Ventilation typology

Frank-Dietrich Heidt

The air movement through a building's envelope is driven by a pressure difference created by wind and indoor air to outdoor air temperature differences. It also depends on the air exchanging openings of a building's envelope, which may be intentional (such as windows, inlet valves or ducts) or not (for example, porosity, cracks or leakage). This means that natural ventilation rates are very unpredictable, given the unknowns of weather, the size and location of leakage in the building envelope and the behaviour of the occupants (opening windows and doors). These factors are additive and define the natural air exchange rate nnat:

nat inf user L J

with ninf as the air change rate due to inflltratlon/exfiltratlon and nuser as the air change rate due to the user-intended opening of windows, door(s) and other appliances.

Mechanical ventilation always acts in addition to the already existing natural ventilation and should dominate the air exchange rate. The overall air change rate is then a superposition of natural and mechanical ventilation:

with nnat as defined above and nmech as an air exchange rate (in 1/h) due to motor-driven air movement. For energy-related considerations, the electrical power for fan operation and control has to be accounted for.

Natural ventilation is totally inadequate during winter in the tight envelope construction of highperformance housing, where the heating energy consumption is less than 25 kWh/m2a. It must, however, be considered during planning in order to ensure that this heating target is not exceeded. Equally important, natural ventilation should be adequate in summer so that if the occupants desire, the mechanical ventilation can be shut down.

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