Improvement of Indoor Living Environment by Occupants Preferences for Heat Recovery Ventilators in High-Rise Residential Buildings

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1 Original Paper Indoor and Built Indoor Built Environ 212;21;4: Accepted: October 17, 211 Environment Improvement of Indoor Living Environment by Occupants Preferences for Heat Recovery Ventilators in High-Rise Residential Buildings Sang-Min Kim a Ji-Hyun Lee b Hyeun Jun Moon c Sooyoung Kim d a Institute of Technology & Quality Development, Hyundai Engineering & Construction Co., Ltd., Yongin, Kyeonggido, South Korea b Graduate School of Culture Technology, Korea Advanced Institute of Science and Technology, Daejeon, South Korea c Department of Architectural Engineering, Dankook University, Yongin, Kyeonggido South Korea d Department of Housing and Interior Design, Yonsei University, Seoul, South Korea Key Words Heat recovery ventilator E Energy savings E Indoor air quality E Ventilation rates E Operating schedule E Residential building Abstract This study examined the influence of heat recovery ventilators (HRVs) on energy savings and indoor air quality (IAQ) in high-rise residential buildings. Field measurements were performed in four residential units, which were validated by computer simulations and estimated the total annual energy consumption. The operation schedules for HRVs were determined by a survey of residents. Field measurement results indicate that HRVs could effectively improve IAQ and afford effective energy savings. The indoor concentrations of formaldehyde were reduced by 54.6% after HRVs were operated for 24 h. The initial concentration was reduced by 82% after 168 h. Toluene was the dominant volatile organic compounds (VOCs) in the indoor air. Its initial concentration was reduced by 5% and other VOCs were also reduced by 4.1% to 53.1% after HRVs were operated. Annual energy savings of up to 2.26% were predicted when HRVs were operated for 24 h continuously, exchanging sensible and latent heat. HRVs could save energy more effectively in winter than in summer due to the greater temperature difference between outdoor and indoor air. Based on the preferred operation schedules of homes surveyed, an annual energy savings could be as high as 8.52%. ß The Author(s), 211. Reprints and permissions: DOI: /142326X Accessible online at Figures 3 23 appear in colour online Sooyoung Kim, Department of Housing and Interior Design, Yonsei University, Seoul, South Korea. Tel. þ , Fax þ , sooyoung@yonsei.ac.kr

2 Introduction Material with high thermal resistance is generally applied to building envelope with air-tightness to save energy in high-rise residential buildings that have higher window to wall ratios on their fac ade. These building envelopes with appropriate shading devices are often effective in utilising daylight to control electric lighting systems in buildings [1,2]. The tightly sealed envelope would be effective to save energy, but it could reduce air infiltration and deteriorates indoor air quality [3]. Insufficient ventilation rates could increase the concentration of harmful air pollutants such as formaldehyde and volatile organic compounds (VOCs) and this is now an important part of building environmental assessment of green building certification together with the appropriate building services [4]. In high-rise residential buildings where natural ventilation through envelopes is limited due to tightly-sealed material, ventilation is primarily dependent on mechanical systems. Due to this, ventilation strategies are required to improve indoor air quality and save energy effectively [5,6]. Alternatively, heat recovery ventilators (HRVs) that recycle the heat ejected from indoor space could effectively be applied to buildings in some European and Asian countries [7]. Various studies were performed to examine the applicability and contribution of HRVs to building energy savings [8 16]. The results of these studies implied that the annual heating energy could be effectively saved by the application of HRVs, and the energy savings would vary according to the outdoor climatic conditions that affected sensible and latent heat. The recovery of sensible and latent heat could reduce annual energy consumption of up to 4%, and the optimum control strategies depended on the ratio of latent to sensible heat [17,18]. The application of HRVs has been demonstrated by previous studies and would reduce heating energy consumption, but the operation of HRVs in cold climate may not be economical when the cooling set-point was above 248C [8,17]. A study, which was performed to examine the applicability and energy saving by HRVs in several cities, have shown that heating energy could be saved by 2%, although this study was limited to heating season only [19]. Other study that was conducted to investigate the energy performance of HRVs in high latitude regions showed that the energy savings achieved by the use of HRVs would exceed the operational costs of the ventilation system [2]. The contributions of HRVs to the improvement of indoor air quality were also examined under a variety of conditions [21 25]. These studies showed that the application of HRVs in buildings can contribute to improve ventilation rates with significant energy savings. Although the effects of HRVs in energy consumption and ventilation in buildings have been examined in a variety of studies, they were considered separately. Energy saving effects with improved indoor air quality according to the variations of controls for HRVs need to be studied simultaneously when HRVs are applied to high-rise residential buildings in real-world situations. It is well known that effective applications of HRVs are to optimise air supply and minimise energy consumption keeping IAQ within ranges recommended by guidelines [26,27]. However, HRVs are usually controlled individually by residents according to their personal preferences. Continuous operation of HRVs would improve IAQ effectively, but HRVs are usually operated only during limited hours, when residents are at home. Therefore, this