Air mass motion
For different building situations
The air mass flowing into the door region is characterised by three physical variables.
1. As a consequence of thermal laws, the cold air directed to move inwards mixes with the warm air inside the room.
2. The wind pressure directed at the door area – and therefore at the room interior – must be taken into account.
3. In the entry area, the higher the proportion of cold air flowing inwards is compared with the proportion of warm air flowing outwards, the more power is needed to keep the warm/cold shielding constant.
Building situation A:
In a single-storey building without connection to the upper levels and without opened windows/leaks, the neutral zone is at 50% of the door height. Therefore, in the door region, the in-flowing and out-flowing air masses take equal proportions of the cross-sectional area.
Building situation B:
In a building with two storeys connected via stairs, the neutral zone in the door region is offset, depending on the size of the leaks and/or opened windows. This means that, with these local conditions, the cross-sectional area of the in-flowing cold air is greater than that of the warm air, i.e. the neutral zone moves into the upper door region.
Building situation C:
For a multi-storey building with multiple levels connected by stairs and possibly multiple leaks/open windows, even more warm air escapes. Correspondingly, the neutral zone moves even further towards the top of the door. In extreme cases, this means the out-flowing warm air in the entry zone is almost zero. The cold airflow takes over the entire cross-sectional area of the door. This results in an airflow directed inwards.