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JUNE–JULY 2020 · VOLUME 19.5

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Eco: Tell us how a mixed-mode ventilation strategy typically works?

Lecamwasam: It is the fusion of time-tested and proven good passive design principles with the predictable design and operation of mechanical AC systems. Correct design and operation of such systems can significantly reduce operating costs and improve user enjoyment of buildings.

Gaekwad: The most basic mixed-mode system is your typical single dwelling – house or apartment – with operable windows and an air conditioning system. Occupants open the windows when it’s “nice” outside. When the dwelling becomes uncomfortably hot or cold, the windows are closed, and the air conditioning system is switched on.

A mixed-mode system operates very similarly for any other building typology.

The main differences are the control system for the natural ventilation – automatic or manual – and what the natural ventilation openings look like – louvres, operable walls, etc.

Pettifer: When outdoor conditions are favourable, the building is naturally ventilated and when they are not the mechanical systems operate. The mode shift can be fully automatic, with actuators used to open and close windows and/or louvres in the façade. Alternatively, in natural ventilation mode occupants can be alerted, for example using a traffic light system mounted in the space or via personal technology, and they then manually open and close the natural ventilation devices in their vicinity.

Outside of natural ventilation mode, the system works as conventional air conditioning.

McCabe: A mixed-mode comfort strategy works on the basis that heating is used when ambient conditions are cold, air conditioning when hot, and natural ventilation when ambient conditions suit to maintain acceptable thermal comfort.

However, a critical design issue is that the natural ventilation system has to be designed to provide for effective ventilation, as just meeting NCC/BCA openable area will not suffice.

Marklund: A semi-automated approach can be to install reed switches on a manually operable façade element that when opened, turns off the local air conditioning zone. A weather sensor operating a visual indicator light or app can be used to encourage occupants to open windows.

A fully automated system, for say an open-plan office, would typically include

24 JUNE–JULY 2020 • ECOL IBR IUM

Mixed opinionFor a host of reasons, mixed-mode air conditioning systems have both their supporters and

detractors. Sean McGowan explores the issues with Caimin McCabe, M.AIRAH, director at Cundall;

Anthony Marklund, M.AIRAH, principal sustainability engineer at Floth; Jason Gaekwad, regional

manager – building physics and sustainability at Inhabit; Lasath Lecamwasam, F.AIRAH, building

services engineer at ESBS; and Andrew Pettifer, principal and NSW buildings group leader at Arup.

F E AT U R EF E AT U R E

outdoor and indoor sensors linked to the BMS to control electrically actuated natural ventilation devices and the air conditioning systems.

Eco: In what applications does mixed-mode ventilation work best?

Pettifer: We tend to think in terms of office spaces, and there are many examples of mixed-mode systems in commercial buildings around the world. But it can and has been applied to other building types such as sports facilities and even theatres.

The main drivers are to reduce energy consumption and the perceived benefits to occupants – most people like the idea of being able to open the façade, breathe fresh air and hear the birds sing. If the air outside is polluted and all you can hear is traffic noise it’s not quite the same effect! So it works best in suburban or rural locations with temperate climates.

Gaekwad: The application of mixed-mode ventilation is wholly dependent on climate and outdoor environment, building form and use, and indoor environment. East coast cities tend to have a mild and desirable climate for the majority of the year, which has led to an “indoor/outdoor” type of lifestyle. Mixed-mode systems can contribute to this design and lifestyle philosophy. During summer in Melbourne, the ambient temperature is generally within the comfort range; natural ventilation could be utilised to remove heat load from, for example, densely occupied office buildings. For regions with a high diurnal variation in temperature, a night-purge system can be used, where cool air is brought into the building at night.

McCabe: Subject to prevailing climate being suitable and functional usage, there are not many building types or areas where a mixed-mode air conditioning approach cannot be adopted.

There are examples of school, university, commercial, retail and even hospital buildings that have successfully adopted mixed-mode air conditioning.

Marklund: Mixed-mode air conditioning has been successfully applied to many types of buildings internationally.

When designed, installed, operated and maintained correctly, particularly in conjunction with an adaptive thermal comfort strategy where individuals have visual connection to the outdoors and are appropriately educated – and allowed control over their environment – international studies indicate that occupant satisfaction and productivity can be improved while significantly reducing air conditioning energy use, cost and associated carbon emissions.

