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Mixed opinion For 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.

Eco: Tell us how a mixed-mode The main differences are the control McCabe: A mixed-mode comfort ventilation strategy typically works? system for the natural ventilation – strategy works on the basis that heating is automatic or manual – and what the used when ambient conditions are cold, Lecamwasam: It is the fusion of natural ventilation openings look like – air conditioning when hot, and natural time-tested and proven good passive louvres, operable walls, etc. ventilation when ambient conditions suit design principles with the predictable to maintain acceptable thermal comfort. design and operation of mechanical AC Pettifer: When outdoor conditions systems. Correct design and operation are favourable, the building is naturally However, a critical design issue is that of such systems can significantly reduce ventilated and when they are not the natural ventilation system has to operating costs and improve user the mechanical systems operate. be designed to provide for effective enjoyment of buildings. The mode shift can be fully automatic, ventilation, as just meeting NCC/BCA with actuators used to open and close openable area will not suffice. Gaekwad: The most basic mixed-mode windows and/or louvres in the façade. system is your typical single dwelling Alternatively, in natural ventilation Marklund: A semi-automated approach – house or apartment – with operable mode occupants can be alerted, for can be to install reed switches on a windows and an air conditioning system. example using a traffic light system manually operable façade element that Occupants open the windows when mounted in the space or via personal when opened, turns off the local air it’s “nice” outside. When the dwelling technology, and they then manually conditioning zone. A weather sensor becomes uncomfortably hot or cold, open and close the natural ventilation operating a visual indicator light or app the windows are closed, and the air devices in their vicinity. can be used to encourage occupants to conditioning system is switched on. open windows. Outside of natural ventilation mode, A mixed-mode system operates very the system works as conventional air A fully automated system, for say an similarly for any other building typology. conditioning. open-plan office, would typically include

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Caimin McCabe Anthony Marklund Jason Gaekwad Lasath Lecamwasam Andrew Pettifer outdoor and indoor sensors linked to At present though, the risks to the BMS to control electrically actuated consultants through adopting mixed natural ventilation devices and the air mode makes such systems a rarity. Sadly, conditioning systems. building occupants have developed some Eco: In what applications does very expensive habits and expectations mixed-mode ventilation work best? over the past 50 years or so, and in an There are examples environment now facing extreme weather Pettifer: We tend to think in terms of school, university, events more commonly, the challenges of office spaces, and there are many commercial, retail and to implementing mixed mode have examples of mixed-mode systems in increased. commercial buildings around the world. even hospital buildings Eco: And what are some of the But it can and has been applied to other that have successfully building types such as sports facilities advantages and benefits? adopted mixed-mode and even theatres. Lecamwasam: The main advantages The main drivers are to reduce energy are reduced operational costs, user consumption and the perceived benefits There are examples of school, university, satisfaction and enhanced green to occupants – most people like the commercial, retail and even hospital credentials – if being environmentally idea of being able to open the façade, buildings that have successfully adopted friendly is a key ethos of the end users. breathe fresh air and hear the birds mixed-mode air conditioning. McCabe: Mixed mode can provide for sing. If the air outside is polluted and all you can hear is traffic noise it’s not Marklund: Mixed-mode air improved energy efficiency – assuming quite the same effect! So it works best conditioning has been successfully effective controls are in place such as in suburban or rural locations with applied to many types of buildings avoiding windows being open and air temperate . internationally. conditioning is on – improved indoor air quality through higher rates of When designed, installed, operated and Gaekwad: The application of mixed- outside air when natural ventilation is maintained correctly, particularly in mode ventilation is wholly dependent in operation, and improved health and on and outdoor environment, conjunction with an adaptive thermal comfort strategy where individuals wellbeing of occupants by providing building form and use, and indoor greater connection to outside, as well as environment. East coast cities tend have visual connection to the outdoors and are appropriately educated – and providing them greater control over their to have a mild and desirable climate environment. for the majority of the year, which allowed control over their environment has led to an “indoor/outdoor” type – international studies indicate that But it is critical that the natural of lifestyle. Mixed-mode systems can occupant satisfaction and productivity ventilation part of the mixed-mode air contribute to this design and lifestyle can be improved while significantly conditioning system be designed to philosophy. During summer in reducing air conditioning energy use, cost provide effective ventilation and not just and associated carbon emissions. Melbourne, the ambient temperature meeting NCC/BCA openable area, as this is generally within the comfort range; Lecamwasam: Mandated building will not suffice and lead to complaints. natural ventilation could be utilised to rating schemes such as NABERS and Gaekwad: If done well, mixed-mode remove heat load from, for example, Green Star could complement the densely occupied office buildings. For adoption of mixed mode very well. ventilation can provide a greater regions with a high diurnal variation in connection to the outdoors, including temperature, a night-purge system can If end user groups such as the to thermal variability and ventilation be used, where cool air is brought into Commonwealth and state governments, velocity variability. This can dramatically the building at night. retail, education, hospitality industries improve comfort, and represents a move etc., redefined lease requirements for towards a more “human centred” method McCabe: Subject to prevailing climate more environmentally friendly minimum of HVAC design. being suitable and functional usage, there service requirements for temperature are not many building types or areas and occupant comfort, then mixed However the building operation should where a mixed-mode air conditioning mode would be far widely adopted by also reflect this design philosophy when approach cannot be adopted. consultants. the HVAC system is running.

