Mechanical ventilation's role in improving indoor air quality

[edit] Introduction

Ventilation has become a key concern in our buildings. Ventilation has progressed from the primary concerns around condensation and mould prevention; to reducing overheating in the more airtight new homes we began to build in response to the Code for Sustainable Homes; through to addressing the issue of harmful indoor air pollution. Most recently, ventilation is being talked about in the light of reducing virus transmission, elevating the subject to something that the everyday person now discusses and is aware of.

[edit] Ventilation and Coronavirus

Coronavirus, which causes the COVID-19 disease, is primarily spread in indoor environments by people breathing in infected droplets and smaller ‘aerosol’ particles in the air that have been exhaled from the nose and mouth of an infected person. Whilst masks help to limit the spread of these droplets, good ventilation is essential to disperse them.

A short film released by the Department of Health and Social Care in November 2020 shows how ventilation can reduce the risk of infection from Coronavirus by over 70%, as fresh air dilutes the particles. The video focuses on natural ventilation through opening windows, but also acknowledges the role mechanical ventilation systems have to play when used correctly and regularly.

[edit] The benefits of mechanical ventilation in reducing transmission

Whilst extract fans in bathrooms and kitchens provide a basic level of ventilation and are low in cost, they are only one step above opening a window. Using these fans, replacement fresh air is provided via background ventilators and normal air leakage.

For a more effective solution that ensures fresh air reaches all rooms in a home and pollutants are directly extracted without losing costly heat from the property, whole house mechanical ventilation solutions can’t be beaten, especially if you opt for a mechanical ventilation with heat recovery (MVHR) system.

MVHRs combine supply and extract ventilation in one system. They work on the principle of extracting and re-using waste heat from ‘wet rooms’ (kitchens, bathrooms, utility spaces). They efficiently pre-warm the fresh air drawn into the building with waste stale air using a heat exchanger. The filtered, pre-warmed air is then distributed around the home, effectively meeting part of the heating load in energy efficient dwellings.

Domus Ventilation’s HRXE-HERA™ and HRXE-AURA™ MVHR units feature advanced heat exchange proficiency enabling up to 95% of waste heat to be recovered. They come with 100% thermal (summer) bypass which automatically activates when the air temperature reaches a pre-set level, allowing in cooler, fresh, filtered air without warming it through the heat exchanger.

MVHR systems provide effective ventilation, are energy efficient, extremely effective at reducing the risk of virus transmission, condensation and cold air draughts and, with their built-in air filters, are particularly useful in more polluted urban areas.

A lower cost, easier to install alternative to MVHR, mechanical extract ventilation (MEV) systems are also available. These actively extract air from wet rooms via ducting to a central ventilation unit which exhausts to the atmosphere. The systems are typically two speed, providing low speed continuous trickle ventilation and high-speed boost flow. Replacement fresh air is drawn into the property via background ventilators located in the habitable rooms and through air leakage.

Both types of systems have been recognised by the World Health Organisation (WHO) as providing a line of defence against the spread of Coronavirus. Because of the nature of these systems, which require extensive ducting, they are mostly suited to new build properties.

[edit] Lest we forget: air pollution

Ventilation in our homes has never been more important, but it would be a mistake to focus solely on Coronavirus as the only threat to our health. We have been in the midst of a health crisis for some time, caused by pollution in the air we breathe.

According to NHS England, 30% of preventable deaths in England are due to noncommunicable diseases specifically attributed to air pollution. Particulate matter, notably PM2.5 and NOx, are the biggest threats here. This has been sadly brought to the fore by the recent recognition of air pollution as a cause of a person’s death for the very first time in the UK and possibly the world.

However, much of the focus on air pollution has been on polluted external air, whereas our exposure to air pollution mostly happens indoors, where we typically spend 90% of our time. With Coronavirus lockdown periods, this figure has increased, along with the risk to our health.

Indoor air pollution sources are widespread and vary dramatically from house to house. Sources include cooking, cleaning, fires, candles and even building and decorating materials. Outdoor air pollution is another source and features some of the more lethal types of pollution including nitrogen oxides, which are tiny particles that can easily enter our homes around closed doors.

For reducing indoor air pollutants, experts agree that both source removal and ventilation are key.

We have already looked at how whole house mechanical ventilation systems work and how effective they are in distributing fresh air throughout the home. But in more polluted areas, such as cities, bringing air into the home also brings in dangerous pollutants, especially if the property is located near a busy road. In these instances, ventilation has to be combined with filtration.

The Domus Ventilation NOX-FILT, for example, works on the supply leg of the ducting system of a mechanical ventilation system and prevents up to 99.5% of NO² pollution from entering a home. There are two units in the range with the second one having the added benefit of a PM2.5 pre-filter.

Whilst the unexpectedly hot spring and summer months of 2020 saw external air pollution drop dramatically as we lived through the first Coronavirus lockdown, sadly this has not been repeated in the second nationwide lockdown. With even more time spent indoors out of the bad weather, not only have we sadly seen a rise in COVID-19 infections, but also air pollution. This has come from the large numbers of people working from home and who are using their gas boilers – a major source of local pollution – during the day, when normally they would be in their workplace and the heating at home would be off. At the same time, many workplaces will continue to be heated for those staff who are in. NOx emissions from cars are unlikely to drop much as, although fewer people are going to the workplace, many are using cars when previously they would have taken public transport.

[edit] The years ahead

No one is pretending that 2021 is a good year, but there is light on the horizon for sure. We are witnessing one of the biggest vaccination programmes in the history of the world in a bid to fight Coronavirus. We are also seeing governments across the world taking air pollution seriously and putting measures in place to reduce it. And we are seeing ventilation being recognised as a valuable and relatively low cost means of improving our indoor environment and our health.

This article originally appeared in the Architectural Technology Journal (at) issue 138 published by CIAT in summer 2021. It was written by Paul Williams, Domus Ventilation Product Manager.

--CIAT

[edit] Related articles on Designing Buildings

• Air quality in the built environment.
• CIAT articles.
• Effective ventilation in buildings.
• Heat recovery.
• IAQ developments accelerated by COVID-19 pandemic.
• Indoor air quality.
• Mechanical ventilation of buildings.
• Ventilation.