HVAC tab in model data
You can include mixed mode natural ventilation and HVAC change-over control by checking the Mixed mode checkbox on the HVAC tab under the Natural ventilation header. Mixed mode is only available when Calculated natural ventilation is selected.
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).
See also the Summary of Mixed mode modelling for background information.
For VAV, CAV and Unitary multizone Compact HVAC system types 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.
Zone-based HVAC systems (Unitary single zone and Fan coil units) don't require a control zone to be set because the zone itself will always be the control zone.
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.
| 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. |
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 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 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 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 below which natural ventilation is shut off when the control mode schedule value = 2 (Enthalpy).
The outdoor enthalpy (in J/kg) above which natural ventilation is shut off when the control mode schedule value = 2 (Enthalpy).
The outdoor dew point temperature 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 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.