In the most recent Passivhaus Trust newsletter (link below) is a link to this document, "Good Practice Guide, MVHR for single dwellings". Its a detailed description of how to design an MVHR system for a Passive House.
The guide: http://passivhaustrust.org.uk/UserFiles/File/Technical Papers/2018 MVHR Good Practice Guide rev 1.2(1).pdf
The newsletter: https://mailchi.mp/passivhaustrust/pht-news-sept-18?e=894c841f59