AIR-HANDLING SYSTEMS
Accurate design and sizing crucial for ‘healthy’AHUs
The way that air-handling units (AHUs) are designed and deployed will play a major part in the UK’s concerted drive to improve the health and wellbeing of building occupants, argues James Henley, Product Development manager at Daikin Applied UK.
The role of ventilation systems in minimising health problems in buildings has been under the microscope this year. Leading medical experts have long recognised the importance of good ventilation in helping to safeguard the health and wellbeing of building occupants – and especially the most vulnerable – but this topic has assumed even greater significance during the COVID-19 crisis. The UK’s Deputy Chief Medical Officer, Professor Jonathan Van- Tam, was particularly clear about this during a press briefing in April, when he said: “There is a definite truism across all of the science literature, that ventilation is a most critical part of reducing transmission from respiratory viruses.” By the same token, there was also a lot of early confusion about the potential for ventilation systems to spread the virus, which could have been very damaging if facilities managers in critical buildings had decided that the safest course of action was to switch systems off.
However, the technical guidance issued by the various UK and European industry bodies took a consistent line, and demonstrated that these systems were not the problem and, in fact, should be seen as a key part of the solution. The HVAC sector as a whole continued to perform a crucial role in keeping essential services like hospitals, supermarkets, care homes, and schools, operating throughout the crisis. Ventilation systems are, of course, going to be even more important as the lockdown restrictions are lifted, and buildings start to re-open.
Transmission via respiratory droplets Public Health England (PHE) quickly established that transmission of the virus was primarily through respiratory droplets generated by coughing and sneezing, and through contact with contaminated surfaces. It did not, though, recommend any special cleaning measures for ventilation and cooling systems. However, all responsible industry bodies would stress that ongoing cleaning and maintenance is always important – and even more so now.
The industry also recommended that, in order to minimise the chance of the virus remaining airborne in an occupied space, extract systems should be used in conjunction with the full fresh air if possible – to ensure a high rate of air changes. Building owners were also advised to switch air-handling units (AHUs) to full fresh air mode, and to temporarily disable any heat recovery recirculation, so that potentially contaminated air was not returned to the occupied space.
Keeping up the required number of air changes
Keeping up the necessary number of air changes – even in partially occupied buildings – will minimise the risk of moisture, which could contain the virus, settling and remaining on internal surfaces. In short, the advice was to increase ventilation as much as possible,
accelerating the flow of outside air, and preventing the creation of any pockets of stagnant air. Recirculation of air within buildings should be avoided to reduce the risk of transmission wherever possible. This does have a short-term implication for energy costs, but most building managers now recognise that is a price worth paying for now to keep people safe. Also, if a building has been ‘locked down’, and before it is reoccupied, there needs to be a thorough re-assessment of air-conditioning and ventilation systems, according to the Chartered Institution of Building Services Engineers (CIBSE). The Institution’s guidance says: “Now is the time to check ventilation in buildings to ensure that there is enough airflow to dilute the virus in the air and improve indoor air quality. Ventilating for longer, and opting for ventilation with higher airflow volumes, will help to reduce the risk.”
A Daikin ‘high-end’ AHU Modular R (ventilation solution with heat recovery wheel) installation with Siemens control.
October 2020 Health Estate Journal 57
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