Extract plenums have been an integral part of office ventilation solutions for a long time. Air is drawn through ventilation openings in ceiling and light fittings, into a void above the ceiling tiles that is kept under negative pressure using open ducts to draw air from the void and into a riser. Eventually, it is discharged via an extract fan. A similar principle can be used to draw air at the highest level, where there are exposed services. In both instances, the amount of air extracted is a close match to the amount of fresh air provided to the space.
The concept became common in the late-1980s, when it was used with high-pressure variable volume systems. Compared with a ducted extract system, the savings were substantial; the costs of large amounts of sheet-metal extract ductwork was saved, and the space saved by the reduced coordination allowed ceilings to be higher and the height of a building storey to be reduced. The concept was simple: an even distribution of supply air gradually eased the return air into the ceiling to balance the pressure.
Mechanical systems continued to develop in the 90s, with fan coil units (FCUs) and minimum fresh air supplies becoming the norm, bringing further savings in costs and space. Fan coils may be served by ventilation ducts to provide fresh air from an air handling unit and, usually, draw additional air from the room or surrounding plenum.
There are other options, such as chilled beams and ceilings, but the reliable familiarity of FCUs makes them the default solution for cooling and fresh air. These more modern systems are routinely used with an extract plenum.
Then came the pandemic. Our industry was quick to respond with guidance that recirculating air should be avoided and equipment such as thermal wheels – which provide a potential contamination route between extract and supply systems – switched off or isolated.
If there is a good outside air ventilation supply (mechanical or natural) to the room or zone, the FCU fan will help destratify the air and reduce the chance of stagnant air pockets, helping to dilute any airborne pathogens. Other advice stated that it is acceptable for a FCU to recirculate air within a single room.
In the short term, we should consider the benefits of using a ducted extract in conjunction with FCUs
If we consider a FCU located in a return plenum, it seems there are risks that are not considered by the advice to date. Typically, FCUs will recirculate a proportion of air from the ceiling void, and this air could be contaminated with virus particles extracted from elsewhere in the office. If this were the case, the air supplied to the office from the fan coil would further spread the virus particles, with the cycle starting again to spread the contaminants further. In the current pandemic, I believe this is a risk that should be considered more carefully.
Things can be done to reduce the risk of such spreading of contaminants. FCUs do not have sufficient fan power to operate with a filter fine enough to remove Covid-19 viruses from the air. Air cleaning technologies are available, and ionisers and ozone cleaners can be fitted into the air stream of FCUs. However, while common overseas – particularly in the US – these solutions are not currently recommended in the UK because of concerns about the risks of secondary chemicals generated by the devices, and of chemical oxidation, photocatalytic oxidation or electrostatic precipitation.
In the short term, we should consider the benefits of using a ducted extract in conjunction with FCUs. This would keep extracted air separated from the supply on its way back to the riser and reduce the risk of wider contamination. In the longer term, consideration of infectious disease transmission needs to be embedded into building ventilation regulations and associated statutory guidance, in the same way that energy, comfort and air quality have been. Part F: Ventilation is under review, so there is an opportunity to consider this immediately.
Many knowledge gaps – including lack of data on the viral load in the exhaled breath of asymptomatic and symptomatic people, the size distribution of particles and how this varies with activity, and lack of data on the dose response – mean uncertainty remains over how to evaluate aerosol transmission effectively and determine the most appropriate interventions.
Building regulations should identify performance standards, and enhanced measures should be taken to ensure compliance is achieved in use. Further regulation and guidance may be required to ensure existing buildings can meet necessary standards. There are no ‘silver bullet’ solutions, however, and a combination of measures is required to reduce the risks of infections indoors.