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ASHRAE RP 1649: Comparing Laboratory Performance and In-Situ Performance of Residential HVAC Filters

Completed Research Q&A: ASHRAE Research Project 1649

Comparing Laboratory Performance and In-Situ Performance of Residential HVAC Filters

From ASHRAE Journal Newsletter, July 28, 2020

Particle filters are used in HVAC systems to protect equipment and reduce exposure to airborne particles. Filtration standard ANSI/ASHRAE Standard 52.2, Method of Testing General Ventilation Air-Cleaning Devices for Removal Efficiency by Particle Size, is used to evaluate filter performance in a laboratory setting.

To determine whether laboratory tests can reflect in-situ performance of filters for residential buildings, ASHRAE Research Project 1649 examined filter performance in both environments.

University of Toronto Researchers Jeffrey Siegel, Ph.D., Fellow ASHRAE, and Tianyuan Li, Student Member ASHRAE, discuss the research in a recent article from Science and Technology for the Built Environment.

1. What is the significance of this research?

Li: ANSI/ASHRAE Standard 52.2 is the most widely used standard for measuring filter efficiency in North America. Although there have been several investigations of ASHRAE Standard 52.2, questions remain whether laboratory tests can reflect the in-situ performance of filters. This is a particularly important question in residential buildings because of variations in many parameters that influence filtration performance. One of the goals of this research project was to explore the difference between the lab-tested performance and the in-situ performance of filters in a sample of homes. The results from this research provide practical information for the application and future development of ASHRAE Standard 52.2 to filters used in residential buildings.

2. Explain the steps of this research project. What did the process look like?

Li: This research project involved three major steps.

  • The first step was to conduct in-situ measurements of filter efficiency and other important filter- and system-related parameters. In this step, we studied 21 homes where four types of high-efficiency filters were installed for three months each in Toronto, Ontario, Canada. A total of 84 filters were deployed over one year in this project. The detailed process is described in the ASHRAE Research Project 1649-RP report.
  • The second step was to send both new filters identical to the ones used in the study and used filters retrieved from the homes in the study to a commercial lab. The new filters were tested according to Appendix J of ASHRAE Standard 52.2, and the used filters were tested with ASHRAE Standard 52.2 initial efficiency protocols.
  • The third step was to conduct a detailed comparison between the lab-tested and in-situ results of filter efficiency and pressure drop. In this paper, we were interested in exploring the differences between the lab results and in-situ results and the contributing factors that might lead to differences.

3. Why is it important to explore this topic now?

Li: Much of the exposure to harmful particulate matter occurs indoors, especially in residences, where people spend much of their time. HVAC systems with properly installed high-efficiency filters can be used to reduce this exposure. However, because residential systems are generally not standardized, the performance of residential filters may not translate well from ASHRAE Standard 52.2 laboratory results. As a result, a filter with high nominal efficiency is not guaranteed to have high in-situ efficiency in a given home. It is essential to investigate the difference between lab-tested and in-situ performance, and more importantly, the factors contributing to this difference, to improve residential filtration.

4. What lessons, facts, and/or guidance can an engineer working in the field take away from this research?

Li: The same filter can perform differently in different homes because of a wide range of variations in system design and operation. Some important factors include bypass (air gaps around the filter), air velocity through the filter and filter loading (how dirty the filter is). As mentioned earlier, a filter with high nominal efficiency is not guaranteed to perform well, for example, when it is not installed correctly. Similarly, for the filters that we tested, there was no strong correlation between filter efficiency and pressure drop. In terms of residential filtration performance and energy use, the system itself is as important as—or even more important than—the filter installed in it. Thus, future residential filtration guidelines should include in-situ measurements and approach filtration performance from a system, rather than a filter, perspective.

5. How can this research further the industry's knowledge on this topic?

Li: In this work, the comparison between lab-tested and in-situ filter performance suggests that lab-tested results could differ greatly from in-situ measurements because of variations in system and loading conditions. Further, the test dust specified in ASHRAE Standard 52.2 is not a good representation of the filter dust in our sample of residential environments. Although the Standard 52.2 results could provide reasonable bracketing of the in-situ filter performance that we saw in all homes, the lab-tested results generally overestimate in-situ efficiency but underestimate in-situ pressure drop.

6. Were there any surprises or unforeseen challenges for you when preparing this research?

Li: One challenge we experienced was our selected sample of homes had relatively low system runtimes (the annual median runtime was 9.6%), which is the fraction of system operation during the year-long study. Because of the low runtimes, many of our test filters had relatively low dust loading. If runtimes were higher, we would expect more dust collected on the used filters and a wider range of pressure drops. This wider range would provide more information on the comparison between naturally and artificially loaded filters and indicates if the final flow resistance specified by the ASHRAE Standard 52.2 is reasonable for homes with high runtimes. Also, our homes were predominantly older and leakier. An important follow-up project would be to do a similar study in more modern homes that were built according to ANSI/ASHRAE 62.2, Ventilation and Acceptable Indoor Air Quality in Residential Buildings.