It may sound unusual but when building an A rated house all consideration and attention is given to the thermal efficiency of the structures wall, glazing and roof also, the permeability of the dwelling but little is given to the ventilation.  If you look at the image below it shows the levels required to achieve an A-rated house and all are measurable.  However, in regard to the ventilation all you need to comply with the build regulations is the minimum airflows as outlined in Part F.  Albeit, the minimum airflow is based on minimum U-value and minimum air permeability – in other words a C1 rated house.  It is critical that no matter what MVHR system is fitted into your dwelling that it meets YOUR requirements and match YOUR build standard (be it B2 or A1).

It is very easy to quantify airflow, velocity and pressure.  This information should be made available BEFORE the system is installed.  Correct design is essential to optimum performance.