Achieving good HVAC outcomes is a challenge in high-rise residential

3 July 2014 — How do you deal with the issue of heating, ventilation and airconditioning in very tall residential buildings? It’s not easy, and the demands are far different to those for commercial towers. We speak to two experts in the field, David Odd, national sustainability engineer for mechanical services contractor AE Smith, and Vince Aherne, Australian Institute of Refrigeration, Air Conditioning and Heating technical advisor.

Making very tall, high density residential buildings sustainable means resolving some major challenges in terms of thermal comfort and energy efficiency, as heating, ventilation and airconditioning systems comprise a substantial part of the overall energy footprint. Unlike the commercial office tower, there is a need to cater for hundreds of individual ideas of comfort, all within a building envelope that can itself add to the level of difficulty.

As part of the development of Section J of the Building Code of Australia, which sets out the minimum benchmarks for energy efficiency and thermal comfort, the Australian Building Codes Board commissioned an energy performance assessment of HVAC systems in multi-residential and commercial buildings.

The lifecycle of equipment, carbon emissions associated with energy use, payback periods and the variations between requirements in different climatic zones were all considered. The analysis found that the use of insulation on pipework and ducts improved energy efficiency, as did the use of variable speed drives, with the savings increasing for larger motor sizes.

Under the most recent 2013 version of the BCA Section J, multi-unit developments must achieve an average energy rating of at least six NatHERS stars, and individually achieve an energy rating of at least five stars. There are two methods to demonstrate compliance, the Deemed to Satisfy provisions or thermal simulation against a reference building, which must be carried out by an accredited energy assessor. Both methods are based on the design and the assessment report lodged either as part of the development application or at the construction certificate stage.

Achieving fresh air in the indoor environment

The building code also sets out specifications for fresh air ventilation, which can be supplied either through operable windows or through a mechanical air handling system. Under the Green Building Council of Australia’s Multi-Residential V1 tool, only passive, non-mechanical fresh air ventilation attracts credit points under the indoor environment quality category.

Even for non-Green Star projects, passive ventilation has become a major selling point for many apartment projects.

Vince Aherne

However, Vince Aherne, AIRAH’s technical advisor, says there can be indoor air quality tensions when naturally ventilated spaces are airconditioned with systems that make no provision for the introduction of outside air. These issues stem from occupant behaviour, building design and regulation.

“Often in residential construction, airconditioning and ventilation are not integrated. This can lead to a lot of related outcomes, including poor indoor air quality, high energy costs and poor comfort,” Aherne says.

“People have choice – for example to open windows – but the designs often do not optimise the potential outcomes. Ideally, in airconditioning mode the ventilation air should be provided through the airconditioning systems, and in natural ventilation mode the ventilation air should be provided through the windows and doors.”

That means when the airconditioning is on, windows and doors should be kept shut in order to achieve the level of comfort and air quality intended by the mechanical system design.

“If the airconditioning system is not supplying conditioned air then some amount of natural ventilation needs to be left open and this goes against all energy efficiency advice and intuitive behaviour. How many people run their airconditioner with the windows wide open? With the windows open on a hot day comfort might be compromised,” he says.

Bad smells and the tricky issue of neighbours

Exhaust systems are another issue for apartments that have a tightly sealed building envelope.

The tightly sealed envelope is positive in terms of managing thermal comfort, but, for example, it means that exhaust systems in bathrooms and toilets need to be provided with a proper air makeup path so air can get in to replace the air being exhausted. In the past this air path was provided by the general leakiness of the building construction.

Cooking-related odours are a common cause of complaint. There is no National Construction Code requirement to provide kitchen exhaust systems for residential and these are often installed as recirculating systems rather than exhaust systems,” Aherne says. Recirculating filter systems are easy to install but are often ineffective.

Exhaust systems are either exhausted through the facade, or through a central shaft. Both, Aherne says, add complications for designers and builders, including fire issues in the case of kitchen exhausts.

Grease build-up inside the ductwork is the main issue. This becomes a fire hazard unless regularly maintained, and there are plenty of ignition sources from the cooking activity. Because of the grease you can’t put fire dampers on the ducts to protect where they pass through fire compartmentation of walls and floors, he says.

“Fire dampers don’t stop fire spread in grease laden ducts – the fire can jump across the damper due to the grease – and multiple exhaust ducts in a common shaft poses a fire risk when they can’t be protected by dampers.” Fire can potentially jump from the inside of one duct to another due to heat alone.

Aherne says that kitchen exhaust can need a lot of maintenance (depending on use) and any grease filters require regular inspection and cleaning. This, however, cannot be effectively mandated in a residential setting, unlike in a commercial setting.

The other option, exhausting at the facade, can cause staining issues, and there is the issue of odour and noise impacts via neighbouring windows and balconies above, below and to the sides to consider.

Generally, the whole issue of exhaust is something of a catch-22, the recirculating systems are easier to install, have aesthetic upsides and don’t have the fire risk or risk of offending other occupants, but they don’t work as well. No exhaust at all means a build up of cooking smells and also condensation issues from steam, which can be detrimental for finishes. Odours moving into common areas, or between apartments, is also a common complaint in multi-unit residential.

Getting everyone on the same page

Overall, in terms of energy use, Aherne says there may be a disincentive for energy efficiency or reduced consumption in common areas, as reductions in service levels (for example lighting levels, spa pool operating times, etc) is problematic.

Some of this can be resolved through design and specification, for example installing carbon monoxide monitors in carparks and automating the exhaust system to only operate when levels exceed the acceptable level, and ensuring the system is right-sized for the space and usage.