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Eco: What types of mechanical systems work best in a mixed- MIXED MODE IN OPERATION mode ventilation strategy? McCabe: ASB HEADQUARTERS, $477 million Cooler Cleaner Schools Mixed-mode designs can program. This could see naturally work with any air conditioning systems AUCKLAND NZ ventilated schools being enabled for – the key is that effective operation is The design of ASB North Wharf in manual mixed-mode air conditioning controlled so that a space or building Auckland reflected the desire of its subject to the teachers being instructed is not being air conditioned with tenant for a world-class building that to operate them this way. the windows or vents are open. The places staff and customers at the only systems where the combination centre of the sustainability equation. SUZANNE CORY of natural ventilation needs to be carefully considered from a controls Featuring a mechanical service HIGH SCHOOL, perspective are chilled ceiling and design by Arup, the seven-storey WERRIBEE, VIC chilled beam systems, so that unwanted commercial office building maximises When designing the mechanical condensation does not occur. daylight, views and the fresh air of the services solution at the Suzanne Cory Waitemata Harbour through the use Pettifer: Chilled beams are High School, Cundall in association of a green light/red light system to problematic because you have to control with Brand Architects adopted a strong alert occupants when conditions are humidity in the building to prevent passive design approach through the favourable for windows to be opened. condensation, and that’s not easy to combination of a high-performance do in a building that is intermittently To read more about the ASB North envelope (including effective use of naturally ventilated. All-air systems Wharf, see Ecolibrium December 2013. thermal mass) as well as a mixed-mode work better. In particular, displacement heating and natural ventilation strategy. systems that work at higher supply‑air QUEENSLAND SCHOOLS The latter is linked to two thermal temperatures and lower system Floth has undertaken engineering labyrinths to assist with providing pressures can complement the natural services and sustainable design for year‑round comfort control. ventilation mode, and the two work numerous state schools that include In addition to passively conditioning more easily together. thermal comfort control via passive incoming outside air in summer by up design, natural ventilation and fans. Marklund: Conventional all- to 10°C and 6°C in winter, the thermal air systems such as fan coil units Seven of these schools have achieved labyrinths are also used as part of and VAV systems make sense for Green Star – Education v1 ratings, a night-purge strategy to prime the mixed‑mode air conditioning, as they including Peregian Springs and thermal labyrinths for the next day are able to manage humidity variation Bayview State Schools, which were as well as remove stored heat in the following a period of natural ventilation. the first in Australia to achieve this. building during the summer months Able to be locally controlled per zone, and periods of warmer weather. In late February, the Queensland they are also suitable for mixed-mode government promised to provide To read more about the operation of dedicated spaces such as a air conditioning to every school in Suzanne Cory High School, perimeter conference room as we have ... and Trust Trafalgar the state by mid-2022 under its see Ecolibrium December 2017. implemented at Floth head office. Eco: What are some of the common ® Suzanne Cory High School features mixed-mode AC and a thermal labyrinth. issues around the adoption of a FyrePEX HP tested for a wide range of fire barriers for -/120/120 including; mixed-mode ventilation strategy? SpeedPanel, Hebel and Plasterboard Marklund: Manual mixed-mode air conditioning relies on consistently motivated occupants. In open-plan Less time and money than traditional environments, occupants can disagree collar systems on the operation of natural ventilation Fully approved to AS1530.4-2014 and mode due to personal attitudes, AS4072.1 Control cables and drain pipes alongside the Pair Coil through each penetration Clusters of up to 3x Pair Coil approved 50mm apart Manual mixed-mode Suitable for FR and PE insulated Pair Coil air conditioning Suitable for single layer residential plasterboard walls for -/60/60 relies on consistently motivated occupants tfire.com.au 1800 888 714 26 JUNE–JULY 2020 • ECOLIBRIUM FIRE ‘FyrePEX® and are trademarks of Trafalgar Group Pty Ltd’

