MVHR insulation

I was in a meeting for the Existing Homes Alliance Cymru and we had a good presentation on the Project CALEBRE research from Loughborough University. One of the issues that they had in their work reminded me of another couple of cases I have seen with the installation of MVHR in homes. MVHR is Mechanical Ventilation Heat Recovery, so a system that ventilates your home with fresh air, but does so by using the warm and moist extracted air to pre-heat the incoming air.

One issue is that of insulation. In the Loughborough case, and in the two I have seen, the insulation was really poorly fitted around the ducting. It was there, but the corners were not covered properly and the pipe lengths were not wrapped up and sealed properly. This of course means that the warm air that you need to pre-heat the incoming air was not as warm as it could have been and also the then pre-warmed air was losing some of its heat in the loft space. So take care when having MVHR installed and make sure that you check the quality of the installation afterwards.

Two other issues emerged from the Loughborough study that are also really important.

1. It was found that the system was not calibrated correctly for the house as the installer did not check this until they were called back to sort the insulation problem. So it sounds as if this is not necessarily done as a matter of course. So ask your installer to check the settings once the system is up and running (with the insulation correctly fitted).

2. The cost savings that can accrue from using a MVHR do not kick in until you have a really airtight house. So unless you have an airtightness level of around 3 cubic metres per sq m per hour or better you will not get any carbon benefit. Having spoken to Ventilation companies they tend to state that MVHR can be installed in houses with an average 7 – 10 cubic metres per sq m per hour. Whilst this is true they can be fitted, it will not give you any cost or carbon benefit, so it is not worth while. Most new houses are built to a standard of 10 cubic metres per sq m per hour and so using MVHR in retrofit is really tricky to get right.

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