Other measures are more operational. For example, pools and spas often have pumps and other systems operating continuously, and these could be equipped with programmable timers or a switch off protocol established with building managers, caretakers or residents. However, Aherne says leaving matters up to residents can also be problematic.

“Because residents often do not pay the electricity costs for common areas directly – it comes out of common funds – they are less inclined to turn things off,” he says.

It only takes one bad apple to waste energy

In the commercial space, proactive building management has shown significant energy benefits, however in the residential space it is not so clear cut and more negotiation would be required.

Ultimately, Aherne says integration of the builder, services designer and architect at early design stages does assist with resolving many of the issues around HVAC. However, this process of design optimisation does have upfront costs.

Right sizing and fit for purpose

David Odd

David Odd, national sustainability engineer for mechanical services contractor AE Smith, says that one of the big differences between multi-residential and commercial buildings is that commercial settings lend themselves more to efficient, centralised systems with chilled water plant. While these systems can be used for multi-residential, there is a general tendency to install individual air-cooled split system units for each apartment, with the condenser unit located on the balcony, or water-cooled split systems with condensers in the ceiling space, so that each apartment takes care of the majority of their airconditioning energy costs.

He says this distributed equipment is generally less energy efficient than central plants, but with the diversity of occupancy times and loads, this can be the most efficient design overall.

“A centralised system has lower running costs in a commercial setting,” Odd says.

“In larger residential developments, particularly where there is mixed use such as retail, there can also be the opportunity to use trigeneration, geothermal or seawater cooling solutions. Although you still have the problem of how to apportion the costs fairly for individual apartment owners.”

A downside of the very tall residential buildings is that the amount of roof space available for installing solar photovoltaics is small compared to the number of apartments, making it difficult for solar to meet a substantial proportion of energy needs.

Also, Odd says the curtain wall facade style, with its extensive glazing, creates a higher solar load in terms of heating up the building, which in turn increases the demands on the airconditioning system. There is, however, some mitigation from having apartments above, below and to either side, which reduces the direct thermal transference, and where a development includes balconies, these can provide a degree of shading for the apartment below.

By comparison, in a medium-rise development, there may be more solar gain from the roof and from external walls, but also more opportunity to effectively harness that sunshine for power.

Design, space and appropriate solutions

Cross flow ventilation is an area Odd identifies as a challenge in high density developments, with the floorplate layouts of some projects precluding some apartments from being able to achieve passive through-flow of fresh air.

The premium on space in the overall project means systems that require extensive riser space or large ductwork in the ceiling, as well as central plant equipment, take up space that cumulatively could add up to a whole other floor of apartments within the permitted height envelope.

Then there is the issue of air distribution within each apartment using a single ducted system or a fan coil unit in each room.

“Individual units can prevent unoccupied rooms from being conditioned. There are also losses in pushing air through ductwork throughout a whole apartment,” Odd says.

“If you have a fan coil system or split system head in each room, ductwork can be avoided. Variable refrigerant flow systems comprising one air-cooled or water-cooled condenser unit servicing multiple indoor heads can also be very efficient.”

These systems can transfer “free” heat from one side of the dwelling to the other, warming up the cold western side in the mornings while also cooling the eastern side simultaneously. This process is automated, based on the thermostat setting of each room.

The other advantage these systems have is in terms of space, as the two insulated pipes required for the refrigerant flow are compact enough to fit in a small space, taking up only a fraction of the space required for a ducted system.

Post-occupancy behaviour also plays a role in energy efficiency

“If each resident is in control of their airconditioning, even if it has been programmed to operate automatically, do they remember to switch it off when they go away?” Odd says.

The individual systems also become less efficient if regular maintenance is not carried out in terms of changing or cleaning the filters. Unlike commercial systems, where regular maintenance is mandated for health and safety reasons, there is no requirement for individual residential dwellings to do so.

Retrofits can be difficult

Where a multi-residential building has inefficient centralised equipment installed, Odd says it can be difficult to achieve an energy efficiency upgrade.

“In strata title buildings, where there are many, many owners, the owners committee can find it difficult to get agreement from all the owners to do things to increase efficiency and get people to spend money to do it,” he says.

“Whereas when you have individual owners you can just do it, with strata title you have so many different types of owners – some are owner occupiers, some are looking to sell, some are investors – everyone has their own agenda.”

There are also the logistical issues of needing to either replace central plant and possibly hundreds of individual fan coil units within apartments, or where there is a more distributed system, upgrade compressor units which are spread out throughout the apartments.

Natural ventilation is also hard to retrofit. Odd says there have been projects where operable windows or louvres were offered as an option for off-the-plan buyers, and now owners who declined the option have moved in, they want to get them retrofitted.

“Then there are issues with the balancing of the ventilation system, and also issues with structural wind loads. If someone was on the 20th floor, retrofitting operable windows could result in creaking walls,” Odd says.

“For operable windows there needs to be appropriate structural engineering, and involvement by the mechanical services consultant and the contractor at the early design stage.”

Creating the benchmarks

In terms of benchmarking operational efficiency, Odd says the new Green Star Performance tool will be useful for multi-residential buildings as well as all other types of occupied buildings.

“The whole building is rated, including any mixed use component. That would be the way to benchmark, and drive improved performance, as you have to demonstrate how you manage energy efficiency.

“When I do energy audits [predominantly for commercial and industrial sites] I recommend energy monitoring, and the same would apply to residential of any size. But with high-rise residential, if there is centralised energy consumption monitoring in place, someone still has to be looking for anomalies, so if they get a spike in energy use they will go and investigate.

“It is very useful when someone is looking at doing energy efficiency upgrades to have a baseline so they can monitor use, justify upgrades and verify the savings.”