study examined the effect of HRVs on IAQ and energy savings under various control schemes in high-rise residential buildings. The HRV operation schedules preferred by real high-rise residents were examined to determine the associated energy savings and most appropriate control options for HRVs in real-world settings. Annual energy savings, according to preferred operation schedules, are estimated and discussed. Field measurements were performed in high-rise residential building, and computer simulations were conducted to validate field results and predict annual energy savings. A survey was also performed for high-rise building residents to determine frequently-used operation schedules for HRVs. Additional computer simulations were then performed to assess the energy savings of HRVs under these preferred operation schedules. Research Methods Field Measurements The high-rise residential buildings examined in this study were located in Seoul, South Korea (latitude: N, longitude: E), and were built in 23 with steel reinforced concrete structures. The building that used for summer measurements (Building A ) has 69 floors, and two identical units on the 39 th and 4 th floors were used for measurements. The building used for winter measurements (Building B ) has 46 floors, and measurements were performed in two identical units on the 1 th Indoor Living Environment in Residential Buildings Indoor Built Environ 212;21:

3 Fig. 1. Floor plan (Building A ). Fig. 2. Floor plan (Building B ). and 11 th floors. The floor plans of the units used for the measurements are shown in Figures 1 and 2. All units were prepared for general residential use. Built-in wooden cabinets and bookshelves were installed in kitchens and living rooms, respectively. The cabinet and bookshelves were manufactured by a particular company according to a standard specification for general application to residential units. Hence, approximately equal chemical compounds were embedded in them, and the emission rates of chemical compounds from them were considered to be equal, although the rates were not exactly monitored. The cabinet and bookshelves were installed in each residential unit on the same day of the field measurements. There were no neighbouring buildings along the main fac ades of this residential building, and no shadows cast over the building by any nearby structures. Venetian blinds with 2.54 cm between slats were installed in all windows. The floor was furnished with flooring on top of the Ondol, which is a radiant floor heating system commonly used in Korea [28 31]. The thermal properties of the buildings that are relevant to energy consumption are summarised in Table 1. A ceiling-mounted individual air-conditioning system was used in each unit during the summer, and a district heating system was used for the Ondol during the winter to keep the indoor temperature within comfortable ranges as suggested by the guidelines [32]. The air-conditioning system supplied air to each room of each unit, and a centralised ventilation system was applied to return the air to the outdoors. Sensible and total heat exchange types of HRVs were installed in units and controlled to modulate ventilation rates. The HRV control conditions are summarised in Table 2. Air supply diffusers were installed in the living rooms and the four bedrooms of each unit. Diffusers for returning air were installed in the kitchens, dining rooms, 488 Indoor Built Environ 212;21: Kim et al.

4 Table 1. Building thermal properties Properties Building A Building B Floor area (m 2 ) Ceiling height (m) U-value of window (W/m 2 K) U-value of wall (W/m 2 K) Ratio of window to wall (%) Table 2. Conditions of HRVs Item Bldg. A Bldg. B Heat exchange type Sensible and Sensible latent Efficiency of latent heat exchange (%) 39.3 N/A Efficiency of sensible heat exchange (%) Table 3. Control settings for heat recovery ventilators (HRVs) Case Bldg. Floor HRV control condition Season Operation Heat Exchange Core part Air passed 1 A h ON Exchanged Installed Passed Summer 2 A h OFF Not exchanged Installed Not passed 3 A 4 24 h ON Not exchanged Removed Passed 4 A 4 24 h OFF Not exchanged Removed Not passed 5 B 1 24 h ON Exchanged Installed Passed Winter 6 B 1 24 h OFF Not exchanged Installed Not passed 7 B h ON Not exchanged Removed Passed 8 B h OFF Not exchanged Removed Not passed and living rooms of all units and connected to the HRVs by ducts. The layouts of the ducts and diffusers are shown in Figures 1 and 2. To examine the influence of HRVs on energy savings and IAQ, the HRVs installed in buildings A and B were operated according to the control settings shown in Table 3. In Case 1, both the supplied and returned air passed through the HRV and participated in heat exchange. The ventilation rate by the HRV was set at.5 air change rate per hour (ACH), satisfying the national building code of Korea, 23, during the time period when the field measurements were performed [33]. Different countries have different ventilation rates set for buildings [34 38]. In Korea, the recommended ventilation rate set in the Building Codes, 23 for the residential buildings was.5 ACH, when this study was performed. It should be noted that the revised Building Code, announced in 26, would require the rate to be not less than.7 ACH, not including natural ventilation [38]. Since this study was performed in 23, the ventilation rate controlled in measurements was based on.5 ACH. For Case 2, the HRV was shut off, so that no air passed through it. Thus, infiltration through envelopes was the only source of ventilation. For Case 3, the HRV was operated without a core part where heat exchange occurs. Accordingly, outdoor and indoor air passed through the HRV without exchanging heat. The ventilation rate was set at.5 ACH. For Case 4, the HRV was shut off for 24 h and the core part was removed. Thus, no air passed through the HRV, and no heat exchange occurred. The source of ventilation was equal to that of Case 2. For all cases, the indoor temperature was kept at 268C. The HRVs for Cases 5, 6, 7 and 8 in building B were