As a first principle, the dilution of internal air streams should reduce the risk of transmission through a reduction of building occupant exposure time to any airborne virus aerosols, whilst also reducing the potential for the virus to settle on any internal surfaces.

As such, the over-riding advice is to reduce internal air recirculation and supply as much outside air to the building as reasonably possible. The key here is the delivery of optimal levels of fresh air provision per person, and maintenance of social distancing between staff that will return to the office to a minimum of 2-3m between individuals.

Any ventilation or air conditioning systems that are normally run with a recirculation mode should be adjusted to run on full outside air where possible. This includes systems which are fitted with return air filters, as it is understood that these will not filter out any viruses themselves.

Any recirculation of air between spaces, rooms or zones occupied by different people should be completely avoided – the primary objective is to maximise the air exchange rate with outside air and to minimise the risk of any pockets of stagnant air.

With regard to HVAC control strategies, it is recommended that extended operation times are implemented, and that ventilation rates should only be reduced down when the building is fully unoccupied. Ventilation systems should not be fully switched off when unoccupied, but ventilation rates can be reduced during full vacancy.

Similarly, CO2 set-points for demand control systems should be set to 400ppm, to optimise the delivery of outside air to internal spaces.

Exhaust ventilation systems of toilets should be left on 24/7.

There will, of course, be implications for energy efficiency through the adoption of these measures – however, it is clear that the health & safety of building occupants is vital, and hopefully adopting this approach in the current season will only result in a relatively limited uplift in consumption.

Whilst it is noted that increased duct cleaning will have no practical effect, as virus particles will not deposit easily in ventilation ducts with appropriate air flow, and it is not necessary to implement an increased regime of changing outdoor air filters, as clogged air filters are not a contamination source, there is potential for virus particles in extract air to re-enter the building – for example in heat-recovery devices that may carry over the virus attached to particles from the exhaust airside to the supply airside via leaks.

In rotary heat exchangers (including enthalpy wheels) particles deposit on the return airside of the heat exchanger surface, after which they might be resuspended when the heat exchanger turns to the supply airside. Based on current evidence, the recommendation is to turn off rotary heat exchangers temporarily.

However, please note that virus transmission via heat-recovery devices is not an issue when a HVAC system is equipped with a twin-coil (‘run around’ coil) or other heat-recovery device that guarantees air separation between return and supply side.