Mixed-mode ventilation for buildings

The term mixed mode can be applied to various buildings and configurations but the term really took hold in the 1990s with the advent of holistic designs that attempted to minimise energy use through the symbiotic application of mechanical and natural ventilation systems. A mixed-mode building may work seasonally altering the proportions of mechanical and natural ventilation – such as the original 1980 BRE Low Energy Office1 that switched from predominantly natural ventilation using windows in summer to tempered, ducted mechanical ventilation in winter – or it may continuously monitor the outside and indoor conditions to determine the most effective ventilation strategy as in the Brooklands Road office building discussed below.

Benefits of mixed-mode systems

Whatever the particular application, the design intent is the same – to reduce carbon emissions by improving the environmental performance of the building while maintaining (and potentially improving) occupant satisfaction. The revisions to Part L2 (2006) firmly lay out the requirements of building systems in terms of a Building Carbon Dioxide Emission Rate (BER) (see over the page for details of the calculation methods to show compliance with the new requirements). By considering mixed-mode systems, it ensures that the designer will need to have a high degree of interest and input both in the building envelope design (air permeability, natural ventilation pathways, fabric thermal performance), as well as the active mechanical systems. The resulting holistic design should have great opportunities to meet or exceed the requirements of the 2006 revisions to Part L2 of the Regulations.

Components of mixed mode

Before any design is undertaken, it is vital to ensure that the client has appropriate expectations for the mixed-mode building. Occupants who have a more active influence on their environment tend to adapt better to more extreme conditions. Hence the design conditions for naturally ventilated buildings are customarily less rigid than for the fully air-conditioned alternative.

The key aspect for the success of low-energy, mixed-mode buildings is to minimise the heating and cooling loads. In a new building, this would influence the shape and form of the development as well as its aspect. It is essential to properly consider the sources of heat losses and gains early in the process and design the building to minimise those loads.

This would include:

• reduction of undesirable fabric heat transfer
• altering the time of peak loads using building thermal mass – this can improve cooling and heating load profiles both by attenuating peaks and providing peak gains when the building is unoccupied
• reduction of infiltration by improving air tightness/reducing building fabric permeability
• control solar gains – also ensure that incoming solar irradiance offsets the need for electric lighting wherever possible (eg, light shelves, prismatic glazing)
• minimise lighting gains – improve daylighting and employ low-energy luminaires
• reduce internal gains from office equipment and other appliances – not only through low-energy appliances but by locating equipment where gains are removed directly from the space before they become cooling loads.

The three principal natural ventilation strategies that are used as part of a mixed-mode solution are:

• Single sided – the use of an opening (or pair of openings), one higher than the other, on the outside wall of the occupied space. This will allow effective ventilation to a depth of between 2 and 2.5 times the room height.
• Cross flow – two openings on opposite external faces of a space. This will allow effective ventilation to a depth of up to 5 times the room height.
• Stack driven – two openings on opposite external faces of a space, one of which is connected to a vertical riser (chimney or atrium). This will allow effective ventilation to a depth of up to 5 times the room height.

Whichever method is employed, the “ventilation effectiveness” must be considered – this is how well a ventilation system works in terms of delivering the supply air to the occupants of a building. The system will be successful only if the supply air not only removes the excess heat, but also provides fresh (as opposed to stale or already contaminated) air to the occupants.

It is this consideration that will affect the practical effective depth of a naturally ventilated space.

At night, the differences between internal and external temperature are greater, enhancing the effects of natural ventilation. Night ventilation can flush the building of warm air, stored heat and pollutants that have accumulated during the day. By lowering the temperature of the building mass at night, the peak internal temperature can be reduced by around 2 to 3K the following day.

Some systems use hollow core concrete slabs as part of the floor structure so that the night ventilation air can cool deeper into the building mass.

However, it is generally agreed that natural ventilation systems can meet total heat loads averaged over the day of around 30-40 W/m2 and it is inevitable that with the coincidence of gains in some applications (such as offices) that the loads may exceed this value. By applying a mixed-mode solution, the advantages of natural ventilation can work with the benefits of air conditioning.