Lecamwasam: Mandated building rating schemes such as NABERS and Green Star could complement the adoption of mixed mode very well.

If end user groups such as the Commonwealth and state governments, retail, education, hospitality industries etc., redefined lease requirements for more environmentally friendly minimum service requirements for temperature and occupant comfort, then mixed mode would be far widely adopted by consultants.

At present though, the risks to consultants through adopting mixed mode makes such systems a rarity. Sadly, building occupants have developed some very expensive habits and expectations over the past 50 years or so, and in an environment now facing extreme weather events more commonly, the challenges to implementing mixed mode have increased.

Eco: And what are some of the advantages and benefits?

Lecamwasam: The main advantages are reduced operational costs, user satisfaction and enhanced green credentials – if being environmentally friendly is a key ethos of the end users.

McCabe: Mixed mode can provide for improved energy efficiency – assuming effective controls are in place such as avoiding windows being open and air conditioning is on – improved indoor air quality through higher rates of outside air when natural ventilation is in operation, and improved health and wellbeing of occupants by providing greater connection to outside, as well as providing them greater control over their environment.

But it is critical that the natural ventilation part of the mixed-mode air conditioning system be designed to provide effective ventilation and not just meeting NCC/BCA openable area, as this will not suffice and lead to complaints.

Gaekwad: If done well, mixed-mode ventilation can provide a greater connection to the outdoors, including to thermal variability and ventilation velocity variability. This can dramatically improve comfort, and represents a move towards a more “human centred” method of HVAC design.

However the building operation should also reflect this design philosophy when the HVAC system is running.

There are examples of school, university,

commercial, retail and even hospital buildings that have successfully adopted mixed-mode

JUNE–JULY 2020 • ECOL IBR IUM 25

Caimin McCabe Anthony Marklund Jason Gaekwad Lasath Lecamwasam Andrew Pettifer

Eco: What types of mechanical systems work best in a mixed-mode ventilation strategy?

McCabe: Mixed-mode designs can work with any air conditioning systems – the key is that effective operation is controlled so that a space or building is not being air conditioned with the windows or vents are open. The only systems where the combination of natural ventilation needs to be carefully considered from a controls perspective are chilled ceiling and chilled beam systems, so that unwanted condensation does not occur.

Pettifer: Chilled beams are problematic because you have to control humidity in the building to prevent condensation, and that’s not easy to do in a building that is intermittently naturally ventilated. All-air systems work better. In particular, displacement systems that work at higher supply-air temperatures and lower system pressures can complement the natural ventilation mode, and the two work more easily together.

Marklund: Conventional all-air systems such as fan coil units and VAV systems make sense for mixed-mode air conditioning, as they are able to manage humidity variation following a period of natural ventilation. Able to be locally controlled per zone, they are also suitable for mixed-mode operation of dedicated spaces such as a perimeter conference room as we have implemented at Floth head office.

Eco: What are some of the common issues around the adoption of a mixed-mode ventilation strategy?

Marklund: Manual mixed-mode air conditioning relies on consistently motivated occupants. In open-plan environments, occupants can disagree on the operation of natural ventilation mode due to personal attitudes,

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MIXED MODE IN OPERATIONASB HEADQUARTERS, AUCKLAND NZThe design of ASB North Wharf in Auckland reflected the desire of its tenant for a world-class building that places staff and customers at the centre of the sustainability equation.

Featuring a mechanical service design by Arup, the seven-storey commercial office building maximises daylight, views and the fresh air of the Waitemata Harbour through the use of a green light/red light system to alert occupants when conditions are favourable for windows to be opened.

To read more about the ASB North Wharf, see Ecolibrium December 2013.

QUEENSLAND SCHOOLSFloth has undertaken engineering services and sustainable design for numerous state schools that include thermal comfort control via passive design, natural ventilation and fans.

Seven of these schools have achieved Green Star – Education v1 ratings, including Peregian Springs and Bayview State Schools, which were the first in Australia to achieve this.

In late February, the Queensland government promised to provide air conditioning to every school in the state by mid-2022 under its

$477 million Cooler Cleaner Schools program. This could see naturally ventilated schools being enabled for manual mixed-mode air conditioning subject to the teachers being instructed to operate them this way.

