COVID-19 | HOARE LEA HQ OFFICE the zonal VAV boxes, speed control of the fans had to be implemented to allow the coils to temper the air to the desired temperature, recognising that the desired supply duct pressure may drop at very high or very low ambient air conditions. During unoccupied periods, the controls interventions permitted a greater period of time when neither the heating nor cooling coil were energised, while also lowering the fan speed to lessen the amount of outside air being provided. The observed flowrates at night are above the normal minimum outside air supply design flowrate and around 20 times that recommended within the REHVA guidance of 0.15L.s-1m-2 during unoccupied periods. Stable fan speed operations limit the minimum supplied volume. As the AHUs operate under a variable air volume control, the units have outside air intake velocity measurements to ensure the desired minimum design outside air flowrate is maintained. This is reassuring to have, as it provides a means to monitor flowrates. Hoare Leas HQ employs sensors, including for air quality All the above is logical in principle, but how do you prove, in practice, that the office space is ventilated to a high level to reassure employees that the environment they occupy for long periods of time is of good quality? The answer lay with the installation of air-quality monitoring within the space. CO2 monitoring is known to be a very good indicator of how well indoor air is diluted with outside air. Since 2016, we have used our office as a Living Lab to demonstrate and test bed new technology, to continually evaluate the post-occupancy performance of our spaces. As part of this, we have multiple sensors around the office, including for air quality. The air-quality sensors data are collected constantly and reported via our in-house app, which displays trend logs of the conditions. At the time of commissioning the new AHU control interventions, the app showed a climb in CO2 when the supply air was interrupted to the space. The sensitivity and response to ventilation being cut is very quick and is a cost-effective way of reassuring employees of the ventilation rates within the space. Reopening offices to employees following government guidance is not as easy as one would think. The airborne transmission route of Covid-19 is by far the greatest challenge to understand and to minimise, especially with the ever growing prevalence of those who are asymptomatic. As a result, the ventilation of indoor spaces has never been so important to understand, control and monitor. CJ STEVE WISBY is a partner at Hoare Lea Control modifications when office is occupied When outside air and mixed air temperature is within + or - 2C of supply air set point, fan speed should be normal pressure control. When more than + or - 2C of supply set point, limit fan speed output to 80% max, but step down 5% if either heating or cooling control value percentage is greater than 90% for five minutes. Likewise, if lower than 75% for five minutes, step up fan speed by 5%, but always limit it to 80% of maximum. OCCUPIED Control modifications when office is unoccupied Space temperature set points to be overridden to 26C cooling set point and 20C heating set point. When outside air and mixed air temperature is + or - 1.5C of supply set point, normal pressure control, but close cooling/heat valve. When greater than + or - 1.5C of supply set point, limit fan speed to 50% max, but step down 5% if either heating or cooling valve percentage is greater than 90% for five minutes. Likewise, if lower than 75% for five minutes, step up fan speed by 5%, but always limit it to 80%. UNOCCUPIED 36 October 2020 www.cibsejournal.com CIBSE Oct20 pp34-36 Hoare Lea Covid-secure.indd 36 25/09/2020 16:37