controlled according to the same settings that were applied to Cases 1, 2, 3 and 4, respectively, except that the indoor temperatures were kept at 238C for all four cases. For all eight control cases, natural ventilation rates through windows were measured in Room 3 and in the living room of each unit using the tracer gas concentration decay method, which has been used effectively to determine ventilation rates by infiltration and mechanical systems in buildings [39,4]. In this study, the ventilation rates by the tracer gas concentration decay method were measured using a multigas monitor and multi-point samplers. In this study, the tracer gas concentration decay method was used to determine ventilation rates in the space. A multi-gas monitor and multi-point samplers were used to monitor the concentration variation in CO 2. Three samplers were installed at the height of 1.2 m in the Room 3 and living room. One sampler was positioned at the centre of each room, and the other two samplers were positioned along a diagonal line of the Room 3 and living room. The distance between each sampler was 1.5 m. Before data monitoring procedures for the concentration began, CO 2 gas was sprayed and introduced into the Indoor Living Environment in Residential Buildings Indoor Built Environ 212;21:

5 tested rooms and mixed by a fan. Once the CO 2 gas was mixed completely with the air in the space, the reduction in CO 2 gas concentration was monitored. The data monitoring was performed for 6 h with a monitoring interval of 12 min. The mean value of CO 2 concentration monitored by the three samplers was used to determine ventilation rates in the space. The determination was performed based on the theoretical background that has been approved and effectively used in other previous research [39 43]. The concentrations of indoor air contaminants were monitored for Cases 1, 4, 5 and 8 to examine the effects of HRVs on the dilution of air pollutants. The concentration of formaldehyde was measured in Room 3 and in the living room of each unit. The concentrations of VOCs were monitored in the living room of each unit. The measurement was performed at a height of 1.2 m in the centre of each room. Data monitoring began 1 month after construction was completed in each unit. Data monitoring intervals for formaldehyde were: 5 h, 1 h, 1 day, 5 days and 7 days after the conditions for HRVs outlined in Table 3 were initiated. The concentrations of VOCs were monitored once, 5 h after the initiation, based on Korean building codes used to assess indoor air quality [33,38]. To examine cooling and heating energy consumption, the total amount of electricity consumed by the airconditioning system, fans, and HRV controllers was measured. The energy used by the Ondol was also calculated based on input calories of district hot water used for heating in each unit. Data monitoring in Building A was performed from June 1 to August 3, 23, and monitoring in Building B was performed from January 1 to February 28, 24. Computer Simulation In this study, field measurements were performed for Building A in summer and Building B in winter. No measurement data are available for either building for the remainder of the year. Therefore, computer simulations were used both to validate the results of field measurements and to predict energy savings by HRVs in seasons when measurements were not performed. TRACE 7 was used in simulations to determine energy consumption under various control conditions for HRVs. TRACE 7 uses analyses of dynamic load calculations to simulate heating and cooling loads according to design alternatives, systems, equipment and economic analysis. TRACE 7 was pre-programmed with common design parameters for construction materials, equipment, base utilities, weather conditions and scheduling [44]. Loads were calculated using the response factor method, which considers heat storage effects occurring on sealed environmental envelopes. Infiltration rates, irradiance and heat generation by lighting and occupants are also considered in the computation algorithms. Due to these features, TRACE 7 was considered an effective tool to perform energy analysis for buildings [45,46]. The input data for simulations were equal to the conditions applied in both buildings used for field measurements. The area and height of each unit, heat transfer coefficients of windows and walls and lighting loads were considered. Standard weather data for Seoul, Korea were used as input data [47]. The specific conditions used to control HRVs during field measurements were applied across all simulations to determine the effects of HRVs on energy savings. Under these conditions, monthly simulations were performed for Cases 1, 3, 5 and 7 during a period from January to December. Survey of High-rise Residents to Determine Operation Schedules A survey was conducted with the high-rise residents to determine practical operation hours of HRVs, since HRVs installed in the buildings are controlled individually by residents according to personal preferences. A total of 72 female and 42 male high-rise residents, living in apartment units fitted with HRVs, participated in the survey. Their education levels ranged from high school to postgraduate education. The number of family members in each unit ranged from one to six. Surveys were conducted personally by interviews with the residents. The surveys included both general and specific questions. The general questions were intended to collect participants information such as gender, age, number of family members, education level, occupation and which floor of the building they lived on. The specific questions solicited information about the participants preferences for using HRVs, including operation hours, situations in which they typically used HRVs, usual operating modes, and satisfaction levels. Survey data were analysed to determine frequently-used operation schedules for HRVs in real-life contexts. Levels of HRV energy consumption were calculated according to these operation schedules using TRACE Indoor Built Environ 212;21: Kim et al.

