1、 P. Azimi and D. Zhao are Ph.D. candidates in the Department of Civil, Architectural and Environmental Engineering, Illinois Institute of Technology, Chicago, IL. B. Stephens is an assistant professor in the Department of Civil, Architectural and Environmental Engineering, Illinois Institute of Tech
2、nology, Chicago, IL. Modeling the Impact of Residential HVAC Filtration on Indoor Particles of Outdoor Origin (RP-1691) Parham Azimi Dan Zhao Brent Stephens, PhD, PE Student Member ASHRAE Student Member ASHRAE Associate Member ABSTRACT Exposure to ambient fine particles (PM2.5: particles 2.5 m in di
3、ameter) and ultrafine particles (UFPs: particles 100 nm in diameter) has adverse effects on human health. Residential buildings greatly impact human exposure to outdoor particles because people spend more than 90% of their time indoors (and much of that time at home), and outdoor particles can infil
4、trate through building envelopes and mechanical ventilation systems with varying efficiencies. In this work, we model the impact of various combinations of central forced air HVAC filters and mechanical ventilation systems on indoor concentrations of fine and ultrafine particles of outdoor origin in
5、 six types of residential buildings including old, existing, and new homes relying on infiltration alone as well as new homes with supply-only, exhaust-only, and central-fan-integrated-supply (CFIS) with continuous exhaust ventilation. A typical 2025 ft2 (188.1 m2) one-story single-family home was m
6、odeled in all ASHRAE climate zones in addition to the 15 most polluted cities in the U.S., providing a total of 22 locations. Hourly outdoor pollutant concentrations were obtained from US EPA for each site. Hourly HVAC runtime fractions and air exchange rates were estimated using a combination of BE
7、opt and EnergyPlus. These inputs fed into a dynamic simulation in MATLAB to solve a discrete time-varying mass balance on indoor particles of outdoor origin in a well-mixed single zone based on physical particle source and removal mechanisms. Estimates of annual average PM2.5 infiltration factors ra
8、nged from 0.4 to 0.2, for filters with a minimum efficiency reporting value (MERV) of 5 and high-efficiency particulate air (HEPA) filter in the old homes respectively. In the existing homes relying on infiltration alone, as well as in the new homes with supply-only ventilation systems, the annual a
9、verage PM2.5 infiltration factor ranged from 0.2 to 0.15 with MERV 5 and HEPA filters respectively. PM2.5 infiltration factors were less than 0.05 for new homes relying on infiltration only, with exhaust-only ventilation, and CFIS ventilation for all HVAC filters. The modeled infiltration factors fo
10、r UFPs were lower in comparison with PM2.5 infiltration factors, but with similar trends ranging from 0.22 and 0.15 for old homes with MERV 5 and HEPA filters, respectively, to under 0.02 in new homes relying on infiltration alone for all filters. These data clearly demonstrate that higher-efficienc
11、y HVAC filters can meaningfully reduce indoor proportions of outdoor PM2.5 and UFPs inside residences, but both home vintage and climate zone strongly influence the results. In general, high efficiency HVAC filtration appears to have a greater influence on indoor PM2.5 and UFPs of outdoor origin in
12、older, less efficient homes with lower airtightness and longer system runtimes. INTRODUCTION Residential buildings can greatly impact human exposure to outdoor particle matter because Americans spend nearly 90% of their time indoors and nearly 70% of their time at home, on average (Klepeis et al. 20
13、01). Further, airborne particles of outdoor origin can infiltrate into residential buildings with varying efficiencies (Chen and Zhao 2011; B. Stephens and Siegel 2012). Therefore, much of human exposure to particles of outdoor origin in the U.S. actually occurs indoors, particularly inside residenc
14、es (Baxter et al. 2013). Relying on ambient concentrations alone without considering the mechanisms that influence indoor concentrations of outdoor particles can result in significant exposure misclassification for a large portion of the population. Moreover, high efficiency particle air filters ins
15、talled in central HVAC systems are increasingly being used to reduce indoor concentrations of particulate matter inside residences (Brauner et al. 2007). Several previous investigations have explored the impacts of HVAC filters on particle concentrations in residences through a combination of measur
16、ement and modeling, but they remain of limited value to ASHRAE membership for a combination of reasons, including: they have (1) considered only a narrow range of particle sizes or classes, (2) relied on filter classifications other than MERV, (3) investigated only a narrow range and variety of filt
17、ers, (4) not considered impacts of different types of mechanical ventilations, and/or (5) relied on static assumptions or modeled values for crucial input parameters (for modeling studies) or often failed to fully characterize the most influential parameters (for measurement studies). In this study,
