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Ionisation technologies are worth considering for higher transmission risk areas such as meeting or conference rooms where social distancing may be difficult to manage, and people tend to congregate for extended periods of time (>15 minutes).
It is worth noting that some products in the market produce ozone as a by-product of the ionization process. Elevated levels of ozone are considered harmful to human health and therefore these types of products should be avoided. ASHRAE recommends that ionisation systems comply with UL2998.
Consideration should also be given to the ongoing maintenance requirements and associated costs for these technologies prior to their installation
Basic split systems with high wall indoor units typically have low efficiency filters that are not effective at capturing aerosols, further these units have limited provisions, if any, to provide conditioned outside air ventilation. Airborne infection risk models show that increasing the outside air ventilation rate is the most effective means of diluting aerosols and therefore the risk of infection transmission.
Consideration could be given to providing additional natural ventilation when ambient conditions are suitable, separate filtered outside air / exhaust systems or provision of ultraviolet germicidal irradiation (UVGI) units which can be easily installed on a wall, ceiling or as a free standing unit.
The key risk associated with installing these types of systems within large air handling systems is considered to be UV exposure induced keratoconjunctivitis that can cause temporary blindness as well as is being extremely painful. It is recommended that safety interlocks be installed to prevent the operation of UV lamps whenever the AHU doors are open. It is also recommended that safety signage be affixed to the AHU doors, and a UV safe viewing portal be provided to allow maintenance personnel to observe the operation of the lamps and to confirm that the lamps are off before entering the units.
UV light can cause premature degradation of some materials used in air handling units so care should also be taken to confirm the suitability of the proposed enclosure when retrofitting lamps to an AHU.
There is increasing evidence that portable units with a fan that recirculates room air through a HEPA filter can reduce levels of suspended droplets and particulates in the air and therefore the risk of COVID-19 transmission. These units may be worth considering for higher risk environments such as conference and meeting rooms where social distancing is difficult or where the provision of such units can improve the building occupant’s sense of safety.
We are not aware of any studies that have concluded that there is a measurable risk that the virus can be spread through a well maintained commercial office air conditioning air handling system.
A study of hospital rooms with COVID-19 infected patients did indicated that return air grilles when swabbed returned a positive result in 2 of 3 rooms with infected individuals.
Whilst the risk appears to be low for infection transmission via central air handling systems it is certainly prudent for building owners to review the system filter efficiencies and the filter installation to assure that return air is not bypassing the filter banks. These checks would involve close examination of the seals and how the filters are sitting in the frames.
It is also worth confirming that your maintenance service providers have safe work method procedures and PPE practices for working within return air and exhaust systems and particularly filter inspections and replacements.
Discharges from 100% outside air systems would reasonably be expected to be at a safe distance from other air intakes and the public as these requirements have been mandated for many years by AS 1668.
High levels of dilution will generally occur when the exhaust or spill air is discharged to outside greatly reducing airborne transmission risks to the public. Furthermore, exposure to UV from sunlight inactivates the virus with time. As a consequence, WHO does not consider air conditioning discharges to be a significant transmission risk factor.
As a minimum we would suggest HVAC systems are checked to assure that they meet minimum standards for separation of discharges from high occupancy public spaces and air intakes.
Specialist applications such as hospital negative pressure isolation rooms typically have HEPA filtered exhaust air to reduce the risk of infection spread to other parts of the hospital or the community. Whilst this is a proven method of infection control, it is unlikely to be feasible to retrofit these types of filters to existing commercial building air conditioning systems due to their significant pressure losses and specialised installation requirements.
These are all good recommendations in theory, their implementation however presents many practical challenges. The majority of commercial air conditioning systems will not be able to maintain comfort conditions if they are set to operate on 100% outside air at all times due to the significant cooling and heating load increases imposed on the AHU coils and thermal plants. In addition, there will be substantial energy consumption penalties with this mode of operation that will adversely impact the building’s NABERS ratings.
For existing facilities, we recommend that a check be completed to ensure minimum statutory outside air rates are being provided. Some forms of outside air measurement across the minimum outside air dampers are crude when compared to current best practice and should be assessed as part of the review process. This forms a first practical step to minimising the risk of infection spread within the building.
Most large systems will have economy cycle systems fitted that, for the majority of the time, will provide higher ventilation rates than the minimum requirement effectively reducing the risk of infection spread with no additional energy consumption.
Whilst the fitting of economy cycle dampers on large HVAC systems has been established practice for more than 40 years, there are many smaller commercial and public buildings that have packaged roof top unitary air conditioning systems without economy cycle dampers. In many cases indoor air quality could benefit from the retro-fitting of economy cycle dampers and controls. The ability to retro-fit these dampers will need to be determined on a case by case basis.
For rooftop unitary equipment replacements, the installation of MERV 13 filters and economy cycle dampers should now be considered minimum practice for all central air handling systems and roof top packaged systems. This may require the installation of larger filter plenums with lower face velocities to stay within the NCC energy efficiency provisions for fan energy.