6 Results Variation of Temperature, Humidity and Ventilation Rate The measured outdoor air temperatures and humidity during data monitoring periods varied but remained within typical summer and winter ranges for Korea. Figures 3 and 4 show examples of such variation during a 3-day period in August and January, 23, respectively. In general, temperature was significantly influenced by solar altitude, remaining high during the day and decreasing as the sun set. The measured outdoor temperature ranged from 23.48C to 31.58C in summer. The temperature typically remained above 268C at night, and reached 29.68C in some cases, which implies that cooling systems must be continuously operated during the summer to keep indoor temperatures within a comfortable range. In winter, the temperature varied from 12.58C to 4.48C, and remained below 8C for the majority of the time. This range indicates that heating must be provided continuously both day and night during the winter to keep indoor temperatures within a comfortable range. The difference between outdoor air temperature and comfortable indoor temperatures was greater in winter than in summer. Accordingly, more energy was used in winter than in summer to keep indoor air within comfortable ranges as recommended by guidelines [32]. Outdoor relative humidity varied from 63% to 99% in summer, such that outdoor air needed to be dehumidified before being supplied indoors to ensure resident comfort. However, dehumidification is not always required in winter, since humidity remained between 25% and 41%. This means that the HRVs function less effectively during the summer in terms of latent heat exchange between outdoor and indoor air. Indoor air temperatures, controlled by HRVs, ranged from 25.38C to 26.48C in the summer, and from 19.68C to 23.68C in winter. These ranges meet the target temperature ranges set for both seasons in this study. The effects of HRVs in terms of energy savings were expected to be weaker during the summer than winter, since the difference between outdoor and indoor air temperature could have an effect on the reduction of energy consumption when HRVs are used. Figure 5 shows an example of measured CO 2 concentration in the Room 3 for the Case 5 and Case 6 in Table 3, and the ventilation rates which were determined using the gas concentration decay method based on the monitored CO 2 concentration. Overall, the concentration of CO 2 decreased significantly for the two cases over the time Temperature [ C] Temperature [ C ] CO2 Concentration [ppm] Winter-OA Temp. Winter-1th Temp. Winter-RH Summer-OA Temp. Summer-39th Temp. Summer-RH Time [hr] Fig. 3. Variation of temperature and humidity (2 4 August) Time [hr] Fig. 4. Variation of temperature and humidity (22 24 January). CO2 variation-case 5 CO2 variation-case 6 Ventilation rate-case 5 Ventilation rate-case Accumulated time [minutes] Fig. 5. Example of measured CO 2 concentration and ventilation rates by tracer gas concentration decay method (Room 3, Cases 5 and 6). period after data monitoring began. The concentration of CO 2 decreased from 4572 ppm to 17 ppm and from 4745 ppm to 1363 ppm for the Cases 5 and 6, respectively. After the test began, the reduction in concentration during each time interval was greater for the Case 5 than that of Case 6 due to the influence of HRVs on ventilation Relative Humidity [%]. Relative Humidity [%]. Ventilarion rate [ACH] Indoor Living Environment in Residential Buildings Indoor Built Environ 212;21:

7 Ventilation Rates [ACH] Room # Livingroom Case Fig. 6. Ventilation rate by tracer gas concentration decay method. Concentration [µg/m 3 ] Case 1-Room 3 Case 4-Room 3 Case 5-Room 3 Case 5-Livingroom Case 8-Room 3 Case 8-Livingroom Time [hr] Fig. 7. Concentration variation of formaldehyde. The natural infiltration rates ranged from.19 ACH to.32 ACH for Cases 2, 4, 6 and 8, in which HRVs were shut off and no air passed through them. The differences between Room 3 and the living room in each unit ranged from.1 ACH to.4 ACH, indicating that the recommended ventilation rate was not satisfied fully by natural infiltration alone, and that HRVs must be operated in order to achieve the recommended rates. This result also indicates that natural infiltration rates were not equal for different apartment units located on different floors due to fluctuations in outdoor air pressure and unpredictable airflow. Under those natural infiltration conditions for the all residential units, the HRVs were operated according to the control settings given in Table 3, and provided additional ventilation rates to meet the required ventilation rates of the Korean Building Code. When HRVs were operated for 24 h, the ventilation rate would vary from.44 ACH to.58 ACH and rarely failed to meet the ventilation rate requirement as given by the Korean Building Code in 23 [33]. The differences between the ventilation rates of the odd and even numbered-cases in Figure 6 were the contributions of the HRVs to the final ventilation rates in Room 3 and living room of each unit. The ventilation rates provided by the HRVs ranged from.16 ACH to.3 ACH. The minimum contribution occurred in Room 3 for Cases 3 and 4, and the maximum contribution was in the living room for the Cases 1 and 2. The reduced concentration ranged from 4 ppm to 292 ppm and 56 ppm to 236 ppm for the Cases 5 and 6, respectively. Based on the reduction in CO 2 concentration for each data monitoring point, ventilation rates were determined [39 41]. The ventilation rates for the Cases 5 and 6 ranged from.4 ACH to.49 ACH and.18 ACH to.2 ACH, respectively. For the entire monitoring period, the mean ventilation rates for Cases 5 and 6 were.44 ACH and.19 ACH, respectively. These procedures were equally applied to the 8 cases summarised in Table 3 to determine ventilation rates using the tracer gas concentration decay method. Figure 6 shows the measured ventilation rates using the tracer gas concentration decay method for all eight cases in relation to the HRVs and natural ventilation used. In each case, ventilation rates were similar for Room 3 and the living room in each unit. For all cases, the differences between these two rooms ranged from.1 to.1 ACH. Concentrations of Air Pollutants The concentrations of indoor air pollutants showed noticeable differences according to ventilation rates and the volume of space in each unit. The concentrations of formaldehyde for Cases 1, 4, 5 and 8 are shown in Figure 7. Overall, the concentrations of formaldehyde were stronger when HRVs were shut off and the ventilation was depended on natural infiltration only. The slope of the decrease for a given interval was not steeper under this condition. Therefore, more time was required to dilute the concentrations of formaldehyde when HRVs are not operated. For all cases, the concentrations of formaldehyde in Room 3 were expected to be stronger than in the living room due to the surface to volume ratio of each space, assuming equal ventilation rates. However, the concentrations of formaldehyde were stronger in the living room than in Room 3 for the entire data monitoring period, probably because unpredictable amounts of formaldehyde 492 Indoor Built Environ 212;21: Kim et al.

