The energy and emissions impact of mechanically ventilated buildings

Where buildings are mechanically ventilated, the rate of air change that is employed in each room or space to provide ‘comfortable healthy air’ is often quoted in Air Changes per Hour (ACH).

Rule of thumb ACH guidance is typically provided by reference to a ‘standard’, most commonly Standard 62.1, published by ASHRAE (the American Society of Heating, Refrigerating and Air-Conditioning Engineers), whose membership spans 130 countries worldwide.

The ACH can vary widely depending upon what is going on inside the building: for example – it is generally considered that 4 ACH’s is the minimum, whereas for a classroom the rate is from 6 - 20 and a computer room from 15 - 20.

While the primary factor used to determine the ACH is the occupational density, this is often supplemented by other aspects of the internal environment, such as a the likelihood of, or sensitivity to, known contaminants , including bio contamination (including from crowding) and odours.

The target air change rate is important because if your air change rate is higher than necessary, you’re wasting heating or cooling and air circulation energy and carbon emissions. If the target air change rate is too low your air will becoming stale and stagnant, which can also lead to a buildup of toxins, pathogens, and the like.

Sustainability - Airora vs ASHRAE 241

ASHRAE 241 and Airora represent two entirely different approaches to reducing the risk of high indoor cross infection, each of which has an entirely different profile in terms of effectiveness and capital and running costs.

ASHRAE 241 infection control

During the Covid-19 pandemic, as the primary source of indoor Covid-19 infection was an airborne virus and, as Airora was not yet available, HVAC engineers developed a new ventilation Standard, ASHRAE 241, to ‘establish minimum ventilation requirements for control of infectious aerosols to reduce risk of disease transmission’ through the dilution of airborne virus concentrations.

Note the wording, these are the ‘minimum requirements’ necessary to ‘reduce risk’ and there is no indication in the standard of what the target level of risk reduction is. However, ASHRAE recently published a cost benefit analysis of 241^[1]whichis based on an assumed 25% reduction in infections due to long range aerosol transmission.

Sustainability impacts of the 241 infection control standard

Compared to a traditional HVAC system, say any commissioned before 2022, enhancements needed to meet the additional requirements of 241 involve, either:

Importing far more air from outside (most existing systems take only a little air if any from outside due to expense of changing the temperature up / down of the outdoor air to the target indoor temperature), or
Creating a constant supply of ‘clean’ air by passing sufficient internal air through high quality MERV 14 filters.

And, in either case, the quantities of ‘clean’ air to be sourced will be very substantial, and the ventilation rates, depending on the space, will typically need to increase by a factor of three to six throughout the system. Indeed, the required air changes per hour typically need increasing in healthcare and schools to 10, waiting rooms to 30, restaurants to 40 and lecture halls to 50!

The scale of the ACH increases required by 241 to achieve its limited impact on infections is testament to just how inefficient a ‘dilution’ strategy is in terms of reducing airborne germs, allergens and pollutants.

The authors of 241 fully appreciated the impracticality / high costs of routinely implementing 241, including the cost of sophisticated filters such as MERV 14, which is why, even where implemented, 241 systems are only expected to be run in this enhanced mode during emergencies.

Sustainability impacts of 241 infection control

In terms of sustainability, 241 raises the obvious sustainability concerns that result from both the increased embodied carbon in the ventilation system and increased emissions arising from far more aggressive air change rates, especially as typically some 40% of all GHG emissions arise from buildings.

Further, as such systems are only designed to be run during ‘emergencies’, and their impacts are modest in comparison to Airora, the additional carbon emissions represented by the embodied carbon in the system may have been accrued to little or even no purpose.

Airora infection control

Airora provides an entirely different approach, through the indoor creation of a natural hydroxyl cascade, as occurs outdoors, which can all but eliminate long range aerosol infections by reducing airborne concentrations by 99.9999% in minutes and then maintaining that level of sanitisation 24/7 with people present.

Unlike 241, which is designed to only reduce the concentration of long range infection aerosols, Airora practically eliminates such aerosols, while also sanitising all surfaces and acting as a real time barrier to person to person infection through exhaled germs.

Clearly, Airora totally eclipses 241 in infection prevention terms, costs far less, and involves substantially lower embodied carbon and emissions in use.

Sustainability impacts of Airora infection control

In terms of sustainability, Airora devices have a far lower embodied carbon content than 241 systems, and require no associated ducting, as they simply diffuse the necessary ingredients into the air, without attempting to achieve air change rates as high as 50.

Indeed, Airora’s in-use energy requirements, and associated carbon emissions are but a fraction of those required by 241, with a typical fan assisted air change rate of 1 - 2, compared to the up to 50 associated with 241.

The future

Ventilation standards, for types of rooms, from domestic to office to restaurants, to healthcare, such as those set by ASHRAE, set air change rates (often quoted as Air Changes per Hour – ACH) based on factors such as:

The likely occupant density
location type, differing for structures such as homes, hotels, offices, stores, schools, sports facilities, or restaurants.
Thresholds for activity related special substances
Potential concentration of odors and irritants (including odours from people)
Control of moisture related issues
Dilution of infectious material

At the simplest level ventilation systems are designed to provide a fixed rate of ventilation based on how the space being ventilated is classified as above. In the ‘real world’ such approximations, based on a relatively crude rating system, can clearly lead to spaces being either under or over ventilated.

The sustainability impact of Airora

Airora can directly mitigate some of the factors accounted for by their currently being including in traditional fixed air change rate targets, so that the fixed air change rates can be reduced, using less energy and resulting in lower carbon emissions.

For example, employing higher than would otherwise be required air change rates to address such issues as removing odours, irritants, allergens, infectious agents and the like is far less efficient than Airora dealing with them directly.

It will take research, and field trials, to establish just how far air change rates can be reduced, and under what circumstances, but change is coming.

Demand Controlled Ventilation (DCV)

Under pressure to reduce carbon emissions and rising energy costs, and with the development of new technologies, there is already a movement toward controlling ventilation rates dynamically based on the current use and activity within a space, not at a fixed level unrelated to its current actual use.

An example of this is the movement toward reducing air changes rates, reducing carbon, and saving energy in research laboratories^[2].

DCV requires an active contaminant sensing system. The components include air sampling sensors, software to interpret the data and air handling system with modulating dampers that uses the data to regulate outside air supply and exhaust. The contaminants that might be actively monitored include:

CO, In loading docks, and spaces suffering from nearby vehicular traffic.
CO_2,Used to gauge occupancy level in assembly spaces such as auditorium, lecture halls and conference rooms.
VOCs, Which come from many commonly used products and indoor furnishings.
Formaldehyde
Nitrogen Dioxide

As Airora actively reduces the concentration of some of the above, where it is present it will automatically increase the impact of DCV systems and further decrease energy costs and carbon emissions

As, in the future, Airora may well already be present to address such issues as infection control in healthcare, we are actively investigating its potential role in monitoring contaminants in active support of energy reducing DVC systems.

References

Bruns R, Cost-Benefit Analysis of ASHRAE Standard 241, ASHRAE Journal, October 2023.
Chan J et al. Reducing Air Changes Rates, Reducing Carbon, And Saving Energy In Research Laboratories. https://www.wbdg.org/resources/reducing-air-changes-rates-reducing-carbon-and-saving-energy-research-laboratories

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