18、 we have modeled the impact of a variety of central forced air HVAC filters on the indoor proportion of outdoor PM2.5 and UFPs in six common types of single-family homes located in multiple locations in the U.S., including three vintages of homes relying on infiltration alone (e.g., older, existing,
19、 and new homes) as well as a new home with three types of mechanical ventilation systems. METHODS Selection of Cities The selection of cities for modeling was designed to capture all 15 U.S. climate zones (thus yielding a wide variety of heating and cooling system operation), as well as the top 15 c
20、ities with the highest annual average outdoor PM2.5 concentrations summarized in the most recent Integrated Science Assessment for Particulate Matter (data coverage of 2005-2007) (US EPA 2009). A total of 22 cities were selected for modeling since some of the most polluted cities were in the same cl
21、imate zone as shown in Table 1. Table 1: List of Selected Cities, States, and Climate Zones City, State, (Climate zone) City, State, (Climate zone) City, State, (Climate zone) City, State, (Climate zone) Boston, MA, (5A) Birmingham, AL, (3A) Bismarck, ND, (7A) Riverside, CA, (3B) New York, NY, (4A)
22、St. Louis, MO, (4A) Colstrip, MT, (6B) Los Angeles, CA, (3B) Philadelphia, PA, (4A) Chicago, IL, (5A) Pinedale, WY, (7B) San Francisco, CA, (3C) Pittsburgh, PA, (5A) Miami, FL, (1A) Denver, CO, (5B) Seattle, WA, (4C) Detroit, MI, (5A) Houston, TX, (2A) Phoenix, AZ, (2B) Atlanta, GA, (3A) Blaine (Min
23、neapolis), MN, (6A) Albuquerque, NM, (4B) Model Home Characteristics and Beopt Modeling The same basic home geometry was used in each climate zone, although envelope characteristics differed by location and vintage. A 2,025 ft2 (188.1 m2) single-family home with three bedrooms, two bathrooms, 8 foot
24、 high ceilings, a natural gas furnace, and a central forced-air air-conditioning system was chosen as the basis for all vintages in all locations as it represents a very typical size and geometry for homes in the U.S. Basic home characteristics were all first input into BEopt for each home type, and
25、 equipment was sized using typical meteorological year (TMY3) data from the chosen locations. Initial simulations were first run in BEopt linked to EnergyPlus, which served to generate EnergyPlus input files that were then edited to reflect smaller changes to inputs such as rounding up to appropriat
26、e equipment sizing and selecting actual weather files for the simulation year. Modern High-Efficiency Home. The new homes were selected to represent new, energy efficient homes that have lower outdoor particle infiltration by incorporating well-insulated building envelopes (meeting modern code requi
27、rements), high airtightness equal to 3 air changes per hour at 50 Pascals elevated air pressure (3 ACH50), and properly sized high efficiency heating and air-conditioning systems for each climate zone. Each city was assigned to a region of the U.S. (i.e., West, Midwest, Northeast, or South), and the
28、 most typical type of foundation for each region was identified for single-family homes in the area (US Census Bureau 2015). Homes had crawlspaces, basements, or concrete slab foundations, depending on location. All homes were modeled with wood frame construction, with R-values of walls, ceilings, a
29、nd foundations and U-values and SHGC values of windows varying by climate zone according to 2012 International Energy Conservation Code. Windows were air-filled double-pane, low-gain (U-value = 0.35 BTU/hrft2F = 1.99 W/m2K and SHGC = 0.3), and low-e with nonmetal frames. An energy simulation program
30、 was also used to properly size the central air-conditioner and gas furnace, although more realistic air-conditioner and furnace sizes were chosen based on knowledge of commonly available incremental capacities. In the new high-efficiency home, the central air-conditioner was modeled as a SEER 16 1-
31、stage DX unit and the gas furnace had a 98% AFUE. Duct insulation was modeled as R-8 with 7.5% duct leakage. Typical Existing Home. Next, typical existing, older, and less efficient homes were designed to have higher, yet moderate, outdoor particle infiltration by incorporating moderately insulated
32、building envelopes, typical airtightness (10 ACH50), and larger and less efficient heating and air-conditioning systems for each climate zone based on typical existing home characteristics in each area. Envelope characteristics were taken from two primary surveys of existing housing characteristics
33、for homes built after 1979 (J. Huang, Hanford, and Yang 1999; Y. J. Huang et al. 1987). Windows were air-filled double-pane with nonmetal frames (U-value = 0.49 BTU/hrft2F = 2.78 W/m2K and SHGC = 0.79). The central air-conditioner was modeled as a SEER 10 1-stage DX unit and the gas furnace had a 90
