THERMAL WHEELS
Myth busting: thermal wheels and virus transmission
As many countries across Europe are experiencing their second or even third lockdown, facilities managers may wish to take the opportunity to review how their ventilation and air conditioning systems are operating in preparation for the re-opening of buildings. Josh Emerson, marketing manager at Swegon explains.
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t is now widely understood that maximising outdoor air, increasing air change rates and therefore effectively diluting the air inside a premises is the most effective way to minimise the risk of building occupation. However, one of the most asked questions we receive about building re- occupation is on the topic of rotary heat exchangers. The operating benefits of properly installed and
maintained rotary heat exchangers (thermal wheels) should far outweigh any risks, particularly when air change rates can be so much higher when compared with many other ventilation strategies such as opening windows and 'split' systems. Until recently, some guidance suggested that rotary heat exchangers were switched off or bypassed as a precaution during the Covid-19 pandemic because of fears about the potential of cross-contamination of supply and return airflows with virus laden droplets. In practice, this is was not a realistic recommendation for physical engineering reasons, and would have no tangible impact on the amount of 'return air' being brought back into a room.
The subsequent guidance from REHVA – the Federation of European Heating, Ventilation and Air Conditioning Associations, Chartered Institute of Building Services Engineers (CIBSE) and the UK’s Building Engineering
16 March 2021
Services Association (BESA) have clarified that rotary heat exchangers pose an extremely low risk of transferring viruses (including Covid-19) if they are 'properly constructed, installed and maintained'. REHVA said rotors should not be switched off because the leakage rate is not influenced by whether they are rotating or not. In fact, normal operation of rotary systems was a good way to allow outside air to be brought into the building to maintain air change rates, which will reduce the concentration and potential transmission of virus laden water droplets, according to the guidance. Bypassing makes it difficult to achieve levels of comfort cooling for occupants during summer months – or to meet heating requirements in the winter months. There are also effects on the planet - thanks to the increased energy consumption - and the building owner, who will experience increased energy bills having missed out on the considerable energy efficiency benefits of this heating and cooling recovery, unless the systems are returned to normal operation.
REHVA said properly operating and maintained thermal wheels would have similar leakage rates to plate heat exchangers which are below 2%. 'Under certain conditions virus particles in extract air can re-enter the building…[but] there is no evidence
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