by changing the VAV Minimum Cooling Airflow value on the Create Room – Airflows screen. In cooling mode, the supply air dry bulb is fixed while the supply airflow is varied to match the load. While the thermostat signal is in the deadband region (essentially, no load condition), the unit will continue to deliver minimum airflow at the SADBC setpoint. However, once the control thermostat senses that the room temperature has fallen below the heating thermostat setpoint, the cooling coil is deactivated and the heating coil becomes active. In heating mode, the supply airflow is maintained at its minimum setting and SADBH is varied to meet the space load. However, once the supply air dry bulb heating has reached its maximum (design) value, the airflow is increased to match the space load. The maximum heating mode airflow is based on the VAV Minimum Heating Airflow value defined on the Create Rooms – Airflows screen.
3. Since the fan coil heating and cooling supply air temperatures respond to the room thermostat, supply air reset control is not possible.
Application Notes
● The program assumes that the fan coil system is four-pipe (heating and cooling available all year round). To input a two-pipe fan coil system, the user should indicate which months the heating and cooling functions are to be locked out when creating the main cooling and heating schedules.
● If the fans are to be cycled during unoccupied hours, enter the fan cycling information on the availability manager tab in the system properties.
VAV PTHP
The design is block air and block cooling capacity.
A separate cooling/heating heat pump is located in each zone. Each unit can draw return air either from a return air plenum above the particular room (return air assignments greater than zero) or recirculate air within the room (return air assignments are zero). The design cooling and heating supply air temperatures are determined from either the Design Phase or user input and cannot be overridden by outside air reset schedules. The fans are variable volume.
System Simulation
When the room drift temperature rises above the room cooling thermostat, the cooling coil is energized (at a constant cooling supply air temperature) for a percentage of the hour that it takes to bring the room temperature down to the cooling thermostat temperature. This heat is rejected to the condenser loop. For the portion of the hour that the cooling coil is de-energized, the supply air will remain at the return/outside air dry bulb temperature (plus supply fan heat).
When the room drift point temperature drops below the room heating thermostat, the heating coil is energized (at a constant heating supply air temperature) for a percentage of the hour that it takes to bring the room temperature up to the heating thermostat temperature. The heat pump will remove heat from the condenser loop, thereby lowering its temperature. For the portion of the hour that the heating coil is de-energized, the supply air will remain at the return/outside air dry bulb temperature (plus supply fan heat).
When the room drift temperature lies within the deadband, the supply air will initially remain at the return/outside air dry bulb (plus fan heat).
System Options
1. The value of minimum ventilation airflow will be proportional to the percentage defined by the outside air schedule. The value of outside airflow may be greater than the nominal value if an economizer or nighttime purge is activated.
The value of outside airflow may be less than the nominal ventilation value if the supply fan (entered as the main cooling fan) has been scheduled at less than 100%; the outside air percentage for a particular hour will be multiplied by the main cooling fan utilization percent for that hour. If the supply fan has been scheduled off for a particular hour, no outside air can be delivered to the rooms.
2. The supply 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 minimum cooling airflow defaults to 30% and can be overridden by changing the VAV Minimum Cooling Airflow value on the Create Room – Airflows screen. In cooling mode, the supply air dry bulb is fixed while the supply airflow is varied to match the load. While the thermostat signal is in the deadband region (essentially, no load condition), the unit will continue to deliver minimum airflow at the SADBC setpoint. However, once the control thermostat senses that the room temperature has fallen below the heating thermostat setpoint, the cooling coil is deactivated and the heating coil becomes active. In heating mode, the supply airflow is maintained at its minimum setting and SADBH is varied to meet the space load. However, once the supply air dry bulb heating has reached its maximum (design) value, the airflow is increased to match the space load. The maximum heating mode airflow is based on the VAV Minimum Heating Airflow value defined on the Create Rooms – Airflows screen.
3. Neither the cooling or heating supply air temperature may be reset using supply air reset controls since the cooling supply air temperature is cycled at a constant temperature and the heating supply air temperature responds to the room thermostat.
Application Notes
● For PTHP systems, the unit can be oversized to include a safety factor by entering the cooling design capacity and heating design capacity (on the sizing tab of the system properties) as something larger than 100% of design cooling or heating capacity respectively.
● If the fans are to be cycled during unoccupied hours, enter the fan cycling information on the availability manager tab in the system properties.
VAV WSHP
The design is block air and block cooling capacity.
A separate WSHP unit is assumed for each room. Each unit can draw return air either from a return air plenum above the particular room or recirculate air within the room. The fans are variable volume.
In the heating mode, the evaporative coil acts as a condenser coil by drawing heat from the condenser loop. If not enough heat is available from the condenser loop, i.e., if the loop temperature has dropped below 60 F, a backup heat source (electric, gas, oil, etc.) is assumed to provide sufficient heat to the condenser loop. In the cooling mode, condenser heat is rejected to the loop. If the loop temperature exceeds 90 F, the heat is rejected to a closed loop cooling tower.
System Simulation
When the room drift temperature rises above the room cooling thermostat, the cooling coil is energized (at a constant cooling supply air temperature) for a percentage of the hour that it takes to bring the room temperature down to the cooling thermostat temperature. This heat is rejected to the condenser loop. For the portion of the hour that the cooling coil is de-energized, the supply air will remain at the return/outside air dry bulb temperature (plus supply fan heat).
When the room drift temperature drops below the room heating thermostat the heating coil is energized (at a constant heating supply air temperature) for a percentage of the hour that it takes to bring the room temperature up to the heating thermostat temperature. The heat pump will remove heat from the condenser loop, thereby lowering its temperature. For the portion of the hour that the heating coil is de-energized, the supply air will remain at the return/outside air dry bulb temperature (plus supply fan heat).
When the room drift temperature lies within the deadband, the supply air will initially remain at the return/outside air dry bulb temperature (plus supply fan heat).
System Options
1. The value of minimum ventilation airflow will be proportional to the percentage defined by the outside air schedule. The value of outside airflow may be greater than the nominal value if an economizer or nighttime purge is activated.
The value of outside airflow may be less than the nominal ventilation value if the supply fan has been scheduled at less than 100%; the outside air percentage for a particular hour will be multiplied by the main cooling fan utilization percent for that hour. If the supply fan has been scheduled off for a particular hour, no outside air can be delivered to the rooms.
2. The supply 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 minimum cooling airflow defaults to 30% and can be overridden by changing the VAV Minimum Cooling Airflow value on the Create Room – Airflows screen. In cooling mode, the supply air dry bulb is fixed while the supply airflow is varied to match the load. While the thermostat signal is in the deadband region (essentially, no load condition), the unit will continue to deliver minimum airflow at the SADBC setpoint. However, once the control thermostat senses that the room temperature has fallen below the heating thermostat setpoint, the cooling coil is deactivated and the heating coil becomes active. In heating mode, the supply airflow is maintained at its minimum setting and SADBH is varied to meet the space load. However, once the supply air dry bulb heating has reached its maximum (design) value, the airflow is increased to match the space load. The maximum heating mode airflow is based on the VAV Minimum Heating Airflow value defined on the Create Rooms – Airflows screen.
Application Notes
● Although the equipment configuration is different, the airside simulation of water source heat pump (WSHP) and packaged terminal air conditioners (PTAC) is similar.
● For WSHP systems, the unit can be oversized to include a safety factor by entering the cooling design capacity and heating design capacity (on the sizing tab of the system properties) as something larger than 100% of design cooling or heating capacity respectively.
● 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.