Mixed mode ventilation – designing for the use of opening windows and mechanical ventilation in combination or as contingency to each other is typical of the work that we do. From providing ventilation solutions for retrofitting existing buildings through to new Passive House buildings, mechanical systems are used alongside opening windows and secure vents to produce a lean solution to ventilation whilst meeting targets for good internal air quality.
‘Mixed mode’ is a term used to describe servicing strategies that combine natural ventilation with mechanical ventilation and/or cooling in the most effective manner. It involves maximising the use of the building fabric and envelope to achieve indoor environmental conditions, and then supplementing this with degrees of mechanical systems, in all or parts of the building. To date the approach has been used most widely in offices; however, it is suitable for a wide range of building types. CIBSE Application Manual AM13 Mixed Mode Ventilation
In low energy buildings, heat recovery is often incorporated as part of the mechanical ventilation system to ensure efficient operation during the heating season, something that cannot be achieved with natural ventilation alone. Refrigerated cooling is avoided where possible through use of night purge ventilation and careful design for cross-flow and stack ventilation between windows for summer use.
Designing for mixed mode ventilation changes the way that design is carried out compared to the typical processes used in the construction industry. More effort must be put into design of the building fabric and envelope to set indoor environmental conditions, and this is typically achieved by highly insulated and airtight fabric design supported by the use of the PHPP (Passive House Planning Package) and other thermal modelling tools. With a greater onus on fabric design and to ensure the building services are the minimum needed, the building services engineers work as part of an integrated team with the architect, structural engineer and other specialists in close collaboration with the client to produce a holistic design based on a thorough understanding of the needs of the people using the building.
Good integration of the design becomes necessary, from the initial stages through to the final details, so that the fabric and services can work together effectively. Engineers and architects must therefore work together more closely, and with their clients, to obtain a good result. The aim should be robust integration, with features that add functionality, not complication. CIBSE Application Manual AM13 Mixed Mode Ventilation
Achieving a good mixed mode ventilation design starts with careful consideration of passive features of the building:
- siting and orientation can influence solar gain, but also noise and other issues that affect use of opening windows;
- building form, orientation and fabric can stabilise internal temperatures through avoiding unwanted gains and losses;
- internal arrangement of the building can maximise potential for daylight and natural ventilation while avoiding excessive areas of glazing (for example, glazing below 800mm from the floor provides little additional daylight, but is hard to shade to avoid unwanted solar gains).
Design then seeks to limit the heating and cooling loads within the building:
- using energy efficient lighting;
- selecting IT and audio visual equipment for efficiency;
- only providing equipment that is required.
The remaining heating and cooling loads are then:
- serviced using the minimum amount of equipment with the maximum efficiency;
- controlled effectively and in a way that is intuitive for the people who manage, use and maintain the building;
- monitored using appropriate systems for efficiency and to detect faults.
The systems should be thoroughly handed over to those who will work with them.
- Effective commissioning is vital, but systems should also be easy to set up, to fine tune and to alter.
- The way the system is intended to work should be made clear to the people who manage, use and maintain the building and those who may adapt, fit out and change the building in the future, such as space planners and interior designers.
Once in use, performance should be monitored:
- Post occupancy evaluation of the building enables feedback to be gathered to optimise the system settings and to inform future building designs.
Blog by Richard Spencer, Certified Passive House Consultant at LEDA
