Performance-based design of ventilation systems. Potential and limitations regarding indoor air quality.

Licentiatavhandling, 2011

It has been increasingly common to design for increased ventilation rates above legislative requirements specified in building codes and other norms as the most optimum way to improve indoor air quality. However, this concept has recently been challenged, for example, by studies demonstrating that the emissions or pollution from used traditional particle filters increases proportionally with air flow through the filter. At the same time ventilation is the biggest single energy user in buildings. Therefore, it has been recognized that there is a genuine need to shift from prescriptive approach to performance-based approach in specification of ventilation requirements which would give the opportunity to properly address both indoor air quality and energy efficiency. The first part of the thesis comprises description and simulation of case studies where performance-based approach is used. The analysis shows that substantial energy and cost benefits in form of lower investment costs due to smaller HVAC systems and lower operational costs are feasible. Simulation results strongly suggest that temperature and relative humidity should no longer be ignored in specifying required hygienic ventilation rates. Under certain conditions temperature and relative humidity along with source control can be much more effective than higher ventilation rates in improving perceived air quality and far more energy efficient. It has been shown that energy savings up to 64% are possible while maintaining or improving indoor air quality. The second part of the thesis involves an extensive a state-of-the-art review of the impact of recirculation on indoor air quality, a state-of-the-art-review of different air cleaning technologies and a questionnaire study directed to practitioners in HVAC field in Sweden regarding their experience with recirculation air and air cleaning technologies. It is shown that no firm conclusions can be made about effects of recirculation on perceived indoor air quality and sick building syndrome. There are theoretical positive impacts on health as recirculation in combination with efficient air cleaning can contribute to lower exposures to particles and ozone as these are often in higher concentrations outdoors than indoors. A steady-state mass balance model that includes the ratio of recirculation and outdoor air flow is evaluated and used to simulate the effect of recirculation rates and air cleaning on redistribution of pollutants within a building. It is shown that recirculation air flow rate should be at least two to three times the outdoor air flow rate to avoid local indoor air quality deterioration. A classification of air cleaning technologies by functionality and pollutant selectivity is presented and it is recognized that several technologies may be needed for optimal performance.