In mixed mode buildings natural ventilation is used as the primary means of providing cooling and, when this is inadequate to provide comfort conditions, active cooling is introduced. A concise definition of a mixed mode building is:
It is a building in which occupants can open windows, and which is designed with effective passive strategies for limiting the effects of the external climate. The passively designed building is utilised to provide acceptable conditions for the majority of the year, and is supplemented by a mechanical system, either on an ‘as and when required’ basis, or on a seasonal basis.
Source: Cooper, V.A., “Occupancy Comfort and Energy Consumption in Naturally Ventilated and Mixed Mode Office Buildings”, PhD thesis, UMIST, 1998
“Mixed-mode” refers to a hybrid approach to space conditioning that uses a combination of natural ventilation from operable windows (either manually or automatically controlled), and mechanical systems that include air distribution equipment and refrigeration equipment for cooling. A well-designed mixed-mode building begins with intelligent facade design to minimize cooling loads. It then integrates the use of air-conditioning when and where it is necessary, with the use of natural ventilation whenever it is feasible or desirable, to maximize comfort while avoiding the significant energy use and operating costs of year-round air conditioning.
Source: Mixed mode page on Center for Built Environment website, contains background, case studies and a database of 150 mixed buildings.
Some useful background documents on mixed mode design and simulation:
When modelling mixed mode systems in DesignBuilder you should ideally use Compact HVAC to avoid the limitations explained below of Simple HVAC. You may also want to use Calculated natural ventilation to calculate natural ventilation air and heat flows based on window opening, stack and wind driven flows etc.
The steps to setting up a mixed mode model are:
Include active cooling and natural ventilation by switching on the 'Natural ventilation on' and 'Cooling on' options on the HVAC tab.
Also, include mechanical ventilation if there is outside air introduced through the HVAC system.
Ensure that the setpoints for natural ventilation and cooling operation are correct. Generally the natural ventilation (window/vent opening) setpoint should be lower than the cooling setpoint to ensure maximum use of natural ventilation and to avoid the active cooling system being used unnecessarily. There is currently no way to link the natural ventilation and cooling systems (see limitations below).
DesignBuilder uses the EnergyPlus zone ventilation object for modelling natural ventilation (when using Scheduled natural ventilation) and mechanical ventilation (when using Simple HVAC). Only one ventilation object can be included per zone in EnergyPlus so if your model uses Scheduled natural ventilation and Simple HVAC you will get an error message like:
Building 1 - Mechanical ventilation and Natural ventilation in same zone not allowed with Simple HVAC + Scheduled natural ventilation
This message will also be generated for Cooling design calculations when mechanical and natural ventilation are selected even if you have selected Calculated natural ventilation or Compact HVAC because Scheduled natural ventilation and Simple HVAC are used by default in Design calculations.
There are various workarounds:
If the problem is occurring in Cooling design calculations, it is possible to select one of the Compact HVAC cooling design sizing methods to use detailed modelling of mechanical ventilation instead of the simple Ventilation object. This allows mechanical ventilation in combination with Scheduled natural ventilation.
Again for Cooling design calculations you can put all of the ventilation in either natural or mechanical category to avoid having both switched on at the same time. Once you have sized your system switch the HVAC sizing option to Manual to avoid carrying sizing calculations before every simulation.
Use Calculated natural ventilation or Compact HVAC for simulations.
Note 1: The problem does not arise in Heating design calculations because natural ventilation is not included in Heating Design calculations.
Note 2: In current versions of DesignBuilder it is not possible to couple the natural ventilation and cooling systems so that windows shut when the cooling switches on but this should be possible in the future.