8 molecules were emitted from the built-in furniture such as cabinet and bookshelves. When HRVs were shut off, the strongest concentrations of formaldehyde were detected in the living room in Case 8. These concentrations were still lower than those given by the National Building Code in Korea in 23. According to the Building Code, the concentration of formaldehyde in newly-constructed residential buildings should not exceed 21 mg/m 3 [48]. However, formaldehyde should still be diluted to prevent any potential hazards to residents who will be exposed to the pollutants continuously as long as they live in the building. The concentrations of formaldehyde in each space decreased gradually with time up to 168 h. When the ventilation rate by natural infiltration in Case 8 was.26 ACH, the initially measured concentration was mg/ m 3 in the living room, which was reduced by mg/m 3 after 168 h. Meanwhile, the concentration in Room 3 was reduced by mg/m 3 during the same period. This suggests that the dilution occurred faster in the living room within this limited time period. The concentrations of formaldehyde appeared to be diluted continuously beyond 168 h due to lower infiltration rates, which were not sufficient to dilute the air and decrease the concentration. The differences between the concentrations in the living room and in Room 3 became smaller as time passed, varying from mg/m 3 to 17.5 mg/m 3 after 168 h. This suggests that the concentrations in both rooms continued to become lower beyond that time point, and that the difference between the rooms would continue to decrease. In cases in which HRVs were operated, the concentration of formaldehyde began to reduce significantly after 24 h. Compared with Case 8, the dilution of air in both Room 3 and the living room was more effective, and formaldehyde molecules were removed more quickly. The concentration did not appear to continue to decrease noticeably after 168 h. However, the concentration was expected to decrease stably beyond this point, showing very narrow decreasing ranges. In Case 5, in which the ventilation rate was.45 ACH, the initially monitored concentration was mg/m 3 in the living room and 52.5 mg/m 3 in Room 3. After 168 h, the concentrations in the living room and Room 3 were reduced by 81.1% and 82.1%, respectively. This result suggests that more formaldehyde was removed from the living room than from Room 3, although the ratio of initial concentration to final concentration was not significantly different between the two rooms. The initial concentrations in the living room and Room 3 in Case 5 were narrower than in Case 8. The differences in concentrations during the initial stage in both cases were reduced with time. The difference between monitoring periods varied from 6.73 mg/m 3 to mg/m 3 in Case 5, and ranged from mg/m 3 to mg/m 3 in Case 8. After 168 h in Case 5, the concentration of formaldehyde in the living room was reduced 1.62 times greater than that in Room 3, reduced by7 mg/m 3 in the living room and 43.8 mg/m 3 in Room 3. In Case 8, the decrease in concentration was mg/m 3 in the living room and mg/m 3 in Room 3. This result implies that smaller ratios of surface area to volume could help to dilute the concentrations. These results may be explained by differences in surface area and volumes of spaces. The ratio of surface area to volume in each room was a critical factor that had an effect on the concentration of pollutants under approximately equal ventilation rates. Space with larger floor area and therefore greater surface areas, would have a higher amounts of pollutants emitted from the surfaces. In addition, larger spaces would require more air to be supplied by the ventilation rates. Room 3 was smaller than the living room, which was open to the dining room and kitchen. The ratios of surface area to volume were 1.92 for Room 3 and 1.47 for the living room. This means that the surface area per amount of air supplied to the living room was less than that supplied to Room 3. Accordingly, this resulted in more effective reduction of formaldehyde molecules in the living room than in Room 3. In general, the concentrations of formaldehyde and VOCs in indoor space are determined by the emission rates from the material and ventilation rates which should be controlled to maintain comfortable environments. In this study, the emission rates from the materials, such as the built-in cabinet and book shelves were assumed to be equal since they were manufactured by the same manufacturer according to standard specifications for them. In addition, they were installed in each residential unit on the same day, and without being altered or changed during the field measurements of this study were completed. Due to these assumptions, the emission rates from the material were not measured in this study. This point might be considered as a research limitation, but the assumption provided reliable grounds for the reduction of formaldehyde and VOCs concentrations when the ventilation rates were controlled by the HRVs. Logarithmic regression models were developed for Cases 1 and 5 to predict the relationship between formaldehyde concentration and the accumulated time Indoor Living Environment in Residential Buildings Indoor Built Environ 212;21:

9 Decreased Concentration [%] Case 1, Room 3 R 2 =.948 Case 5, Livingroom R 2 =.94 Case 5, Room 3 R 2 =.9442 Case 1-Room 3 Case 5-Room 3 Case 5-Livingroom Accumulated Time [hr] Fig. 8. Correlation between accumulated time and reduced amount of formaldehyde concentration (Cases 1 and 5). Decreased Concentration [%] Case 4-Room 3 Case 8-Room 3 Case 8-Livingroom Case 8, Room 3 R 2 =.8793 Case 8, Livingroom R 2 =.9641 Case 4, Room 3 R 2 = Accumulated Time [hr] Fig. 9. Correlation between accumulated time and reduced amount of formaldehyde concentration (Cases 4 and 8). Table 4. ANOVA test results for model Model Variable Unstandardised coefficients T Sig. ANOVA test B Std. Error F test Sig Case 1, Room 3 ln (Time) F(1,4) ¼ (Constant) Case 5, Room 3 ln (Time) F(1,4) ¼ (Constant) Case 5, Living room ln (Time) F(1,4) ¼ (Constant) Case 4, Room 3 ln (Time) F(1,4) ¼ (Constant) Case 8, Room 3 ln (Time) F(1,4) ¼ (Constant) Case 8,Living room ln (Time) F(1,4) ¼ (Constant) which can be applied to the amount of air supplied to each space. The time elapsed from the beginning of data monitoring was considered an independent variable in the model. The difference between initial formaldehyde concentration and the formaldehyde concentration at each time point was considered as a dependent variable. The predicted relationship is shown in Figures 8 and 9 and Table 4. Each data point represents the decrease in formaldehyde concentrations. Overall, a strong relationship was demonstrated between the two variables for all cases considered in the regression analysis. The concentrations monitored in Cases 1 and 5 appeared to decrease, showing stable patterns within limited ranges, and forming a plateau beyond 168 h. This suggests that the decrease in concentration would stop at some time beyond that point when a constant volume of air was supplied to the space continuously under a constant ventilation rate. As discussed previously, the decrease in the living room in Case 5 was more efficient than in Room 3. The coefficients of determination were.9442 and.94 for Room 3 and the living room, respectively. This means that the variation of the decreased concentration of formaldehyde was reduced by 94.42% and 94% over the time period during the monitoring. The relationship for Room 3 in Case 1 was also strong. The regression model was tested using ANOVA to determine whether a logarithmic relationship existed between elapsed time and formaldehyde concentration. Table 4 demonstrates that the logarithmic regression models were acceptable under the significance level of.5, since the levels calculated for all cases were less than.1. These models imply that the formaldehyde emitted from indoor spaces could be removed completely after 26 h when ventilation rates were maintained at.45 ACH by HRVs. 494 Indoor Built Environ 212;21: Kim et al.

