Using ventilation to mitigate COVID19 

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Published: 04 May 2021


Crossflow heat exchanger – energy exchange without mixing air streams Crossflow heat exchanger – energy exchange without mixing air streams

 

Increasing building ventilation rates appears to be a logical step to improving indoor air quality and a person’s wellbeing. Hern Yau, ventilation product manager at Mitsubishi Electric, highlights the required considerations when maintaining an effective, efficient ventilation process.

The news is full of fresh air talk at the moment, which makes a nice change from it being full of hot air! Joking aside, there’s a serious reason why fresh air and ventilation is so prevalent in the headlines as we need to find ways to mitigate the effects of the COVID19 pandemic. 

As we all look hopefully towards a re-opening of society, it is worth exploring why ventilation is such an important issue and explaining how a simple tweak of existing equipment can immediately make a difference.

Everyone is aware now of how fresh air can help mitigate the risks of the pandemic, which is why one of the first things to ‘open up’ is the idea of meeting out in the open air. 

Getting fresh air into our interior spaces is a different issue though and although we’ve had some lovely spring days, it is simply too cold at the moment to think about just opening windows. 

At the same time, many buildings already have some form of ventilation systems, so can these can be adapted to increase ventilation? 

To use demand control or not? 

There’s a lot of talk about whether or not to use CO2 sensors during the pandemic as a way of tracking occupants inside building areas and adjusting the ventilation rate to suit. 

When you first think of it, it sounds like a great idea. There are no sensors available to detect COVID in the air so the closest thing we have is measuring air that comes out of occupants and assuming all breath can be contaminated. 

To do this it is advised that you alter the ventilation systems to adjust the target CO2 ppm level to outside levels of 400-500 parts per million (ppm). 

After all, if we are keeping the levels indoors the same as outdoors, then aren’t we creating an indoor environment with all the benefits of reducing risk as being outside? 

Potentially not.

Let’s take a 10m x 10m and 2.5m high room designed pre-pandemic to hold 30 people. A ventilation system capable of 300 l/s would have been installed to meet current Building Regulations Part F 2013.
If someone is COVID positive in the room alongside just one other person, in order for the ventilation system to meet the demand of removing the CO2 it only needs to operate at a low rate compared to the size of the system, such as 20 l/s. 

If we equate that to the volume of the room it would take a whopping 3.5 hours to fully change the air. In a well-mixed space it can take five complete air changes of a room to reduce concentrations by 99%. This means that the other occupants of the room can be potentially exposed to infectious particles for an extended period of time, as could new occupants entering into the room.

However, what would happen if we didn’t use CO2 levels as a measure and ran the system at its full potential of 300 l/s? 

It would take just 15 minutes to change all of the air which would drastically reduce the potential of virus transmission. 

By using CO2 as the measure then we won’t necessarily see this level of ventilation unless the room was completely full – which it can’t be in these socially distanced times.

CO2 sensors definitely have their place for saving energy and controlling ventilation in a non-pandemic world. 

However, in the current environment it makes more sense to utilise the full ventilation capacity of a space if it is available. 

And the best way of doing this of course, is with MVHR (Mechanical Ventilation with Heat Recovery), in the most energy efficient way possible, by recovering up to 90% of the heat energy that would otherwise be lost.

These ventilation systems will also have been designed and installed to deliver occupancy comfort as a first thought, and that includes noise levels. This is simply because we do not want the fan speed noise to affect the occupants. We don’t want the ventilation to disturb anyone.  

With the arrival of COVID19, and the need for increased ventilation though, we can change the focus from comfort and prioritise safety first.   

Whilst we don’t want any noise from the ventilation to disturb occupants, if we change the designed fan speed from low to high, then you can often double the ventilation and double the amount of fresh air being supplied into that space even further.

Noise increase

But what would the increase in noise actually mean for the occupants? In addition to fan speed, noise from ventilation systems depends on a number of other factors, such as ducting and distance and this will affect the actual flow of air into the space.

Regarding noise from increasing the fan speed, the best modern ventilation systems generally include more efficient fan motors which mean higher performance with lower power consumption, and lower noise. Modern ventilation units (such as our own Lossnay range) can also benefit from a wider air volume range, generally with variable fan speeds of the rated air volume of each unit.

Units can be quickly and easily switched from full fan speed (at times when spaces are unoccupied) to 75% or 50%, if noise is likely to be more of an issue for occupants.

Noise wise, these modern systems offer fan speed noise levels from 17dBA (for the lowest fan speed in the smallest unit) to 40dBA (for the largest model on high fan speed).

A study room or library is considered to be at a level of about 20dBA, whilst a whispered conversation is recorded at anywhere between 25 and 35dBA. 

Overall a suburban area at night registers at 40dBA, so even with an increased fan speed, the units can still provide the quietness of operation that will deliver significantly increased air flow into the space, without disturbing the occupants. 

At the moment, the current consultation on Part-F is calling for changes in the way buildings are designed so that we can increase ventilation by an extra 50%.

Hopefully this article helps to highlight that for many buildings, the systems are already in place, and with a tweak of the control or the fan speed, they can increase the amount of fresh air, without compromising on comfort. 

And if your building or budget doesn’t allow for a complete upgrade in your ventilation system, then there are individual wall-mounted MVHR systems that can be easily retro-fitted to immediately increase air flow. These require an outside wall and electrical connection to offer a quick and cost-effective remedy. 

How does MVHR work?

Mechanical ventilation with heat recovery allows the stale air in a room to be removed but captures heat energy from the outgoing air to reduce the amount of energy needed to heat up the incoming air. The best systems can recover between 80-90 per cent of the heat energy and they do this without the two air flows (exhaust and supply) touching each other.

In the case of the Lossnay system, a specially designed paper core ensures the maximum crossover of heat energy from one air flow to the other. The use of paper also allows both latent heat and sensible heat, meaning the moisture levels in the room are maintained and the air doesn’t become too dry and static.
In the case of Lossnay’s paper core, this has also been tested using the E-Coli bacteria, which is significantly smaller that the SARS (COVID-2) virus.

Not only can MVHR therefore increase the amount of fresh air inside our buildings, it can also safeguard the occupants by filtering out harmful or toxic things in the air.

In conjunction with BESA, Mitsubishi Electric has produced a free Guide to Indoor Air Quality which can be downloaded here.