SUZANNE CORY HIGH SCHOOL, WERRIBEE, VICWhen designing the mechanical services solution at the Suzanne Cory High School, Cundall in association with Brand Architects adopted a strong passive design approach through the combination of a high-performance envelope (including effective use of thermal mass) as well as a mixed-mode heating and natural ventilation strategy.

The latter is linked to two thermal labyrinths to assist with providing year-round comfort control.

In addition to passively conditioning incoming outside air in summer by up to 10°C and 6°C in winter, the thermal labyrinths are also used as part of a night-purge strategy to prime the thermal labyrinths for the next day as well as remove stored heat in the building during the summer months and periods of warmer weather.

To read more about the Suzanne Cory High School, see Ecolibrium December 2017.

Manual mixed-mode

air conditioning relies on consistently motivated occupants

Suzanne Cory High School features mixed-mode AC and a thermal labyrinth.

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perceived thermal comfort, noise intrusion, allergies etc. These factors can easily contribute to the default being “air conditioning on”.

Lecamwasam: The difficulty is to make clients and end users aware of the advantages of such systems and to make these a key requirement in the project brief. If solely left to individual design teams to implement, mixed-mode air conditioning is likely to be a non-starter or worse still, likely to fail in operation.

McCabe: User buy-in is a critical factor in the successful adoption of mixed-mode comfort control. Also while not supported or personally advocated, many mechanical engineers and clients alike will try to eliminate the need to provide ducted outside air to the air conditioning systems by using the loophole of the NCC/BCA that allows this approach to be adopted if the minimum required openable windows/doors are provided.

Under this approach, occupants are denied outside air during the colder and warmer months when the air conditioning is operating – a very poor indoor air quality approach particularly in buildings such as aged-care facilities.

Gaekwad: Mixed-mode ventilation design requires a systems approach to building design, where the interaction between HVAC, architecture, occupant comfort, occupant behaviour, BMS control, natural ventilation and aerodynamics is well understood and integrated. There are few projects that enable the design team to function in such an integrated fashion.

Pettifer: It should be noted that mixed-mode systems are not cheap, as you are effectively providing two types of system and the additional controls needed to operate them both. The climatic conditions have to be favourable to justify the additional cost and complexity of this approach. But in the right location and climate they definitely have their place.

Eco: What are the challenges, particularly given the human factor?

Pettifer: The natural ventilation openings need to be really carefully sized and located to provide the best internal environment, and this is an art in itself. Openings need to be at high and low level and on different facades to use thermal and wind effects respectively as the motive forces. Low-level openings need to consider the risk of draughts, and it is

F E A T U R E

It should be noted that mixed-mode systems

are not cheap, as you are effectively providing

two types of system

generally better if these are louvre-style devices. Clerestory glazing above windows and thermal chimneys are commonly used at high level.

We do know that people are more tolerant of their internal environment if they have some level of control over it, which is another good reason to adopt a simple system with manual control in natural ventilation mode.

Gaekwad: The short, and potentially controversial, answer is that sometimes the environment outside isn’t all that desirable but the industry seems to ignore this case.

A well designed and operated building would be able to cope with two cases – naturally ventilated and mechanically conditioned. Unfortunately, the way we build and operate mechanically conditions buildings is still really poor, with significant air leakage, poor thermal comfort, condensation and mould issues in a range of building typologies and climates. The real challenge around mixed-mode AC is

actually getting each mode to work; the majority of buildings fail at both.

Beyond the significance of the above, manual control can be affected by the range of human perception, and therefore disagreement between people as to how to operate the building. I also think the assumption that people know when to operate natural ventilation openings is erroneous unless people are well connected with the place, environment and climate. This sort of connection must come with both education and time.

Lecamwasam: A key ingredient to the successful implementation is user awareness, and mixed mode being a client-driven requirement. Once these parameters are defined, the other challenges are relatively easy. These include the importance of correctly defining the form and function of the building and the zones by the architect, considering the soft – but important – aspects of design, including operable windows, prevailing wind directions, orientation, the outdoor air quality considering gaseous and solid pollutants, noise, insect screens, rain, security.

The actual mechanical systems are not complicated – the trick would be to automate the controls without restricting manual intervention by the occupants, which provides a sense of independence and room for adaptive comfort.