10 In Cases 4 and 8, the formaldehyde concentrations in each space were reduced by approximately 5% after 168 h. This result occurred despite the fact that HRVs were not operated and ventilation was depended on natural infiltration only. Ventilation rates were not high enough to dilute the formaldehyde molecules that were being accumulated in the space after emissions from surface areas such as wall, floor and ceiling. The reduction in concentration occurred effectively in the living room within a given time interval. This result is consistent with those of Cases 1 and 5. The coefficient of determination varied from.8793 to This implies that the variation in the decrease of formaldehyde concentration was reduced from 87.93% to 96.41% when the elapsed time changed. Table 4 demonstrates that the models used were acceptable under the significance level of.5. Unlike Cases 1 and 5, a period of at least 12,831 h was necessary to dilute all formaldehyde molecules emitted from indoor spaces. The results for Room 3 in Case 4 were even worse than those results. The concentrations of VOCs measured in the living room of each building are shown in Figures 1 and 11. Outdoor air contained toluene up to 38 mg/m 3, and the concentrations of other pollutants were weaker. The concentrations of VOCs in the living room were affected by the ventilation rates of HRVs and infiltration. For Cases 4 and 8, when HRVs were not operated and infiltration was the only source of ventilation, the concentration of toluene was 345 mg/m 3 and 298 mg/m 3, respectively. The concentrations of ethylbenzene and mpxylene varied from 268 mg/m 3 to 344 mg/m 3 in both cases. Benzene does not seem to be a very critical pollutant. As with formaldehyde, no VOC pollutants exceeded the concentration given by the National Building Code of Korea, 23, which specifies permissible concentrations of benzene, toluene, ethylbenzene and xylene as 3 m/g, 1 m/g, 36 m/g and 7 m/g, respectively [48]. Although the monitored concentrations did not violate these codes, they should still be diluted to improve indoor air quality. In particular, attention should be paid to reduce the concentration of ethylbenzene. The concentrations of all VOC pollutants were reduced significantly as HRVs were operated. In Case 1, the concentrations of toluene, ethylbenzene and mp-xylene were 59.6%, 46.9% and 49.3%, respectively, in Case 4. The concentrations of the three VOCs in Case 5 were reduced by 5.1%, 6.1% and 5.5%, respectively, as compared with the concentrations in Case 8. The amounts of each pollutant that were removed were not equal in Cases 1 and 5 due to the differences in initial Concentration [µg /m ] Outdoor Case 1 Case 4 Benzene Toluene Ethylbenzene m, p-xylene o-xylene Pollutant Fig. 1. Concentration of VOCs (Cases 1 and 4). Concentration [µg/m 3 ] Outdoor Case 5 Case 8 Benzene Toluene Ethylbenzene m, p-xylene o-xylene Pollutant Fig. 11. Concentration of VOCs (Cases 5 and 8). concentrations and ventilation rates. In Cases 1 and 5, benzene was completely removed from the space with the aid of ventilation by HRVs. However, the concentration of o-xylene was reduced by only 3%. It appears that the decreases of pollutant concentrations were influenced by the ratio of surface area to volume in the living room and by ventilation rates. Under equal ventilation conditions, the decreases in concentrations occurred more efficiently when the ratio of surface area to volume was smaller. Those ratios for the living room were 1.75 in Building A and 1.47 in Building B. Therefore, decreases in concentrations occurred more effectively in the living rooms of Building B than in Building A. Although the ventilation rates for Building A were greater than for Building B by.11 ACH, the ratio of surface area to volume of the living room was a more influential factor in reducing the concentrations of VOCs. This implies that the removal of pollutants emitted from indoor surfaces was influenced critically by this ratio when ventilation rates were not significantly different. Indoor Living Environment in Residential Buildings Indoor Built Environ 212;21:

11 In summary, the pollutants emitted from various materials are important factors in deteriorating IAQ. They should be removed or diluted by ventilation to maintain indoor air quality. However, when ventilation rates are set to maintain the required quality of indoor air, more energy consumption would occur. Under these circumstances, the HRVs considered in this study should be a good alternative for improving IAQ with associated energy savings. Energy Consumption [kwh] Case 1 Case 4 Case 5 Case 8 Energy Consumption Measurements and Validation Energy consumption in each assessed residential unit of the high-rise buildings was reduced by the operation of HRVs to maintain ventilation rates in each unit. Figure 12 shows the monthly energy consumption required to keep indoor air temperatures within target ranges. Overall, less energy was consumed in Cases 1 and 5 than in Cases 4 and 8, because outdoor air passed through the HRVs and exchanging heat with exhausted indoor air at the target temperatures, which were maintained during the monitoring period. Heat exchange by HRVs was a meaningful factor in energy consumption when the temperature difference between outdoor and indoor air was large. When the HRVs were shut off and no heat exchange occurred between exhausted air and outdoor air (Case 8), the amount of heating energy consumed in January and February was 1996 kwh and 1864 kwh, respectively. However, the heating energy consumption was saved by 11.55% on average when the HRVs were operated for 24 h exchanging heat in winter (Case 5). This result occurred since the sensible heat that is expressed in terms of temperature difference between air and air was a significant contributor to the energy savings. While the energy saving effect in winter was effective, the savings in summer was not efficient. In particular, the HRVs had contributed to save cooling energy up to 3.76% in the summer. Since the temperature differences in summer were smaller than that during the winter, less efficient energy savings were achieved by HRVs during the summer. In this study, the indoor air temperature was set at 268C in summer, and the temperature difference between outdoor and indoor air did not exceed 78C. It appears that the contribution of latent heat recovery was not significant in energy savings in winter. The portion of latent heat exchange for heat recovery ventilators should be considered to improve energy savings. In general, the results of this study were consistent with previous studies which were conducted to examine the influence of heat recovery systems on energy savings in Jan. Feb June July Aug Month Fig. 12. Measured energy consumption. buildings located in two different climatic conditions [19]. The result showed that the operations of heat recovery systems saved heating energy effectively in winter when the outdoor air temperature ranged from 12 to 88C. However, the use of heat recovery system was ineffective when the cooling set-point in indoor space was above 248C for a particular climatic region where outdoor temperature was 338C. Other study showed undesirable influence of uncontrolled heat recovery systems on cooling loads in mild and cold climate region [17,49]. The results revealed that temperature-based control strategies should be necessary to reduce cooling energy consumption. Additional research also proved that higher cooling energy demand occurred for particular outdoor conditions during summer when indoor temperature is higher than the outdoor temperature and cooling is still necessary to meet thermal comfort for residents [5]. Although the heat recovery ventilators were not effective for particular outdoor conditions in summer, they significantly reduced heating energy consumption in winter. Total energy consumption was lowest in June and highest in August. For Case 1, the energy consumed in June was 4.54% of that consumed in August. The solar altitude is highest in June, and the influence of solar radiation on the cooling load is significant. However, the mean temperature profile in Korea indicates that the outdoor temperatures and humidity are greater in August than in June. This resulted in more cooling energy consumption in August. In this study, the energy consumption by HRVs in highrise residential buildings was measured during a limited period, not year-round. To examine the influence of HRVs on energy savings for entire seasons, computer simulations were performed using TRACE 7. Experimental data 496 Indoor Built Environ 212;21: Kim et al.

12 were used as input data in the simulations to predict energy consumption for periods during which measurements were performed. Standard weather data were also used for simulations [47]. The results of both experimental measurements and simulations were examined using linear regression analysis to validate simulation results. The relationships between these data are shown in Figure 13. ANOVA tests were performed to identify significant relationships. A summary of the tests is shown in Table 5. The test results indicate that an acceptable linear relationship existed between measured and simulated energy consumption for Case 1 (F(1,23) ¼ 37.64, p5.5) and Case 3 (F(1,23) ¼ 71.15, p5.5). The coefficient of determination was.8862 for Case 1 and.8887 for Case 3. This implies that the variation in simulated results was reduced by 88.62% and 88.87% for Cases 1 and 3, respectively, when measured results were used to predict simulated results. Since the validation was acceptable, the energy consumption for the rest of the year was predicted using simulations. The predicted energy consumption for each month under various control settings is shown in Figures 14 and 15. Positive and negative values indicate heating and cooling energy consumption, respectively. Overall, the Simulated Energy [kwh] Case 1 Case 3 Case 1 R 2 =.8862 Case 3 R 2 = Measured Energy [kwh] Fig. 13. Correlation between measured and simulated energy consumption. energy consumption for each month was less for Cases 1 and 5, when the HRVs were operated to exchange heat between outdoor air and the exhausted air from indoors. Specifically, Cases 1 and 5 resulted in annual energy savings of 23.29% and 18.25% as compared to Cases 3 and 7, respectively. These results were consistent with previous research, which revealed that heating energy could be reduced by 2% when heat recovery ventilators were employed during winter [17,19]. In summary, efficient energy savings were achieved when heating was necessary, since heat exchange occurred effectively in the HRVs due to the temperature differences between outdoor and indoor air. The HRV systems could achieve effective energy savings and ventilation rates with improved IAQ in high-rise residential buildings, where natural ventilation is limited due to tightly-sealed envelopes. Determination of Preferred Operation Schedules and Energy Savings A total of 72 female and 42 male residents of a high-rise building participated in the survey. The ages of participants ranged from 18 to 8. Overall, 87.7% of participants were older than 4, and 54.4% of those were females. A detailed distribution of participants ages is shown in Figure 16. A total of 43% of