Flushing with outside air would be expected to be more beneficial at the end of the occupancy period rather than morning start-up as any aerosols are likely to have settled during the off cycle.
Most commercial air conditioning systems will start on a morning warm-up/cool down cycle before the occupied time starts. These typically operate on minimum outside air or completely close the outside dampers to conserve energy. It may be appropriate to review options to maximise the ventilation rate during this mode of operation whilst not overly penalising energy usage.
We believe for well designed, commissioned and maintained systems, the probability of droplets spreading via the AHU systems remains low.
For new designs, owners should be seeking a comprehensive design response that addresses not only energy efficiency but also Indoor Air Quality. We expect to see greater adoption of air-to-air heat exchangers enabling increased outside air with improved energy efficiency. We note that the use of air-to-air heat exchangers are already required by the NCC as of 2019 in many applications.
Appreciating that this is a contentious issue in terms of balancing building occupant health and well-being with energy consumption and global warming, it is difficult to provide a simple answer.
As noted above, the central plant is unlikely to be able to maintain comfort conditions once the ambient conditions approach the design conditions for the building.
Our view is that a practical approach to reducing the risk of COVID-19 transmission within the occupied space should focus on assuring minimum levels of outside air ventilation and that the economy cycle is working correctly. Further reductions in transmission risk may be achieved through a review of the filtration standards and reduction of filter bypass.
Filter technology has improved over time and there may be options to improve the filter selection to increase its efficiency. Newer filters with more pockets and more surface area woven into the pockets can reduce filter pressure drops whilst also improving filter efficiency.
By increasing the face area of the filters, it is possible to reduce the filter pressures losses. Dropping the filter face velocity from 2.5m/s to 1.8 m/s should deliver filter pressure loss reductions of almost 50% and lower the associated fan energy consumption by almost 70%
Increased surface area can be achieved with larger housings to accommodate more filter cells, in some instances the existing filter frame can be modified to accommodate more filters by simply removing blanked filter frames. Another option is arranging the filters in a Vee pattern that enables more filters to be installed in a given cross section, however this requires more length.
Should an air handling unit be approaching a mid-life renewal, this is the best opportunity to undertake a comprehensive review of the air handling unit as part of the system to improve both its energy efficiency and air filtration performance.
In some instances, this is an option, though it is generally more likely to be achievable in older style built up air handling units rather than newer modular type units.
Testing aerosol transmission of virus and microbiological contaminants in-situ is challenging and the results achieved are dependent on a wide range of factors. Assessment of the transmission risk must also consider the complexity of types or airborne particles, their uniqueness (for ease of identification in the environment where they may already exist), their decay via settling, evaporation, inactivation as well as the difficultly in measuring or sampling techniques, has not allowed for a method robust enough to provide definitive conclusions. There are simply too many variables!
Tracer gas techniques have been used for years and are sometimes suggested as a suitable means of testing virus transmission through the air handling system. It is relatively simple means of testing by injecting a uniquely identifiable gas into a space, and measuring in real time its concentration in the return air. This is effectively measuring the outside air change rate. The important point to note however is that this is a gas, and will therefore behave differently to particles which will vary in size and thus airborne ‘range’. It also doesn’t include the decay that will occur for virus particles over time.
It would be better to use a model for Airborne Transmission Risk of which there are number in development around the world responding to the COVID19 pandemic. The main influencing factor in these tools will be the outside air ventilation rate, which can be measured using the tracer gas technique, or even CO2 concentrations with less accuracy.
Copper based antibacterial coatings have been proven to be effective in many studies related to bacterial surface transmission in healthcare settings and many hospitals are starting to specify copper based finishes for touchpoints and equipment.
These studies indicate that the copper neutralizes bacteria and viruses within minutes.
Where practicable, risks associated with human touch points within the built environment should be reduced by embracing a multi-layered approach. This can include utilising technology and increasing cleaning hygiene standards. For example, Request-to-Exit buttons can in many cases be easily and cost effectively replaced with REX PIRs.
COVID-19 has brought new focus to the indoor environment – how do we transform and manage our existing buildings to provide safe and healthy indoor environments that minimise the risk of COVID-19 transmission while supporting occupant health and wellbeing?
What are the practical approaches we can use with existing systems, what are the worthwhile upgrade and technology options, how to achieve improved indoor conditions while minimising energy usage, how do we get more out of building technology systems to help us monitor and manage the indoor environment.
In the second of our Virtual Seminar Series we bring together three experts to address this issue and answer your questions:
- Operating, Maintaining and Improving HVAC systems for COVID
Matt Peacock, A.G. Coombs Advisory
- Optimising the Cleanliness of the Indoor Environment
Joe Scholz, QED
- Occupant Access and Management for a COVID-Safe Facility
Darian Mozley A.G. Coombs Advisory