HVAC tab in model data
You can include mixed mode natural ventilation and HVAC change-over control for zone equipment by checking the Mixed mode checkbox on the HVAC tab under the Natural ventilation header.
The mixed mode control serves two purposes:
The mixed mode change-over controller links the associated HVAC system with the operable windows and vents in the zones served by the HVAC system. The controller overrides the natural ventilation controls for windows and vent openings, closing these openings
under certain conditions and allowing the HVAC system to operate. If the controller determines that conditions are favourable for natural ventilation, then the HVAC
system is disabled and the openings are able to operate based on the
standard natural ventilation control
mechanism (natural ventilation setpoint, maximum opening area, modulation etc).
Mixed mode controls can be used when using either Calculated or Scheduled natural ventilation, Simple, Compact or Detailed HVAC.
When controlling natural ventilation together with a Detailed HVAC air loop through change-over mixed mode you should enter the mixed mode controls on the Air handling unit, Unitary heat cool or Unitary heat pump dialog.
When using Detailed HVAC the mixed model controls on the HVAC Model data tab are specifically used to control zone equipment such as fan coil units, PTAC, PTHP etc and are not used to control AHUs.
See also the Summary of Mixed mode modelling for background information.
When using Compact HVAC only, you must select a control zone to define which zone conditions are used by the mixed mode controller for the HVAC system. Do this by going to the zone level and checking the Mixed mode control zone checkbox located near the top of the data on HVAC tab. You must select just one control zone per AHU. The selected mixed-mode control zone should generally have conditions representative of all of the zones served by the HVAC system.
Select a Linear or Quadratic performance curve that parameterizes the variation of opening factor as a function of outdoor wind speed. The output of this curve is multiplied by the opening factor of opening objects to give the final openness. This field only works with the AirflowNetwork (Calculated natural ventilation) opening objects.
Indicates whether the rain indicator in the hourly weather data is used to shut off natural ventilation. This can help simulate conditions where one would normally close windows to avoid rain penetration in a space. Any possible rain getting into a space will not be counted as a zone load.
The wind speed (in m/s or ft/min) above which natural ventilation is shut off. This can help simulate conditions where one would normally close windows to avoid wind problems in a space (papers blowing around, etc).
The outdoor temperature (in °C or °F) below which natural ventilation is shut off when the control mode schedule value = 1 (Temperature). This lower temperature limit is intended to avoid overcooling a space, which could result in a heating load.
The outdoor temperature (in °C or °F) above which natural ventilation is shut off when the control mode schedule value = 1 (Temperature). This upper temperature limit is intended to avoid overheating a space, which could result in a cooling load.
The outdoor enthalpy (in J/kg or Btu/lb) below which natural ventilation is shut off when the control mode schedule value = 2 (Enthalpy).
The outdoor enthalpy (in J/kg or Btu/lb) above which natural ventilation is shut off when the control mode schedule value = 2 (Enthalpy).
The outdoor dew point temperature (in °C or °F) below which natural ventilation is shut off when the control mode schedule value = 3 (Dew point). This lower dew point temperature limit is intended to avoid dehumidifying a space.
The outdoor dew point temperature (in °C or °F) above which natural ventilation is shut off when the control mode schedule value = 3 (Dew point). This upper dew point temperature limit is intended to avoid humidifying a space.
This schedule determines whether or not for a given time the mixed mode ventilation control is to be applied. Schedule values equal to zero indicate no ventilation control, resulting in natural ventilation and HVAC system operation being performed based on their own controls. Schedule values equal to one denote temperature control for either cooling or heating, which is determined internally based on thermostat set point. The temperature control is restricted between the minimum and maximum outdoor temperatures provided in two additional input fields (below). Schedule values equal to two denote enthalpy control, which is restricted between minimum and maximum outdoor enthalpy values given in two additional input fields (below). Schedule values equal to three denote dew point control for either dehumidification or humidification. Schedule values equal to four represent outdoor ventilation air control.
Tip: Although basic DesignBuilder natural ventilation options don't allow controls based on absolute outdoor air temperature, it is possible to apply upper and/or lower outdoor air temperature limits for natural ventilation through use of the Temperature control schedule mixed mode and the Mixed mode minimum and maximum temperatures.
| Schedule value | Operation mode |
| 0 | No mixed mode control - HVAC and natural ventilation operate separately according to their own controls. |
| 1 | Temperature control for either cooling or heating - temperature control is restricted between minimum and maximum outdoor temperatures. When the outside air temperature falls below the minimum value or goes above the maximum value the openings are closed and the HVAC system operates. |
| 2 | Enthalpy control, outside enthalpy is restricted between minimum and maximum outdoor enthalpy values. |
| 3 | Dew point control, outside dew point temperature is restricted between minimum and maximum outdoor dew point values. |
| 4 | The minimum outdoor ventilation air for a given time in ac/h. The program calculates the natural (outdoor) ventilation in the controlled zone first and compares the amount of outdoor air brought by opening windows or doors and other small openings to this minimum value. If the amount of outdoor air from natural ventilation is less than the minimum value, the natural ventilation is shut off (i.e., the window or door openings are closed) and the HVAC system may operate if needed. Otherwise, the natural ventilation is on and the HVAC system is shut off. The amount of outdoor ventilation air entering the controlled zone is determined as air from outdoors and not from other adjacent zones. Therefore, this option is only applied to a zone having a window or door exposed to outdoors. |
The schedule is only used if Ventilation Control Mode = 4 (Outdoor Ventilation Air Control). This schedule determines the minimum outdoor ventilation air for a given time in the units of air change per hour (ac/h). The program calculates the natural (outdoor) ventilation in the controlled zone first and compares the amount of outdoor air introduced by opening windows or doors and other small openings to this minimum value. If the amount of outdoor air from natural ventilation is less than the minimum value, the natural ventilation is shut off (i.e., the window or door openings are closed) and the HVAC system may operate if needed. Otherwise, the natural ventilation is on and the HVAC system is shut off. The amount of outdoor ventilation air entering the controlled zone is determined as air from outdoors and not from other adjacent zones. Therefore, this option is only applied to a zone having a window or door exposed to outdoors.
This schedule is available when Calculated natural ventilation is set. It determines for a given simulation timestep how the opening objects respond to the hybrid ventilation control when the hybrid ventilation control allows the objects to operate.
Schedule values equal to zero indicate individual ventilation control based on the control requirements specified for each individual AirflowNetwork opening object. Schedule values equal to one denote ”group” control. The opening objects exposed to outdoors in the controlled zone served by the primary air loop will be considered as a master to provide group control.
This schedule is available when the Scheduled natural ventilation model option is set. It works with ZoneVentilation:* and ZoneMixing objects only. This schedule determines for a given simulation timestep how the ZoneVentilation:* and ZoneMixing objects respond to the hybrid ventilation control when the hybrid ventilation control allows the objects to operate.
Schedule values equal to zero indicate the individual ventilation control based on the control requirements from their own objects. Schedule values equal to one denote group control. The ZoneVentilation:* and ZoneMixing objects in the zones served by the primary air loop are controlled by a single object, whose name is provided in the following input field.