The air distribution in buildings with combined natural and mechanical ventilation

Abstract: This work describes result from both measurements on anumber of one family houses, an analytical study of a one-zonemodel and multi zone studies of a two storey building. Thesimulations are performed as both parametric studies, withcombined values of outside temperature, wind velocity and winddirection, and whole year simulations. For the latter, aclimate file for the northern Swedish city Östersund isused.The results, for the whole year simulations, are presentedas ventilation availabilities. The ventilation availability isdefined as the relative time of the heating season during whicha specified airflow is exceeded. This specified airflow maye.g. be a Building Code requirement if such exists.The influence of different measures, and combinations ofmeasures, on the ventilation availability has been determinedfor the different rooms. It is found that acceptableventilation availability is possibly to achieve with naturalventilation. However, it requires large supply and overflowopenings and extended ventilation chimneys. These chimneys maybe difficult to accept from an esthetical point of view. Thenatural system is also very sensitive for changes in winddirection.To ensure required airflows at all times, an exhaust orhybrid ventilation system may be necessary.Some recommendations may be based on this study.-Consider the predominating wind direction. It’san advantage to have more supply openings on the leeward side,i.e. to place“humid”rooms towards the knownwindward side.-Use different chimney heights from the different“humid”rooms, to balance the internal airflows. Ifmechanical exhaust is used, it may be used only from some ofthe“humid”rooms, preferable the ones with closeddoors.-Use as large supply and overflow openings aspossible. Different opening areas may be used to balance theairflows, especially if the predominating wind direction isknown. Acoustic problems may be a limiting factor for theopening area. There may also exist a maximum opening area abovewhich stability problems occur.-Construct ventilation chimneys and chimney outlets ina way, that the windgenerated pressure at the outlet is alwaysnegative and independent of wind direction. Insulate thechimneys to avoid cooling of the air and decreased buoyancyforces.-Use different chimney heights from the different“humid”rooms, to balance the internal airflows. Ifmechanical exhaust is used, it may be used only from some ofthe“humid”rooms, preferable the ones with closeddoors.-Use as large supply and overflow openings aspossible. Different opening areas may be used to balance theairflows, especially if the predominating wind direction isknown. Acoustic problems may be a limiting factor for theopening area. There may also exist a maximum opening area abovewhich stability problems occur.-Construct ventilation chimneys and chimney outlets ina way, that the windgenerated pressure at the outlet is alwaysnegative and independent of wind direction. Insulate thechimneys to avoid cooling of the air and decreased buoyancyforces.