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perceived thermal comfort, noise Under this approach, occupants are intrusion, allergies etc. These factors can denied outside air during the colder easily contribute to the default being and warmer months when the air “air conditioning on”. conditioning is operating – a very poor indoor air quality approach particularly Lecamwasam: The difficulty is to in buildings such as aged-care facilities. make clients and end users aware of It should be noted the advantages of such systems and Gaekwad: Mixed-mode ventilation to make these a key requirement design requires a systems approach to that mixed-mode systems in the project brief. If solely left to building design, where the interaction are not cheap, as you individual design teams to implement, between HVAC, architecture, occupant are effectively providing mixed‑mode air conditioning is likely comfort, occupant behaviour, BMS to be a non-starter or worse still, control, natural ventilation and two types of system likely to fail in operation. aerodynamics is well understood and integrated. There are few projects that McCabe: User buy-in is a critical enable the design team to function in Eco: What are the challenges, factor in the successful adoption such an integrated fashion. particularly given the human factor? of mixed‑mode comfort control. Also while not supported or personally Pettifer: It should be noted that Pettifer: The natural ventilation advocated, many mechanical engineers mixed‑mode systems are not cheap, as openings need to be really carefully sized and clients alike will try to eliminate you are effectively providing two types and located to provide the best internal the need to provide ducted outside of system and the additional controls environment, and this is an art in itself. air to the air conditioning systems needed to operate them both. The climatic Openings need to be at high and low level by using the loophole of the NCC/ conditions have to be favourable to justify and on different facades to use thermal BCA that allows this approach to be the additional cost and complexity of this and wind effects respectively as the adopted if the minimum required approach. But in the right location and motive forces. Low-level openings need openable windows/doors are provided. climate they definitely have their place. to consider the risk of draughts, and it is

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generally better if these are louvre‑style actually getting each mode to work; devices. Clerestory glazing above the majority of buildings fail at both. windows and thermal chimneys are Beyond the significance of the above, commonly used at high level. manual control can be affected by We do know that people are more the range of human perception, and tolerant of their internal environment if therefore disagreement between people they have some level of control over it, as to how to operate the building. I also which is another good reason to adopt think the assumption that people know a simple system with manual control in when to operate natural ventilation natural ventilation mode. openings is erroneous unless people are well connected with the place, Gaekwad: The short, and potentially environment and climate. This sort controversial, answer is that sometimes of connection must come with both the environment outside isn’t all that education and time. desirable but the industry seems to ignore this case. Lecamwasam: A key ingredient to the successful implementation is A well designed and operated building user awareness, and mixed mode would be able to cope with two cases – being a client-driven requirement. naturally ventilated and mechanically Once these parameters are defined, conditioned. Unfortunately, the way the other challenges are relatively we build and operate mechanically easy. These include the importance conditions buildings is still really of correctly defining the form and poor, with significant air leakage, poor function of the building and the thermal comfort, condensation and zones by the architect, considering mould issues in a range of building the soft – but important – aspects of typologies and climates. The real design, including operable windows, challenge around mixed-mode AC is prevailing wind directions, orientation, the outdoor air quality considering gaseous and solid pollutants, noise, THE WHO insect screens, rain, security. The actual mechanical systems are AND WHAT not complicated – the trick would be to automate the controls without OF INFECTION restricting manual intervention by the occupants, which provides a sense of CONTROL independence and room for adaptive According to the World Health comfort. Organization (WHO), adequate Eco: Is mixed-mode ventilation ventilation can reduce the a particularly difficult strategy transmission of infection in to design and model? healthcare settings. Marklund: Firstly, the proposed It says natural ventilation can be building occupancy and location should one of the effective environmental be suitable for a natural ventilation measures to reduce the risk of mode to be fit-for-purpose, with careful spread of infections in healthcare. consideration of intended occupancy, The WHO has published a associated thermal and acoustic guideline that explores the design comfort requirements, local climate, requirements for natural ventilation noise sources, outdoor air quality, in the context of infection control, pests, maintainability and so on. describing the basic principles of Building factors and typical client design, construction, operation and tenant expectations for strict and maintenance for an effective natural ventilation system to control thermal and acoustic design criteria infection in healthcare settings. often mean a mixed-mode approach has poor feasibility. But where it has It also includes detailed general been determined as a viable option, the design guidelines for mixed-mode thermal envelope should be designed ventilation. Check it out at www. to minimise cooling loads and include who.int/water_sanitation_health/ sufficient natural ventilation devices, publications/natural_ventilation/en/ which can include thermal and wind- driven devices. i0522-279 Condair Dehums AU 1/3 Page AD.qxp_Eqolibrium 72x

