External terminals can be close together – often vertically one above the other with exhaust 600mm below intake is a good solution to minimise wall use on a narrow frontage house. The position of the intake terminal is important for air quality – avoid locating near smells or pollution, e.g. bins or car park, and avoid intake through the roof in order to avoid bringing in hot air in summer – even north roofs can get hot. Put the intake 2 m or more above ground level to minimise intake of particulates. It is better to have the exhaust on the same wall as the intake, to minimise impact of wind pressure difference. Using a roof exhaust causes problems of condensate from the damp exhaust air collecting in the duct and needs special drainage.
Ductwork can either be rigid – normally steel – in a branched configuration, or newer ‘semi-rigid’ systems which use plastic ducts that can be bent round corners, but still have a smooth inner bore. For the latter, the ducts are arranged radially running one or two to each room from supply and extract distribution manifolds. ‘Flexible’ ductwork, like old tumble drier hoses, is not acceptable – it gets squashed and has a high resistance to airflow. All distribution ductwork has to be within the airtight thermal envelope – no exceptions.
Ductwork size is determined initially on air velocity to avoid regenerating noise, and the requirement is Higher pressure loss increases fan energy consumption and the test figures for PHI certified MVHR units only apply up to a maximum system pressure of 100 Pa. This should only be exceeded in large non-domestic projects when additional calculation is needed to determine the fan energy usage in PHPP in accordance with actual system pressure loss.
Excerpt from “How to build a Passivhaus: Rules of thumb”, chapter by Alan Clarke, Passivhaus Trust, April 2015.