Most commercial buildings are mechanically ventilated by taking in fresh air, heating or cooling it as necessary, and then distributing the air back to the workspace via grills. However, in order to reduce energy costs, a proportion of the air, perhaps up to 60%, within the building is recirculated. In normal times this was considered best practice. We are now not in normal times and the potential of COVID-19 transmission should now be considered.
COVID-19 is a respiratory virus and is caught from breathing in aerosols expelled from infected people’s breath. In the open air the virus is diluted and dispersed, but inside buildings the aerosols can linger, spread and build up to dangerous levels, in a similar manner to cigarette smoke. Ventilation is therefore the key to prevention because the risk of breathing in COVID-19 is significantly reduced by fresh air.
Comparisons between countries are very difficult but the arguments in favour of ventilation as a critical weapon are compelling, and now confirmed by public health advice in other countries[1].
Building Ventilation Issues
A well-ventilated building is therefore the key to reducing the risk of breathing in the virus. But what does well-ventilation mean? The National Association of Air Duct Specialists recommends[2] that there should be no recirculation of air, the air intake into the building should be 100% fresh air. In addition, the building ventilation system should be operated at normal speed for at least two hours before and after business hours. In addition, where the building ventilation has been previously switched off overnight and at weekends, it should now run continuously during those periods but at a lower speed. The advice from NAAD is also not to change heating or cooling set points during the new extended running hours (the virus prefers the cold).
Previous best practice from SEAI was to vary the ventilation rate to occupancy levels by the use of CO2 sensors[1]. However, this will not achieve the desired ventilation rate to provide protection from COVID-19 transmission and conflicts with current advice from HVAC specialists.
Building energy managers may be dismayed by these recommendations. Thermal energy usage will go up in the winter with increased heating due to the continuous running of the ventilation system. With the continuous running of fans, electricity usage will also increase. The energy rating of buildings will be adversely affected and corporate energy targets will now not be meet. The building carbon footprint will increase as well.
Alternative Options
There are some options to reduce energy usage whilst still having 100% fresh air. The first option is to ensure that there is efficient heat recovery (that is heat is recovered from the extracted air to preheat the incoming fresh air). Many buildings would not have considered heat recovery in the past as energy was saved by minimising the amount of fresh air taken in. In short, the payback would previously have been too long. Heat recovery options should now be reassessed.
The second option to reducing energy costs, whilst still having 100% fresh air, is to review the way in which air flow rate is controlled in the AHUs (Air Handling Units). Many systems will control the air flow rate using a damper. Any sort of flow restriction is a grossly inefficient way to control fluid flow. Ideally, the motors for the fans should be retro-fitted with VSDs (Variable Speed Drives)[4]. These electronically vary the speed of the motor and thus will allow the air changes to be varied day and night as recommended by NAAD.
The author, Bob Sutcliffe, is a Certified Energy Manager and a director at Environmental Efficiency. Bob can be contacted at bobsut@enviro-consult.com
[1] Orla Hegarty, assistant professor, School of Architecture, UCD, Irish times 8 October 2020.
[2] https://www.envirohygiene.ie/naad-covid-19-guidance-for-ventilation-hygiene/
[3] SEAI Energy Audit Handbook, Section 5.4.2
[4] Carbon Trust CTV007v3 April 2018.