The case for MVHR

New guidance from REHVA (the Federation of European Heating, Ventilation and Air Conditioning Associations) highlights the risks of spreading the SARS-CoV-2 virus through building air handling services. Their recommendation is that all ventilation that recirculates air within a building should now be switched off, and as much outdoor air should be supplied to the indoors as possible.

In response, PHT member bere:architects points out that the one ventilation system that can deliver safe 100% fresh air into a building (‘hygiene ventilation’) – while at the same time counteracting heat loss – is a Mechanical Ventilation Heat Recovery (MVHR) system.

These systems extract warm but potentially stale, dirty and humid air from bathrooms, kitchens and utility rooms before passing it over a heat exchanger which transfers the heat to fresh, filtered air from outside the building. Outdoor pollutants are filtered out, and indoor pollutants are expelled. With a cross-flow heat exchanger, extract and supply air streams are fully separated, so any risk of contamination is eliminated. As well as providing a huge improvement in indoor air quality, such a system, when well-designed and installed, can result in a reduction in heating demand of nearly 30%. The latest units can operate at up to 90% efficiency. They're an integral part of a Passivhaus or EnerPHit certified building.

 

MVHR installation at Golcar Passivhaus by PHT member Green Building Store

 

However, MVHR systems remain relatively niche in the UK. Why? One of the main reasons is a perception that they are only effective in more airtight buildings. But a new research report just released by the Trust examines and ultimately debunks this rule-of-thumb. The perception of a need for high levels of airtightness is based on outdated assumptions, and the evidence is that MVHR systems result in not only improved ventilation, but also lower carbon emissions, for all levels of airtightness.

 

As we face the stark realities of life during a pandemic, and at the same time grow increasingly conscious of the urgent need to cut our carbon emissions, both of these are benefits we should grasp with both hands. There is a compelling case that MVHR systems should be fitted in all new dwellings as standard, and strongly encouraged in retrofits where significant reductions in energy demand are being targeted.

 

Further information

PHT research report: The case for MVHR – April 2020

bere:architects: COVID-19 and the risk from air recirculated in buildings – 22 April 2020

CIBSE Journal: Preventing COVID-19 spreading in buildings – March 2020

Previous PHT story: MVHR – is it all just hot air? – 27 January 2016

Technical Guidance: Good practice guide to MVHR for single dwellings

29th April 2020

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