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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 There is risk of environmental conditions being achieved and not highlight any over‑complicating the localised issues. The only way to design and the KISS truly understand the localised impact or ventilation effectiveness (keep it simple stupid) of a natural ventilation strategy is principle is best applied through the use of computational fluid dynamic (CFD) modelling. Gaekwad: The “bulk flow” method Pettifer: There’s a lot to think would be arguably the most common about because you have to work out method of modelling natural how the building will function in ventilation. While the method multiple modes and how/when you itself is robust, the application shift from one mode to another. of the method, especially in There is risk of over‑complicating popular “black box” software, is the design and the KISS (keep it oversimplified. The challenges to simple stupid) principle is best mixed‑mode design are design applied. It’s important that the integration and holistic knowledge occupants understand how the of the fundamental mechanisms by building works and when/if they which the system functions. are required to take action. Eco: In what circumstances With modern sophisticated software is mixed‑mode ventilation it is possible to reasonably accurately likely to fail? model the energy benefits but in simple manually operated systems this will McCabe: Reasons for when a mixed- be based on assumptions about how mode AC typically fails is when there occupants may behave, which is not is too many controls, or too much always easy to predict. reliance on automation in respect to the transition between air conditioning McCabe: In respect to modelling, and natural ventilation, and vice versa. there are a number of well-defined As previously noted the assumption assessment protocols and design that just providing or meeting the guides that provide designers with 5 per cent openable area advocated by guidance on the effectiveness of the NCC/BCA will provide for effective an air conditioning or a natural natural ventilation tends to lead to a ventilation strategy as well as the poor occupant comfort outcome.

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The use of reed switches in windows accepted by clients, and mixed mode designed natural and hybrid systems to control the operation of the air is designed systematically as part of a can achieve better thermal regulation conditioning can be costly both in terms multi‑disciplinary design process, there and infection control than purely of initial capital and maintenance costs, are a lot of positives with mixed mode. mechanical systems, particularly in and can also lead to poor control from And in an industry where we can resource-challenged locations. false readings. contribute so much towards pragmatic McCabe: Mixed-mode AC can solutions towards zero emissions, So while I am a strong advocate for passive provide for improved IAQ through Clean air, our care design and the use of natural ventilation mixed mode is a topic we can no longer higher rates of outside air when as part of a seasonal comfort strategy, ignore or simply make half‑hearted natural ventilation is in operation, the desire for a mixed-mode AC solution attempts to implement. It is much and improved IEQ to occupants needs to be driven by the occupants or bigger than just air conditioning by providing greater connection to users, as they are the critical success factor. systems with operable windows. outside as well as offering them greater control over their environment. Gaekwad: Like any building, the wider Eco: What impact can mixed-mode systems context must be reflected in the ventilation have on IEQ and IAQ? However, like any ventilation strategy design. The contribution of mixed-mode Pettifer: Typically in natural ventilation a natural ventilation strategy needs ventilation to IEQ and IAQ is totally mode there is either no filtration or also to be designed to ensure effective dependent on the quality of the air and very coarse filters to prevent ingress of ventilation is achieved and not just The school provides the best education. environment outside the building. vermin. As such, the quality of the indoor rely upon providing the 5 per