Homes with MVHR have better air quality

Concerns about air tightness leading to poor air quality in our homes are increasing - now including comments from the Royal College of Physicians [1]. However, there is also mounting evidence that mechanical ventilation systems can provide a useful fix. A study comparing homes with and without mechanical ventilation with heat recovery (MVHR) shows much better levels of ventilation in the MVHR group [2].



The Royal College of Physicians seem to be mainly concerned about outdoor pollution sources such as traffic. They say that outdoor air pollution causes 40,000 deaths and year and the cost to health services, society and business could be £20 billion/year. However they also cite indoor pollution sources as an increasing problem. Indoor pollution comes partly from ourselves but also from many other sources - from glues, paints, and solvents in everything from air fresheners to carpets, and of course smoking. All these sources are hard to avoid completely but they need not be a problem as long as we have enough fresh air. As the Royal College of Physicians point out: 'initiatives to conserve energy in homes (in an effort to reduce carbon emissions) have generally led to reductions in ventilation (air change rates) and hence the potential build-up of pollutants from indoor sources.'

So what is the solution? The problem is that we are very bad at recognising when we need more ventilation and doing something about it. Our building regulations require simple ventilation systems such as trickle vents in windows but these are not always enough (especially after they have clogged up with a few years worth of grime). That is why I like MVHR, or at least ventilation systems that operate automatically.

The benefits of MVHR are clear from this study of eight social housing homes in Ireland where air quality was measured for three months in winter. This chart shows levels of CO2 in bedrooms - CO2 levels are a good guide to whether or not ventilation is adequate because we generate CO2 all the time by breathing. It is generally considered that levels below 1000ppm are OK and by that measure the MVHR homes were OK at least most of the time. However the naturally ventilated homes were not - in three our of four cases the average is above 1000ppm and in two cases it is more than double the OK level.

Comparison of bedroom ventilation in 4 homes with MVHR and 4 with natural ventilation [2]

Another interesting finding about this study is that the residents in the homes with MVHR had little idea how it worked. In fact they often didn't even know where the controls were. They also often opened windows, even though you should not need to with MVHR. (The window opening habits were much the same in both groups.) However with MVHR you don't need to do anything - it just happens automatically (see Living with MVHR).

On the other hand, in a naturally ventilated home, it seems that merely relying on the trickle vents is not enough. Not surprisingly the bedroom with the highest levels was one of the cases where the occupants say they kept their windows closed - this chart shows the gradual rise in CO2 level through the night, peaking at over 4000 ppm, four times the recommended limit.

Chart from [2] showing overnight CO2 levels rising to over 4000ppm - four times the recommended level. They had their windows closed.

There is some evidence that they noticed that the air quality was poor - residents in the naturally ventilated homes scored their homes worse for stuffiness and odour. Also they were much more likely than the MVHR group to report lethargy/tiredness, itchy eyes, headache and stuffy noses. However this did not seem to lead them to do anything about it - perhaps they did not associate these symptoms with poor ventilation, or perhaps there were other reasons for not opening the windows such as noise.

Regular readers will know I am a great fan of MVHR - see Modern air tight homes need modern ventilation but this is not because MVHR is mechanical, it is the fact that it happens reliably with no need for me to do anything about it. I would be happy to live with any ventilation system so long as it works automatically, provides good quality air and doesn't waste heat.


[1] Every breath we take - The lifelong impact of air pollution. (Royal College of Physicians) Feb 2016
[2] Case study investigation of indoor air quality in mechanically ventilated and naturally ventilated UK social housing. Grainne McGill, Lukumon O. Oyedele, Keith McAllister (International Journal of Sustainable Built Environment) March 2015