Mixed mode Cooling

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.


Definition of a mixed mode building:


"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 mode buildings.


Some useful background documents on mixed mode design and simulation:


  1. www.cbe.berkeley.edu/mixedmode
  2. www.ibpsa.org/proceedings/BS1999/BS99_D-07.pdf


DesignBuilder can model all three types of mixed mode system defined by CBE in the above reference.


  1. Concurrent (Same space, same time)
    Concurrent mixed-mode operation is the most prevalent design strategy in practice today, in which the air-conditioning system and operable windows operate in the same space and at the same time. The HVAC system may serve as supplemental or “background” ventilation and cooling while occupants are free to open windows based on individual preference. Typical examples include open-plan office space with standard VAV air-conditioning systems and operable windows, where perhaps perimeter VAV zones may go to minimum air when sensor indicates that a window has been opened.
  2. Change-over (Same space, different times)
    Change-over designs are becoming increasingly common, where the building “changes-over” between natural ventilation and air-conditioning on a seasonal or even daily basis. The building automation system may determine the mode of operating based on outdoor temperature, an occupancy sensor, a window (open or closed) sensor, or based on operator commands. Typical examples include individual offices with operable windows and personal air conditioning units that shut down for a given office anytime a sensor indicates that a window has been opened; or a building envelope where automatic louvers open to provide natural ventilation when the HVAC system is in economizer mode, and then close when the system is in cooling or heating mode.
  3. Zoned (Different spaces, same time)
    Zoned systems are also common, where different zones within the building have different conditioning strategies. Typical examples include naturally ventilated office buildings with operable windows and a ducted heating/ventilation system, or supplemental mechanical cooling provided only to conference rooms. For many mixed-mode buildings, operating conditions sometimes deviate somewhat from their original design intent (e.g., a building originally designed for seasonal changeover between air-conditioning and natural ventilation may, in practice, operate both systems concurrently).


Note: Concurrent and Zoned systems can be modelled simply by including natural ventilation with HVAC systems without using DesignBuilder mixed mode control. In DesignBuilder "mixed mode" specifically refers to Change-over systems where the HVAC and the natural ventilation do not take place at the same time.




DesignBuilder supports mixed mode in all combinations of natural ventilation and HVAC model options.


The steps to setting up a change-over mixed mode model in DesignBuilder are:


  1. Include active cooling and natural ventilation by switching on the Natural ventilation on and Cooled options on the HVAC tab.
  2. Also, include mechanical ventilation if there is outside air introduced through the HVAC system.
  3. Ensure that the setpoints for natural ventilation and cooling operation are correct. The natural ventilation (window/vent opening) setpoint should ideally be 2°C lower than the cooling setpoint to ensure maximum use of natural ventilation and to avoid the active cooling system being used unnecessarily and also 2°C higher than the heating .temperature. Typical values might be heating setpoint temperature = 21°C, natural ventilation setpoint temperature = 23°C, cooling setpoint temperature = 25°C. Heating should ideally be switched off in the summer to avoid unnecessary heating caused by natural ventilation overcooling the space.
  4. See the mixed mode section on the HVAC help for detailed information on the data that can be set, including conditions that require windows and vents to be closed such as rain, wind and low outside air temperature.


Note: "Mixed mode" means the same as the term "Hybrid ventilation" used in the EnergyPlus documentation.