255 High Street benefits from natural ventilation. This is increasingly popular and below are some questions and answers to explain the system.

ANSWERS FROM DARREN KEEP Director, MZA Consulting Engineers

Q: Why use natural or mechanical fresh air ventilation?

DK: Natural fresh air ventilation is normally used where the external air quality permits whereas mechanical fresh air ventilation systems can be employed in locations where there is a need to filter the outside air. Considering the type of filters commonly used on mechanical ventilation systems, typical filters (G4) achieve 50% to 65% for large particulates and aren’t designed to filter the finer (ePM10 and ePM2.5) particles or gases. Higher filter grades F7 have a minimum efficiency of 50% in removing the finer particulates (ePM2.5 and ePM10). In summary modern mechanical ventilations systems can remove some of the smaller particulates however a natural solution is suitable where the outside air is of a suitable quality. Air quality in Guildford is good and meets the National Air Quality Standard for NO2.

Q: Can fresh air mechanical ventilation system, provided with increased levels of filtration, help to manage the risk of COVID19, preferable over natural ventilation?

DK: The use of increased filtration on the fresh air system serves no useful purpose as the ventilation fresh air is drawn air from outside. Natural ventilation also draws air from outside.

Q: Does mechanical fresh air ventilation system provide better comfort than natural ventilation?

DK: Mechanical ventilation systems provide a fixed quantum of air into the space whereas natural ventilation allows the occupants to alter the ventilation rate to suit their needs. The heating and cooling to the space is provided by means of separate air source heat pump system to maintain the comfort conditions. Providing occupants with control over their environment is known to help increase levels and the general perception of comfort and productivity.

Q: Does mechanical or natural fresh air ventilation help in managing the transmission of COVID 19?

DK: The prevailing recommendations from BCO, WHO, CIBSE and ASRAE indicate that increased fresh air ventilation rates are desirable to help, alongside a number of management measures, in managing the transmission of COVID 19. With fixed air duct and plant sizes it is generally accepted that it’s not feasible to significantly increase the ventilation rates provides by mechanical ventilation systems. Natural ventilation allows the occupant to increase the ventilation rates by opening windows or ventilation panels.

Q: What are the energy credentials of 255 High Street?

DK: The building achieved an EPC A rating, this is the highest possible rating. This was achieved as a result of the ability to use the space in a mixed mode strategy, i.e. during times of the year opening windows rather than using the heating and cooling system.

Q: Is there a difference between the heating and cooling systems in a mechanical and naturally ventilation building?

DK: Commonly both mechanical and naturally ventilated office buildings are provided with heating and cooling by units, known as fan coil units (FCU’s). This is a common solution used throughout office buildings across the world. The FCU’s take air from the room across a filter which is then heated or cooled before its supplied back into the room. The important factor in office ventilation, heating and cooling systems is the recirculation of air. Currently differing forms of filtration are being reviewed however