For any type of mixed-mode strategy to realise its full potential, a key element is the control system. The aim of mixed mode is that, at any time, the operating method should take account of the external environment as well as the current state of the building and its use. The control system must continually monitor the immediate and past internal and external environments. Using predictive wind algorithms (including wind speed and direction), the facade device opening positions (controlling the natural ventilation rates) are altered to ensure a steady flow through the space. Poorly designed or maintained controls can lead to the systems opposing each other rather than working together, potentially increasing, rather than reducing, carbon dioxide (CO2) emissions.

Ideally, any mixed-mode strategy will be adopted at the early stages of a new building design. In this way, natural ventilation can be used for the greatest benefit, as the building has been created to take advantage of such a system. However, it is also possible to introduce a mixed-mode system into refurbishment projects. In such cases, the addition of comfort cooling is a great advantage, as the natural ventilation elements may not be working in a specially designed space.

Moving mixed mode forward

Mixed-mode systems can provide an excellent basis for buildings that both comply with the 2006 revisions of the Building Regulations and, more importantly, provide usable buildings with lower CO2 emissions.

There are two excellent publications that provide detail on mixed-mode applications. Freely downloadable from the web is GIR 56 – Mixed-mode Buildings and Systems – An Overview and for more extensive coverage there is CIBSE AM133 Mixed Mode Ventilation. For key guidance on natural ventilation, the recently revised CIBSE Applications Manual AM104 is an excellent reference with good practical examples.

© Tim Dwyer 2005

References

1) IP4/88 – The BRE Low Energy Office:

A Longer-term Perspective

2) Guide to Mixed Mode Cooling Systems, Mitsubishi Electric Europe, June 2005

3) Mixed Mode Ventilation, CIBSE AM13: 2000, ISBN 1903287014

4) Natural Ventilation in Non-domestic Buildings, CIBSE Applications Manual AM10, ISBN 1 903287 56 1

Refurbishment Case Study – Brooklands Road Office Building2

Originally built in the early 1900s, the Brooklands Road office building was refurbished in the 1980s. However, with only single-sided ventilation available through manually opening windows, increased use of IT and a growing number of occupants, it suffered from uncomfortable indoor temperatures during the summer. Summer mornings were a particular problem with only the existing single-sided opening windows – internal temperatures rose quickly, as there was no way of removing warm air from the building at night. Studies showed that one day in July saw the office temperature rise to 31ºC. Thermal modelling showed that if only stack-driven natural ventilation was used with these higher heat gains, temperatures would rise above 25°C for 13% of the occupied time and also exceed 28°C for 3% of the occupied time.

It was clear that a natural ventilation-only system would not provide occupants with the comfort they needed to work productively, in this higher heat gain space. However, the seemingly simple solution of switching to an entirely air-conditioned environment would have created high energy costs and resulting carbon dioxide (CO2) emissions. The solution employed was a combination of passive ventilation with spot comfort cooling – mixed mode.

Annual energy use has been reduced by more than 41% for the whole building compared with a full mechanical system – 19% of the savings resulting from the use of night cooling. As daytime temperatures rise, passive ventilation draws external air into the office space. As inside temperatures rise beyond a set point, the building energy management system automatically switches on the spot cooling. As late afternoon temperatures fall, the comfort cooling turns off and the building returns to natural ventilation mode.

Night cooling has played a large part in achieving the outstanding energy reductions. By utilising low night-time external air temperature on a typical day in mid-July, there is a 57% reduction in energy consumption – 39% of this is directly attributed to night cooling.

The natural ventilation system uses the plenum of the ceiling void and discreet vents above the windows to allow outdoor fresh air into the building in a controlled way. Exhaust air is removed by the stack effect via roof mounted terminals. No mechanical component is used for ventilation, yet fresh air still remains within minimum recommended levels. By reducing the mechanical elements in the system, energy is saved, as are maintenance costs.

The air-conditioning system also makes a large contribution to the overall energy efficiency of the mixed-mode system, as it uses inverter-driven technology – the night cooling effect also means that it can work at part load most of the time. Night cooling ensures that there is no need for using the air-conditioning system before occupants arrive. As well as reducing energy costs, the system creates a better internal environment for occupants by maintaining low internal CO2 levels.

This use of a mixed-mode system in the Brooklands Road office building demonstrates that natural ventilation can work in an existing building. Selection of air-conditioning equipment that is energy efficient, even at part loads, has ensured that building energy use is minimised. Figures 1 and 2 demonstrate the load profiles and energy consumption of the mixed-mode system in this application when compared with notional mechanical systems.