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Now let’s bring our buildings out of lockdown

As the government has relaxed lockdown restrictions, facilities managers should take the opportunity to review how their ventilation and air conditioning systems are operating in preparation for the re-opening of buildings, according to indoor environment company Swegon.

With occupancy levels only likely to increase gradually, this is a good time to carry out thorough system checks, the company believes.

“This is an ideal time to carry out surveys of building services equipment as any changes can be made while demand is low as buildings are either empty or partially occupied – so reducing the potential for disruption,” the company said.

Swegon also highlighted an apparently over-cautious approach to rotary heat exchangers that had led to many being disabled. This is despite clear guidance from leading technical bodies that these systems posed only a minimal risk to building occupants and provided considerable energy savings.

“The operating benefits of properly installed and maintained rotary heat exchangers (thermal wheels) should far outweigh any risks,” the company said. “We need to bring these highly valuable systems out of lockdown too.”

A number of rotary heat exchangers were switched off or bypassed as a precaution during the COVID-19 lockdown because of fears about the potential of cross-contamination of supply and return airflows with virus laden particles.

‘Extremely low risk’
However, subsequent guidance from REHVA – the Federation of European Heating, Ventilation and Air Conditioning Associations – and the UK’s Building Engineering 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 bring outside air 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 does reduce the risk of cross-contamination of the airflows, but makes it difficult to achieve target levels of comfort cooling as we enter the summer months – or to meet heating requirements later in the year. Also, users will miss out on considerable energy efficiency benefits 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 i.e. below 2%.

“Under certain conditions virus particles in extract air can re-enter the building…[but] there is no evidence that virus-bearing particles starting from 0.1 micron would be an object of carry over leakage,” states REHVA’s updated guidance.

“Properly constructed, installed and maintained rotary heat exchangers have almost zero transfer of particle bound pollutants including air-borne bacteria, viruses and fungi. It is known that the carry-over leakage is highest at low airflow, thus higher ventilation rates are recommended.”

BESA also stated that: “Virus particle transmission via heat recovery devices is not an issue when a HVAC system is equipped with a twin coil unit or another heat recovery device that guarantees 100% air separation between return and supply sides.”

However, Swegon said that it would be good practice to inspect all heat recovery equipment to ensure any possible leakages from the exhaust air side to the supply side are addressed. Adjusting the pressures can also minimise the potential for cross-contamination, including the use of bypass valves and/or dampers to avoid higher pressure building up on the extract side.

It added that facilities managers should check for common design and installation faults such as fans being mounted wrongly so that higher pressure is created on the exhaust air side. The company is offering technical guidance free of charge backed up by full training to help with this.

“In short, the same concept of good, basic engineering practice applies as in any health and wellbeing context,” said Swegon. “If ventilation equipment is of sufficient quality and has been properly designed, installed and maintained, then the risk of Covid-19 transmission is extremely low.

“What this period has taught us all, however, is to be vigilant and to further focus on the importance of monitoring and maintaining our equipment. In other words, doing what we already should have been doing to keep building occupants safe and healthy.”

 

27 July 2020

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