Optimizing Filter Installations to Reduce Power Consumption and Improve Indoor Air Quality

Background: Epidemiological studies have linked increased concentrations of particulate in air with human health problems. Indoor aerosol concentration is most typically controlled by the HVAC system through dilution with fresh air and removal using air filters. The supply air stream has been shown to be the optimum placement for a filter of given filtration efficiency to reduce particulate concentration in indoor air. Previous optimization methods have not included electrical power requirement as a parameter or compared filters of different filtration efficiency in various installation configurations to determine a true optimum filter installation configuration. The purpose of this research is to determine the configuration of air filter installations to minimize power consumption and indoor aerosol. Method: This investigation was performed by combining the equations governing indoor particle dynamics with a model derived to predict the power consumption and operation cost of HVAC air filters. The indoor-to-outdoor particle concentration, filter power requirement, and annual operating cost were determined for filter installations in the supply air (SA), return (RA), or outdoor air (OA) streams. The analysis was also performed for varying filtration efficiency, air flow rates, and percentage fresh air to compare across a wide range of possible installations. Results: A comparison of the installation of filters with given filtration efficiency in different locations of the HVAC system showed consistency with previous research. The SA was the best location for the filter installation. If the filter was moved from SA to the OA the power consumption was reduced by 75% and 50% for 30% and 50% fresh air systems, respectively, while the concentration of all particle sizes increased by 60-500% which is unacceptable for occupant health. Similar trends were seen if the SA filter was moved to the RA stream. Comparing filter schemes with different filtration efficiencies showed the potential for reducing the concentration of fine and ultrafine particulate in indoor air. If a MERV 13 SA filter was replaced by a MERV 15 OA filter the power required to operate the filter was reduced by 65% while the concentration of particles 0.3-3µm in diameter was reduced by 16% and the concentration of particles 3-10µm in diameter was increased 300% for an air system operating at 30% fresh air. Conclusion: To determine optimum filter installation location both filtration efficiency and energy efficiency must be taken in to account. For certain building systems it was possible to reduce the power requirement and indoor concentration of particles 0.3-3µm in diameter at the expense of increasing the concentration of particles 3-10µm in diameter. As airborne particles with smaller diameter are more strongly associated with health problems there is potential for increasing overall health of building occupants while simultaneously reducing the energy required to operate the system.