In response to this question, the leading advice suggests that direct airborne transmission is the predominate method of transfer (cough and sneeze particles), touching and picking up formites which are then transferred to the body via mouth or eyes is one method. It is also important to remember that virus’ do have a limited lifespan outside a host.

There are a number of variables to be considered such as the location of the supplementary unit, its application (i.e. meeting room, office, etc), and the history (i.e. have filters been changed recently, has there been a recent active case in the room).

The first step to consider is to undertake an inspection of the unit to determine filter condition, combined with other observations and information to make a determination. In many instances replacing the filters would be appropriate, as new filters provide a range of system benefits including a level of occupant comfort around heightened maintenance practices and less likelihood of low airflow generating temperature complaints and alike.

There are probably two key parts to this question – the first is around the effectiveness of killing the virus which relies on the technical information provided by the product provider, which generally comes with laboratory test certificates. The second part, which is more pertinent to the question and which was commented on during our webinar, is that is the virus likely to be present on the coil surfaces.

There are a lot of surface areas in an AHU system, of which the coil only represents a small part, therefore it is not practical to clean a system that was not specifically designed to be cleaned. For this reason, we would suggest that cleaning focus on the interior room surfaces and that disinfecting of any part of the AHU system only be considered by exception where there maybe additional concerns or circumstances raised that require additional measures to be taken.

We do take this opportunity to note that there is no documented evidence, that we are aware, of viable viral particles in AHU’s. However, if there were concerns about viral particles in AHU’s, it may make sense to disinfect the coils because of airflows. However, air velocities through coil pathways probably means that any entrainment is extremely unlikely and if viable particles did impact, they are likely to be unviable by the time you came to disinfect the coils.

At the end of the day, if you or the occupants would feel more comfortable, then there is no harm to disinfect, using appropriate chemicals, methods and PPE.

HEPA filters which are typically used in Clean room type environments such as Hospital operating theatres and pharmaceutical manufacturer do stop the transmission of a virus. The issue is that HEPA filters require very different system design due to the increase in system pressure which requires higher capacity fans. Consideration also needs to be given to how they are typically mounted (mostly used as terminal fittings, i.e. at the ceiling), and the high maintenance requirements and replacement costs.

Most existing HVAC systems have been designed for panel or bag type air filtration which would not be suitable to retrofit to HEPA type filtration. There has been no suggestion, at this stage that offices should upgrade to HEPA filters. Take the opportunity to review the media currently being used (G1 – F9 scale) and look to upgrade where practicable. Also, alternative technologies such as electrostatic and UV are worth monitoring as designs continue to evolve whereby, they may be considered.

Please also refer to the A.G. Coombs Advisory Note – HVAC and Covid-19.

The main focus will be to ensure that if a system has been shut down, that there is a full plan for start-up. Given that most system have remained in operation, the focus will be to reactivate any supplementary systems that were isolated or have simply been inactive for a long period due to reduced occupancy. Reactivating these systems which will have been in effect system dead legs may introduce several issues, particularly on water quality – bacterial growth, and corrosion.

It will be important to be vigilant on water sampling and testing, including corrosion monitoring.

Please also refer to the A.G. Coombs Advisory Note – Cooling Towers Systems During Covid-19 which provided further details and advice in relation to this question.

Many things will change as offices return into service, the new ‘normal’. Some offices will be operating in a low load scenario, however the operating hours maybe extended to accommodate a staged return and to enable better social distancing. This may mean starting plant earlier than normal, and have it operating for longer.

The opportunity to maximise any free cooling from using more outside air should be a focus of course, whilst focussing on indoor humidity levels. Given much of the load will be in fans and general plant, followed by chillers/boilers we need to carefully look at things such as VSD’s which may have been fixed to operate at 100%, but could be now scaled back.

Outside air rates are typically designed for total occupancy so you can either take advantage of increased OA per person or possibly scale back. Replacement of filters will typically yield savings as new media will result in fans not needing to work as hard to generate the required airflow. Looking at how boilers and chillers and associated pumps have been configured to operate as a system will also be important, as we typically focus on creating a low load scenario, but not necessary using it as a semi-permanent solution.

Low load chillers/boilers may need additional focus to ensure they operate at maximum efficiency. There are many things which could be considered; however, it is a matter of trying to list them all and then prioritise on the ones which will yield the best gains based on building specifics.

Please also refer to A.G. Coombs Advisory Note – Returning to Operation from COVID-19.