PHPP works out an overall heat load, in terms of watts and W/m2. This gives a continuous heat load – ie, assuming the heating is on 24/7, and is aimed at seeing
if heating via the air is possible. The numbers are finely balanced, which may be lovely in theory but doesn’t always make for a robust and easy to live with heating system. For other heat distribution methods, such as radiators, you have the option to turn heating off at night, and warm up relatively quickly when cold. An allowance for extra capacity should be included – as a rule of thumb size Passivhaus heat emitters at 20 W/m2 (or in other words at 200% of design heat loss).
For radiator systems you can use a simple room by room steady state heat loss model. You can read off the effective ventilation and infiltration rate from the PHPP sheet – a ventilation rate of 0.5 ACH and 90% heat recovery is effectively 0.5 x (1–0.9) = 0.05 ACH.
Excerpt from “How to build a Passivhaus: Rules of thumb”, chapter by Alan Clarke, Passivhaus Trust, April 2015.