A simple tool to assess the feasibility of hybrid ventilation systems.

Decision about the use of natural, mechanical or hybrid ventilation system in a building should be taken at the early stages of the building design. While for a mechanical system the decision may be taken on the grounds of purely deterministic procedures, the use of natural forces requires a different approach. Therefore, a simple tool is needed which, given a few important and readily known building parameters and information about the local outdoor climate, enables the designer to determine the most suitable ventilation system. This paper describes the development of this tool and its use. Using a multi-zone code, the air changes due to natural ventilation have been calculated in a number of typical buildings for a specific set of weather data. From these simulations, a simplified two-parameter regressive model has been identified, obtaining in this way a set of parameters for each building typology and surrounding terrain. Stability and uncertainty of these parameters for different meteorological data, building envelope permeability, and orientation have been checked. Through the location Test Reference Year the simplified model provides a frequency distribution of effective pressure differences and, given the overall permeability of the building, the cumulated frequency of air changes due to natural ventilation. This information will be used by the designer to decide whether the air tightness of the envelope should be increased or decreased, and/or a mechanical system should backup the natural system.