A side benefit of the reinstalled ventilation system is that we now have a unit that has been certified by the Passive House institute. They replaced the Stiebel Eltron LZW 170 with an LZW 270 Plus. The "plus" means that it has a bypass function, which is the part we wanted to help get rid of that summer heat. The 270 is more powerful than the 170 so it can ventilate more volume, although we already had enough with the smaller system. The other features are the same and it's difficult to beleive the efficiency has changed in any way.
As far as data entry is concerned, though, if you're entering manufacturer data for parts that have not been certified by PHI, you need to take off something like 12%. This is probably entirely justified, but in our case we can now increase the heat recovery efficiency from 78% to 83%. This doesn't sound a lot but brings our score down from 14.6 to 13.8 kWh/m2a.
Further investigation online shows that Stiebel give a heat recovery up to 90%, and they say that the Passiv Haus Institute has a figure of 86%. The database on the PHI website passiv.de, on the other hand, gives a figure of 83%. There is a mistake somewhere!
I suppose a lesson to learn, if the calculations are to be believed, is that ventilation and air tightness make a big difference, and having a well insulated house is not enough.
