The Covid-19 pandemic drew everyone’s attention to the importance of good indoor air quality (IAQ), but there is a danger that these advances in knowledge and action will be lost in the face of the energy and climate crisis. It is possible to deliver good IAQ without creating significant CO2 emissions, but it requires a data-led approach that has not been commercially viable until now.

In the height of the pandemic, the World Health Organization (WHO) released updated guidance on the acceptable levels of air pollution required to protect people’s health. Approved document F – Ventilation (Part F), now requires buildings ventilation systems to limit the ingress of pollutants listed in the WHO guidance into buildings.

A typical ventilation system will be operated via a timeclock, with 100% of ventilation capacity being supplied during all core occupied hours. During the pandemic, building operators were advised to adjust their ventilation systems to run for two hours before occupation, and for two hours after in order to flush out potentially contaminated air. This is an inefficient way to run a ventilation system.

Demand Controlled Ventilation (DCV) is an effective way to reduce the costs associated with running an HVAC system in buildings with varying levels of occupancy. The extent to which the HVAC system runs is typically related to the Carbon Dioxide (CO2) concentration monitored within the building. The CO2 concentration is closely correlated with the number of people in the space. The drawback of these types of systems is that people using the building outside of core hours, for example cleaners and security staff, can be put at risk from pollutants building up inside an unventilated building. This issue can be partially addressed by ensuring that the building has a minimum ventilation rate outside of core hours.

Air Quality Controlled Ventilation (AQCV) is a development conceived decades ago that allows a buildings ventilation rate to vary according to multiple indoor air quality parameters. If IAQ levels are poor, the ventilation system will deliver a higher volume of outdoor air through the Air Handling Unit (AHU) to dilute the pollutants, this way of operating a ventilation has only recently become feasible due to the rise in availability of low cost IAQ sensors. This success of these systems is dependent on the filtered supply air containing lower levels of pollutants than the air already inside the building.

Air Quality Determined Ventilation (AQDV) is a recent development that allows a buildings ventilation system to modulate according to both the indoor and the outdoor air quality (OAQ). This innovation is only now possible due to the availability of both low cost IAQ sensors, accurate granular OAQ data and concessions made in Part F. OAQ varies day by day, and hour by hour, it cannot be assumed to be “fresh air” all the time. Section 2.6 of ADF states  “where sources of pollution vary with the time of day, such as urban road traffic, it may be acceptable, for time-limited periods, to take one of the following actions.

1. Reduce the flow of external air into ventilation intakes
2. Close ventilation intakes when the concentrations of external pollutants are highest”

If levels of OAQ are poor, and the IAQ is good, the ventilation system can switch to re-cycle a larger percentage of the air through the Air Handling Unit (AHU). Re-cycling the air for long periods of time with little dilution with outside air would typically result in unsatisfactory levels of odours and gaseous compounds. To address this, air cleaning technologies need to be deployed within the AHU to remove them including combined ePM10 and molecular filters. This “clean first” principle has been demonstrated in buildings built to ASHRAE 62.1 standards for decades.

IAQ sensors form an integral part of this solution. For large open plan spaces, sensors in the supply and return ducts are sufficient to measure the air quality. In buildings with varying loads throughout the space, a higher density of sensors need to be used in conjunction with volume control dampers. These will need to be retro-fitted in buildings that were previously operated on a timeclock.

Efficient operation of AHUs is seldom high up on the list when an organisation comes up with ideas to reduce its CO2 emissions, but it should be, there are significant savings to be made, particularly if you consider whole life carbon emissions.