Eco: Is mixed-mode ventilation a particularly difficult strategy to design and model?

Marklund: Firstly, the proposed building occupancy and location should be suitable for a natural ventilation mode to be fit-for-purpose, with careful consideration of intended occupancy, associated thermal and acoustic comfort requirements, local climate, noise sources, outdoor air quality, pests, maintainability and so on.

Building factors and typical client and tenant expectations for strict thermal and acoustic design criteria often mean a mixed-mode approach has poor feasibility. But where it has been determined as a viable option, the thermal envelope should be designed to minimise cooling loads and include sufficient natural ventilation devices, which can include thermal and wind-driven devices.

F E A T U R E

THE WHO AND WHAT OF INFECTION CONTROLAccording to the World Health Organization (WHO), adequate ventilation can reduce the transmission of infection in healthcare settings.

It says natural ventilation can be one of the effective environmental measures to reduce the risk of spread of infections in healthcare.

The WHO has published a guideline that explores the design requirements for natural ventilation in the context of infection control, describing the basic principles of design, construction, operation and maintenance for an effective natural ventilation system to control infection in healthcare settings.

It also includes detailed general design guidelines for mixed-mode ventilation. Check it out at www.who.int/water_sanitation_health/publications/natural_ventilation/en/

F E A T U R E

Pettifer: There’s a lot to think about because you have to work out how the building will function in multiple modes and how/when you shift from one mode to another. There is risk of over-complicating the design and the KISS (keep it simple stupid) principle is best applied. It’s important that the occupants understand how the building works and when/if they are required to take action.

With modern sophisticated software it is possible to reasonably accurately model the energy benefits but in simple manually operated systems this will be based on assumptions about how occupants may behave, which is not always easy to predict.

McCabe: In respect to modelling, there are a number of well-defined assessment protocols and design guides that provide designers with guidance on the effectiveness of an air conditioning or a natural ventilation strategy as well as the

associated transitional control logic for a mixed-mode AC solution.

However, most dynamic simulation tools will only provide an indication of the bulk or average internal environmental conditions being achieved and not highlight any localised issues. The only way to truly understand the localised impact or ventilation effectiveness of a natural ventilation strategy is through the use of computational fluid dynamic (CFD) modelling.

Gaekwad: The “bulk flow” method would be arguably the most common method of modelling natural ventilation. While the method itself is robust, the application of the method, especially in popular “black box” software, is oversimplified. The challenges to mixed-mode design are design integration and holistic knowledge of the fundamental mechanisms by which the system functions.

Eco: In what circumstances is mixed-mode ventilation likely to fail?

McCabe: Reasons for when a mixed-mode AC typically fails is when there is too many controls, or too much reliance on automation in respect to the transition between air conditioning and natural ventilation, and vice versa. As previously noted the assumption that just providing or meeting the 5 per cent openable area advocated by the NCC/BCA will provide for effective natural ventilation tends to lead to a poor occupant comfort outcome.

i0522-279 Condair Dehums AU 1/3 Page AD.qxp_Eqolibrium 72x

There is risk of over-complicating the design and the KISS

(keep it simple stupid) principle is best applied

Inspired in part by termite mounds, the Eastgate Centre, Harare, has minimal mechanical AC.

30 JUNE–JULY 2020 • ECOL IBR IUM

The use of reed switches in windows to control the operation of the air conditioning can be costly both in terms of initial capital and maintenance costs, and can also lead to poor control from false readings.

So while I am a strong advocate for passive design and the use of natural ventilation as part of a seasonal comfort strategy, the desire for a mixed-mode AC solution needs to be driven by the occupants or users, as they are the critical success factor.

Gaekwad: Like any building, the wider systems context must be reflected in the design. The contribution of mixed-mode ventilation to IEQ and IAQ is totally dependent on the quality of the air and environment outside the building.

I’ve seen buildings where operable louvres have been value managed and don’t seal properly, causing a traffic noise intrusion into the building even when closed. I’ve also seen buildings that included mixed-mode ventilation in their concept design, but had to be significantly reduced due to air pollution from a new tunnel development. Poorer noise and air quality outcomes are a real risk for inner-city developments.

