How clean new air is incorporated into ventilation systems

The complexity of natural ventilation systems lies in the diversity of factors that cause air movement within a building, both in terms of wind (speed, direction, topography, and surrounding buildings) and external and internal temperatures of the building. These factors influence ventilation in different ways: air stratification, overheating of the roof, introduction of clean air, etc. Therefore, to correctly design a natural ventilation system, it is necessary to validate it beforehand using numerical simulation techniques (CFD) and establish an initial strategy that takes into account the following factors:

• Climate analysis: in areas with very warm climates and large temperature variations between day and night, it is appropriate to promote night ventilation that can be closed during the day to avoid warm outside air. In humid and hot climates, natural ventilation throughout the day is very suitable, encouraging an open building scheme with patios and crawlers that allow the entry of clean new air. In these cases, air movement inside contributes to increasing the comfort index of the occupants. Finally, the installation of a mechanical ventilation system should be considered when natural ventilation is inadvisable, as can happen in areas with very cold climates.

• Building orientation: it is necessary to maximize the ventilation with clean new air that can be introduced into the building from facades perpendicular to summer winds, avoiding introducing obstacles (trees, adjacent buildings, etc.) in that direction. Conversely, to avoid winter winds, vegetative barriers or rows of trees can be placed to intercept their predominant direction.

• Project design criteria: to introduce an efficient natural ventilation, the projected building must be narrow, so that the main facades are close together, allowing for easy cross ventilation. The maximum width of the building in these cases is 13 m.; beyond this length, the installation of a mechanical ventilation system should be considered. In spaces with high ceilings such as industrial warehouses, interior patios, etc., the installation of skylights with ventilation grids is very appropriate, favoring the exit of air through temperature gradients between the interior and exterior of the building.

The Technical Building Code in its Section HS. 'Health' requires the introduction of a ventilation system hybrid or mechanical for housing. The mechanical ventilation becomes especially necessary in housing blocks where a single vertical duct can serve several homes at the same time and thus guarantee their ventilation efficiently. The hybrid ventilation system, on the other hand, is very suitable for low-height buildings with individual ventilation systems, as in the case of single-family homes, terraced, or semi-detached houses. From the point of view of the energy efficiency of the property, its consumption is lower because it uses a natural system at certain times to renew the contaminated air. This operates with pressure or temperature gradients between the interior and exterior of the homes. It has the disadvantage that in designing the air ducts for extraction, many horizontal sections should not be used, as that would greatly hinder the operation of the installation during natural ventilation processes.