cent openable area stipulated in the NCC/ I’ve seen buildings where operable louvres environment is very dependent on the BCA. Relying on an NCC/BCA AFPRO Filters provides the clean air. have been value managed and don’t seal air quality in the vicinity of the building. approach to natural ventilation denies properly, causing a traffic noise intrusion If it is good then flushing the building occupants outside air during the into the building even when closed. I’ve with lots of nice clean air is going to be colder and warmer months when the also seen buildings that included mixed- beneficial, otherwise properly filtered and air conditioning is operating, so the mode ventilation in their concept design, conditioned air from an air conditioning system will create a better internal IEQ and IAQ in a building or space but had to be significantly reduced would in fact, get worse. due to air pollution from a new tunnel environment. Dust and flying insects are development. Poorer noise and air quality other issues that need to be considered Lecamwasam: Good mixed-mode outcomes are a real risk for inner-city for natural ventilation. Introducing fly design principles could also complement developments. screens helps but creates resistance to air good IEQ requirements, such as shallow Everyone is entitled to clean, healthy air. It is an AFPRO A+ filter main quality features are: flow and adds a maintenance burden. floorplates, with window sizes that permit Lecamwasam: Most of my experience important part of a healthy lifestyle and essential ● effective on even the smallest fine dust Marklund: good ventilation, natural light and views. is where clients have engaged us to offer Where outdoor air quality in all public spaces. That is why we take our filters particles (PM1) advice to fix such systems that have not is high, a well-designed and operated Eco: Finally, is mixed mode better to the next level. AFPRO Filters stands for health, ● extremely energy efficient due to low air met the expectations. However, I realise natural ventilation mode can more suited to some climates than others? that this statement is unfair to mixed- effectively remove pollutants than a sustainability, efficiency and innovation. They all come resistance Pettifer: Absolutely – it’s heavily climate mode air conditioning. conventional air conditioning system. together in our latest creation: the AFPRO A+ filter. ● innovative filter media with a durable pre-layer and location dependent. You need to If the advantages and limitations of The World Health Organization (WHO) look at what proportion of the year for easy installation such systems are understood and has also recognised that properly outside conditions are suitable for natural Find out more at www.afprofilters.com.au ● Eurovent guaranteed performance [email protected] +61 3 9312 4058 30 JUNE–JULY 2020 • ECOLIBRIUM FEATURE

ventilation operation. In hot and humid particularly during its winter months climates such as this won’t be when the humidity levels are not very long. Melbourne and are high. Domestically, it is also used better, but still not great compared to in the tropical climate of Darwin New Zealand and the UK. during the dry season, but rarely for Gaekwad: non‑residential buildings. It really depends on building More than this, I think type and use. Pettifer: Adaptive comfort can be the key here (i.e. asking people to accept a Areas around the central east coast – mixed mode is more wider range of internal temperatures such as in Sydney – can have a desirable a cultural concept, and wear clothing to suit). outdoor temperature and humidity for large portions of the year. The same is or maybe an indoor/ If you broaden the acceptable range true for summer in Melbourne, and outdoor state of mind of internal temperatures from the shoulder seasons in and Adelaide. 21°C–23°C that you would expect More than this, I think mixed mode is in a fully air conditioned space more a cultural concept, or maybe an McCabe: While most climates offer some to say 20°C–25°C you extend the indoor/outdoor state of mind. opportunity for adopted a mixed mode hours of operation and improve the feasibility of the approach. As well Marklund: It works best in temperate AC solution, it is generally found that as saving energy by operating in climates, so is more suited to Melbourne they are adopted in temperate climates natural ventilation mode for longer than Brisbane due to outdoor conditions such as experience in Melbourne, you will reduce the demand on the being in the acceptable thermal comfort Adelaide and Hobart, as well as the air conditioning system when it is band many more hours of the year. Mediterranean-like climate of Perth and operating. Introducing additional air the desert climate of Alice Springs. Notwithstanding this, mixed mode air movers (ceiling fans) to operate in conditioning has been successfully applied However, it can also be applied in natural ventilation mode assists with in sub-tropical and tropical climates. the subtropical climate of Brisbane – occupant comfort and acceptance. ❚