Lecamwasam: Most of my experience is where clients have engaged us to offer advice to fix such systems that have not met the expectations. However, I realise that this statement is unfair to mixed-mode air conditioning.

If the advantages and limitations of such systems are understood and

accepted by clients, and mixed mode is designed systematically as part of a multi-disciplinary design process, there are a lot of positives with mixed mode. And in an industry where we can contribute so much towards pragmatic solutions towards zero emissions, mixed mode is a topic we can no longer ignore or simply make half-hearted attempts to implement. It is much bigger than just air conditioning systems with operable windows.

Eco: What impact can mixed-mode ventilation have on IEQ and IAQ?

Pettifer: Typically in natural ventilation mode there is either no filtration or very coarse filters to prevent ingress of vermin. As such, the quality of the indoor environment is very dependent on the air quality in the vicinity of the building. If it is good then flushing the building with lots of nice clean air is going to be beneficial, otherwise properly filtered and conditioned air from an air conditioning system will create a better internal environment. Dust and flying insects are other issues that need to be considered for natural ventilation. Introducing fly screens helps but creates resistance to air flow and adds a maintenance burden.

Marklund: Where outdoor air quality is high, a well-designed and operated natural ventilation mode can more effectively remove pollutants than a conventional air conditioning system.

The World Health Organization (WHO) has also recognised that properly

designed natural and hybrid systems can achieve better thermal regulation and infection control than purely mechanical systems, particularly in resource-challenged locations.

McCabe: Mixed-mode AC can provide for improved IAQ through higher rates of outside air when natural ventilation is in operation, and improved IEQ to occupants by providing greater connection to outside as well as offering them greater control over their environment.

However, like any ventilation strategy a natural ventilation strategy needs also to be designed to ensure effective ventilation is achieved and not just rely upon providing the 5 per cent openable area stipulated in the NCC/BCA. Relying on an NCC/BCA approach to natural ventilation denies occupants outside air during the colder and warmer months when the air conditioning is operating, so the IEQ and IAQ in a building or space would in fact, get worse.

Lecamwasam: Good mixed-mode design principles could also complement good IEQ requirements, such as shallow floorplates, with window sizes that permit good ventilation, natural light and views.

Eco: Finally, is mixed mode better suited to some climates than others?

Pettifer: Absolutely – it’s heavily climate and location dependent. You need to look at what proportion of the year outside conditions are suitable for natural

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ventilation operation. In hot and humid climates such as Brisbane this won’t be very long. Melbourne and Sydney are better, but still not great compared to New Zealand and the UK.

Gaekwad: It really depends on building type and use.

Areas around the central east coast – such as in Sydney – can have a desirable outdoor temperature and humidity for large portions of the year. The same is true for summer in Melbourne, and shoulder seasons in Perth and Adelaide. More than this, I think mixed mode is more a cultural concept, or maybe an indoor/outdoor state of mind.

Marklund: It works best in temperate climates, so is more suited to Melbourne than Brisbane due to outdoor conditions being in the acceptable thermal comfort band many more hours of the year.

Notwithstanding this, mixed mode air conditioning has been successfully applied in sub-tropical and tropical climates.

McCabe: While most climates offer some opportunity for adopted a mixed mode AC solution, it is generally found that they are adopted in temperate climates such as experience in Melbourne, Adelaide and Hobart, as well as the Mediterranean-like climate of Perth and the desert climate of Alice Springs.

However, it can also be applied in the subtropical climate of Brisbane –

particularly during its winter months when the humidity levels are not high. Domestically, it is also used in the tropical climate of Darwin during the dry season, but rarely for non-residential buildings.

Pettifer: Adaptive comfort can be the key here (i.e. asking people to accept a wider range of internal temperatures and wear clothing to suit).

If you broaden the acceptable range of internal temperatures from the 21°C–23°C that you would expect in a fully air conditioned space to say 20°C–25°C you extend the hours of operation and improve the feasibility of the approach. As well as saving energy by operating in natural ventilation mode for longer you will reduce the demand on the air conditioning system when it is operating. Introducing additional air movers (ceiling fans) to operate in natural ventilation mode assists with occupant comfort and acceptance. ❚

F E A T U R E

More than this, I think

mixed mode is more

a cultural concept,

or maybe an indoor/

outdoor state of mind