34、% AFUE. Duct insulation was modeled as R-4 with 15% duct leakage. Typical Older Vintage Home. Typical older vintage homes were designed to have the highest outdoor particle infiltration by incorporating poorly insulated building envelopes, low airtightness (20 ACH50), and larger and less efficient (
35、and often undersized) heating and air-conditioning systems for each climate zone based on typical older vintage home characteristics in each area. Envelope characteristics were again taken from (J. Huang, Hanford, and Yang 1999; Y. J. Huang et al. 1987). Windows were single-pane with nonmetal frames
36、 (U-value = 1.1 BTU/hrft2F = 6.25 W/m2K and SHGC = 0.87). The central air-conditioner was modeled as a SEER 6.8 1-stage DX unit and the gas furnace had a 78% AFUE. Ducts were not insulated and had 30% duct leakage. New Homes with Exhaust-Only Ventilation. Only the new home model types are used to in
37、vestigate the impacts of mechanical ventilation on indoor concentration of PM2.5 and UFP of outdoor origin because older and existing homes are unlikely to have been built to meet minimum ventilation requirements in ASHRAE Standard 62.2 using mechanical ventilation systems. The minimum mechanical ve
38、ntilation airflow rate for the model homes used herein is assumed to be 50 cfm (85 m3/hr) (ASHRAE 2013). In the exhaust-only mechanical ventilation approach, a small exhaust fan is assumed to operate 100% of the time with an airflow rate of 50 cfm (85 m3/hr). Make-up air is provided by infiltration
39、through the building envelope. New Homes with Supply-Only Ventilation. In the supply-only ventilation system approach, outdoor particle penetration occurs through a combination of intentional mechanical supply in addition to incidental infiltration through the building envelope. A small supply fan i
40、s assumed to operate 100% of the time with a constant airflow rate of 50 cfm (85 m3/hr). In these cases, at least 50 cfm (85 m3/hr) of ambient air is supplied directly by the ventilation system and passes through a filter installed inside the small ventilating unit; any additional air exchange is as
41、sumed to occur due to infiltration through the building envelope. Therefore, PM2.5 and UFP penetration factors depend not only on envelope infiltration but also on the removal efficiency of the ventilation system filter. Most manufacturers have not adopted high efficiency filtration systems in small
42、 ventilator units; therefore, we assume that supply-only mechanical ventilation systems utilize only a MERV 5 filter. New Homes with Central-Fan-Integrated-Supply (CFIS) with Continuous Exhaust. In this case, a 50 cfm (85 m3/hr) intermittent outdoor air supply is ducted directly into the return plen
43、um of the existing air handling unit and a 50 cfm (85 m3/hr) exhaust system runs continuously. Therefore, outdoor air enters the indoor environment through a combination of (1) direct supply through the HVAC system and is filtered by the central system filter and (2) infiltration through the buildin
44、g envelope; the portion of each depends on the assumption for HVAC system runtimes. The CFIS system operates with an HVAC system runtime equal to that from the exhaust-only ventilation system scenario (i.e., runtime varies based on location). ASHRAE Standard 62.2 is met based on the 50 cfm (85 m3/hr
45、) continuous exhaust. The total air exchange rate in all cases is assumed to be the same as that modeled for new homes using exhaust-only ventilation systems. Long-term average PM2.5 and UFP penetration factors were assumed to vary depending on the central HVAC system runtime. Indoor Proportion of O
46、utdoor Particles Modeling Methodology For old, existing and new homes relying on infiltration only and modern high-efficiency homes with exhaust-only ventilation, we utilized a discrete time-varying mass balance on particles of outdoor origin inside a single well-mixed zone where, in the absence of
47、indoor sources, the indoor particle concentration (of PM2.5 or UFPs of outdoor origin) at each time step Ci.in(tn) is estimated using Equation 1. One-minute intervals were used to improve model stability (t = 0.01667 hours). Because the supply air is assumed to infiltrate through the building envelo
48、pe with exhaust-only ventilation systems, we assumed that PM2.5 and UFP penetration factors were the same as the penetration factors for the new homes without mechanical ventilation systems. HVAC recirculation rate (HVAC) for all homes with mechanical ventilation is assumed to be equal to the same v
49、alue for new homes relying on infiltration. (1) Hourly PM2.5 and UFP indoor concentration for new homes with supply-only ventilation systems are estimated using Equation 2. As mentioned, in this type of ventilation PM2.5 and UFP penetration factors depend not only on envelope infiltration but also on the removal efficiency of the ventilation system filter (which is assumed to be low efficiency, or MERV 5). (2) Finally hourly indoor PM2.5 and UFP concentration for the new homes with CFIS systems were estimated using Equation 3 which is somehow a combination of supply-only a
