Why We Don’t Believe in Mechanical Ventilation with Heat Recovery (MVHR)

This quotation from the book ‘The New Eco-Architecture’, by Colin Porteous highlights the dangers of restricting ventilation in buildings in order to save energy:

‘Paradoxically the drive of the 1980s and 1990s towards greater energy efficiency and more tightly sealed dwellings, coupled with the earlier move away from heating by open fires, has resulted in relatively poor quality of air and higher levels of humidity. In turn there are correspondingly increased populations of dust mites and aggravated incidence of respiratory ailments such as asthma. This irony has an important general application. Due largely to specialisation, the typical architect of today apparently no longer feels as confident as (the early modern architects)*, or as in control of the aspects of design that have a significant influence on the interaction of occupants with a building. 

In this respect, sunlight, daylight and the quality of air are still at the top of the agenda, but apparently not well understood or acted upon by most of the profession’.

* word substitution for clarity

As a consequence of the specialisation mentioned above, and also fear of liability issues, architects these regularly abdicate an understanding of natural light and ventilation in favour of mechanical ventilation with heat recovery (MVHR) as a solution to their low energy aspirations. And not surprisingly a whole industry has popped up to supply the means with which to enact this solution and in so doing creating its own political pressure to go ‘Passivhaus’ with the mainstream. In short, architects have lost purchase on what they are supposed to know about – light and ventilation and how to garner the best use of these for their clients. Mechanical solutions offer a quick fix liability avoiding way of achieving this. 

In 2015 the Architects Journal published an article by Cartwright Pickard Architects and MEARU (the Mackintosh Environmental Architecture Research Unit). The article was entitled ‘Are our Homes Making People Sick?’ The article drew together statistical data from 20 housing schemes in the Southeast and London and compared the energy use to the energy use that was predicted at their design stage. The diagram below illustrates the huge gaps they found between predicted performance and actual performance. 

The vertical bars represent actual energy usage and the crossing dots on each column represent the predicted energy use. While a couple of the projects had commendable results most badly underpredicted their energy consumption.

Here is a quote from the article: 

‘ Key Findings

CO2 concentrations rose above the threshold level in 95 per cent of the properties we monitored.

Indoor CO2 levels were worst in winter, with concentrations rising above the recommended level of 1,OOO ppm for 54 per cent of the occupied hours. In some properties we found CO, levels over 2,OOO ppm for sustained periods of time, with peaks of 3,250 ppm not uncommon.

We found several factors contributing to inadequate ventilation and high CO, levels: a Ventilation systems were often badly installed and incorrectly commissioned. This was particularly prevalent with MVHR systems, which need to be carefully installed and balanced to gain maximum operating efficiency.

– Ventilation units were also poorly maintained. Most commonly we found the air filters had not been changed or cleaned, reducing the airflow through the system and restricting the ability to extract stale air and supply fresh air.

‘The majority of the MVHR systems tested failed to meet the Building Regulations’ advisory airflow rates.

– Trickle vents above windows were either nonexistent, closed or covered by curtains or blinds, reducing the flow of fresh  air into the properties.

– Most of the properties overheated during the summer months, in part because of excessive solar gain from unprotected full-height windows, but exacerbated by under performing ventilation systems failing to extract sufficient stale, warmed air to keep spaces ventilated.The summer bypass function required by MVHR systems to reduce overheating was often not working or incorrectly set up.’ 

The article continued to the following conclusion: 

‘ Since the mid -1970s oil crisis, the UK’s Building Regulations have steadily increased the levels of insulation and air tightness in our homes without considering the unintended consequences of living in such highly sealed buildings. The UK has one of the highest levels of asthma in the developed world. We believe there is a correlation between this and the increasing air-tightness of new homes, which has resulted in poorer indoor air quality. (my bold text).

MVHR systems have become the ubiquitous default solution to achieving carbon targets and SAP ratings in urban housing. But our survey has shown that, in the dwellings we studied, most of the MVHR systems were installed or commissioned incorrectly and were seldom maintained properly because of the hassle and cost involved. Regulatory dogma has shot the industry in the foot. 

MVHR systems should be carefully scrutinized with regard to life-cycle costs and practicality in use. If not operated and maintained correctly, the air quality in a home quickly deteriorates without occupants realising. There are simpler and more practical alternative mechanical ventilation systems on the market, which do not require filters and that can respond to occupational density and varying conditions within each room or dwelling.

We found that most residents were ill-informed about the controls and technology in their homes.The heating and ventilation controls in most of the homes were over-complex so residents had trouble understanding how to use them correctly. Most residents complained they had been given little or no face-to-face introduction to the controls and systems in their new homes, and that even the property managers had a poor understanding of them.

The problems with air quality outlined in this feature are just some of the many serious problems this research project. ‘

Not only poor air quality but poor energy performance can be the outcome of using MVHR. And such systems also suffer from being complicated to understand and operate, with occupants frequently deciding not to change air filters because of the constant hassle of doing so, for example. 

Combine use of MVHR with manufactured wall and floor paneling, chemical adhesives and cleaners and solvent-based paints etc., and the ‘offgassing’ of chemicals within those building materials that can produce a toxic atmosphere inside a building is enhanced. Add to this the Co2 accumulation mentioned in the article and the likelihood is high for ‘sick building syndrome’. 

So the answer to the rhetorical question asked by the article is: YES, our buildings with MVHR have a higher likelihood of making us sick.

Our belief at Solarity is that human beings evolved to breathe fresh air and not air processed through a mechanical device. Our architecture provides our client the means to do what makes them comfortable, and puts them in charge of the air they breath. Doing this is quite simple through providing a variety of sizes and locations of openable windows with secure trickle vents which give opportunities for cross ventilation in summer and suppression of ventilation in winter.

Likewise we believe in natural light in all its forms and in the use of natural materials inside and outside.

Our attitude to Mechanical Ventilation with Heat Recovery (MVHR) reflects our whole attitude towards how people use our buildings. We believe that natural is best and this is what we work towards in all of our work.

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