the survey participants were women who did not work outside the home and who spent the majority of their time in their residential units. A total of 37.7% of the participants were professional or selfemployed, and the rest of the participants were students and salaried persons who commute regularly. Their education levels ranged from high school to graduate degrees. A detailed distribution of occupation and education levels is shown in Figure 17. The survey participants preferred to operate HRVs between 6 and 12 h per day. The operation hours fell into the range was 55.2% of all the residents surveyed. A total of 8.8% of the participants preferred to use HRVs continuously for 24 h per day, but 35.9% of the Table 5. ANOVA test results for validation Model Variable Unstandardised coefficients T Sig. ANOVA test B Std. Error F -test Sig. Case 1 (Constant) F(1,23) ¼ Slope Case 3 (Constant) F(1,23) ¼ Slope Indoor Living Environment in Residential Buildings Indoor Built Environ 212;21:

13 participants preferred to use HRVs less than 1 h per day. The preferred operation hours for HRVs are shown in Figure 18. The survey participants particularly preferred to use HRVs while they cooked, dined and rested after dining, Energy Consumption [kwh/m 2 ]. Energy Consumption [kwh/m 2 ] Case 1 Case Month Fig. 14. Predicted energy consumption (Building A ). Case 5 Case Month Fig. 15. Predicted energy consumption (Building B ). with 67.5% indicating such a preference. A total of 6.1% of the participants preferred to operate HRVs while they slept. A total of 11.4% of the participants preferred to use HRVs only when they felt it was necessary. The preferred cases for operating HRVs are shown in Figure 19. In this study, preferred operation schedules for HRVs were determined based on survey results to predict energy savings by HRVs for the preferred operation hours. The majority of operation hours preferred by residents did not exceed 12 h per day, and HRVs were used primarily around cooking, dining and resting times. Accordingly, it was determined that operation schedules of 6 and 12 h were assigned for those three activities to perform computer simulations. The determined operation schedules are shown in Table 6, and the shaded areas indicate that the HRVs were operated for the designated time. The procedures used to predict monthly energy consumption discussed in the previous section were applied to the simulations under the determined operation schedules shown in Table 6. Predicted monthly energy consumption under two operation schedules is shown in Percentage [%] Housewife Student Salaryman Selfemployed Professional Occupation Fig. 17. Participants education level and occupation. Highschool Bacholor Master etc male 25 female < > 6 Age Fig. 16. Participants age. Percentage [%] Percentage [%] No use < Operation hour [hr] Fig. 18. Preferred operation hour for HRVs. 498 Indoor Built Environ 212;21: Kim et al.

14 Figures 2 and 21. Positive and negative values indicate heating and cooling energy consumption, respectively. Overall, slightly less energy was consumed when HRVs were used for at least 12 h in the two buildings. Compared with energy consumption during summer, more energy was consumed from December to February when heating was necessary. This result was similar to that for the two buildings in which the two types of HRVs were controlled in Cases 1, 3, 5 and 7. In particularly, the amount of energy consumed in winter was 2.6 times greater than that consumed in summer. The energy consumed by the total heat exchange type of HRVs was slightly greater than that consumed by Percentage [%] No use etc when necessary Case for using HRV Fig. 19. Preferred case for operating HRVs sleep cooking dining After cooking the sensible heat exchange type of HRVs. This means that the sensible heat had an influence on energy consumption in the season. In summary, the amount of annual energy consumption under operation schedules preferred by residents and the other three cases discussed in the previous section is shown in Figures 22 and 23. Overall, heating energy was a major portion of the energy consumption, ranged from 71.9% to 75.93% of the total energy consumption when HRVs were operated according to various control settings. The HRV operated 24 h continuously in Cases 1 and 5 saved energy more effectively than other operation schedules. In particularly, heating energy consumption was reduced by 9.54% and 8.9% compared with Cases 2 and 6, respectively. Cooling energy in Cases 1 and 5 was reduced by 1.63% and 3.39%, respectively. This means that annual energy consumption can be reduced by 2.17% when HRVs are operated for 24 h continuously, exchanging sensible and latent heat. When the total heat exchange types of HRVs were used according to operation schedules preferred by survey participants, total annual energy consumption was reduced by 8.49%. The sensible heat exchanging HRVs reduced energy consumption by 5.64% annually. The worst case scenario for energy savings happened in Cases 3 and 7 when HRVs were operated 24 h continuously without heat exchange. However, such control schedules Energy Consumption [kwh/m 2 ] hr Month Fig. 2. Predicted energy consumption according to operation schedule (Bldg. A ). 12hr Energy Consumption [kwh/m 2 ] hr Month Fig. 21. Predicted energy consumption according to operation schedule (Bldg. B ). 12hr Table 6. Operation schedule for HRVs according to residents preference Operation Time (1 24 h) schedule h œ œ œ œ œ œ g g œ œ œ g g œ œ œ œ g g œ œ œ œ œ 12 h œ œ œ œ œ g g g g œ œ g g g œ œ g g g g g œ œ œ Indoor Living Environment in Residential Buildings Indoor Built Environ 212;21: