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Energy and Buildings 46 (2012) 3–13

Contents lists available at SciVerse ScienceDirect

Energy and Buildings

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etermining operation schedules of heat recovery ventilators for optimumnergy savings in high-rise residential buildings

ang-Min Kima, Ji-Hyun Leeb, Sooyoung Kimc, Hyeun Jun Moond,∗, Jinsoo Choe

Institute of Technology and Quality Development, Hyundai Engineering and Construction Co., Ltd., Yongin 446-716, Republic of KoreaGraduate School of Culture Technology, Korea Advanced Institute of Science and Technology, Daejeon 305-701, Republic of KoreaDepartment of Housing and Interior Design, Yonsei University, Seoul 120-749, Republic of KoreaDepartment of Architectural Engineering, Dankook University, Yongin 448-701, Republic of KoreaDepartment of Computer Engineering, Kyungwon University, Seongnam 461-701, Republic of Korea

r t i c l e i n f o

eywords:eat recovery ventilatorptimum operation schedulenergy savingsatural infiltrationeat exchangeesidential building

a b s t r a c t

This study examines the influence of heat recovery ventilators (HRVs) on energy savings in high-riseresidential buildings to determine optimum operation schedules. Field measurements were conductedin two actual residential buildings, and computer simulations were performed to predict energy savingsby the HRVs. Measurement results showed that energy consumption in each building was reduced whenthe HRVs were operated in line with recommended ventilation rates and comfortable temperature ranges.The HRVs achieved greater savings of energy during winter than summer.

Simulation results showed that the HRVs contributed to the annual savings of heating and coolingenergy by 9.45% and 8.8%, respectively, when the ventilators were operated continuously for 24 h. More

energy was saved as the operating hours of the HRVs increased. The continuous operation of HRVs waseffective for the savings of energy and to maintain recommended ventilation rates. The HRVs achievedeffective energy savings and maintained necessary ventilation rates in high-rise residential buildingswhere natural infiltration was minimal, due to tightly sealed building envelopes. This study suggests thatthe influence of HRVs on the improvement of indoor air quality needs to be examined in conjunctionwith energy savings by HRVs.

. Introduction

A building envelope is the physical separator between thenterior and the exterior environments of a building that helpso maintain the comfortable indoor environment and to facili-ate the micro climate control of the building. The envelopes ofigh-rise buildings constructed in recent decades in Korea areade of materials with high thermal resistance. These build-

ngs have strong air tightness in order to minimize heat loss andain through the envelopes. This design contributes to the sav-ngs of heating and cooling energy in buildings, but it also causesmportant ventilation issues by cutting off natural infiltration rateshrough the envelopes. While the air tightness applied to build-ng envelopes is effective for energy savings, it reduces infiltrationates, and consequently results in the deterioration of indoor air

uality.

Due to these problems, appropriate alternatives have beenpplied to solve the problems caused by the tightly sealed

∗ Corresponding author.E-mail address: hmoon@dankook.ac.kr (H.J. Moon).

378-7788/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.enbuild.2011.10.053

© 2011 Elsevier B.V. All rights reserved.

envelopes of buildings [1–4]. In particular, ventilation systems thatassure necessary ventilation rates with energy savings effectivelyshould be adapted to the high-rise buildings and operated prop-erly, since insufficient ventilation rates are critical factors thatcause severe dissatisfaction in indoor environments. It is commonlyunderstood that heat recovery ventilators (HRVs) are effective forsaving energy and maintaining necessary ventilation rates. The typeof heat recovery ventilators that reuse the heat ejected from indoorspaces have been effectively utilized in high-rise buildings in coun-tries in Asia and Europe [5,6].

A variety of studies have been conducted to examine the influ-ence of heat recovery systems on building energy performance[7–12]. These studies have proved that the application of heatrecovery ventilators conserves energy for heating, but that moreenergy for cooling is necessary to handle particular outdoor con-ditions in summer. Other studies have shown that heat recoveryventilators that are capable of exchanging latent and sensible heathave successfully reduced heating and cooling energy together [13].

However, the operation of heat recovery ventilators is ineffectivewhen the outdoor enthalpy is lower than the enthalpy of indoorair, while outdoor humidity is higher than that of the air suppliedto the conditioned space [9,14].

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S.-M. Kim et al. / Energy

In general, residents control heat recovery ventilators individ-ally, based on their preferences for thermal needs, which varynpredictably. Continuous operation may be satisfactory most ofhe time, but heat recovery ventilators are operated only duringelect hours when residents are at home. Previous studies haveocused primarily on energy savings by heat recovery ventilators.ess attention has been paid to the operation strategies that opti-ally utilize heat recovery ventilators to achieve effective energy

avings as well as satisfy the thermal needs of residents.The contribution of heat recovery ventilators to energy sav-

ngs should be studied according to variable schedules that controlhen heat recovery ventilators are used in high-rise residen-

ial buildings. Therefore, this study examines the influence ofeat recovery ventilators on energy savings under various con-rol schemes in high-rise residential buildings to propose optimumperation schedules for maximum energy savings. Field mea-urements were conducted under four settings of heat recoveryentilators in two high-rise residential buildings. Computer sim-lations were performed to validate the results of the fieldeasurements and to determine optimum control schedules for

he heat recovery ventilators.

. Research methods

.1. Field measurements

The buildings used for field measurements in this study are con-tructed of steel reinforced concrete and located in Seoul, Korealatitude: 37◦34′N, longitude: 126◦58′E). Field measurements per-ormed during the summer period were taken in a building with9 floors located in Mokdong, which is in the western part of Seoulbuilding A). The building used for field measurements performeduring winter has 46 floors, and is located in Seochodong, which is

n the southern part of Seoul (building B). Two identical heat ven-ilator units on the 39th and 40th floors (unit numbers 3903 and003) in building A were used for the measurements. For building, two identical units on the 10th and 11th floors (unit numbers003 and 1103) were used for the measurements. Views and floorlans of each of the buildings used for field measurements in thistudy are shown in Figs. 1 and 2. The floor areas of buildings A and

are 207 and 217 m2, respectively. The window to wall ratio of

uilding A is 43%, and the heat transfer coefficient of wall and win-ow is 2.74 and 3.40 W/m2 K, respectively. For building B, the heatransfer coefficient of wall and window is 2.65 and 3.34 W/m2 K,espectively. The ratio of window to wall on its envelope is 41%.

Fig. 1. Floor plan (

uildings 46 (2012) 3–13

No neighboring buildings exist close to the fac ades of the tworesidential buildings, and the buildings are free from the effects ofshadows from nearby buildings or any other structures. No par-ticular shading devices had been installed on the glazed areas ofthe building envelopes. For the purposes of this research, the heatventilator units were prepared for residential use, and furnituresuch as cabinets and bookshelves were placed in the living roomsand kitchens. The floor was furnished with flooring, specifically,linoleum on top of the Ondol, which is a radiant floor heating systemwidely utilized in residential buildings in Korea.

In order to keep temperatures comfortable for residential usein the summer, individual air-conditioning systems were installedon the ceilings of the residential units. The air supply to each roomof the residential units was provided by air-conditioning systems.After the initial supply of air, the circulated air in the rooms wasmechanically sent outdoors by a centralized ventilation system. Forheating during winter, a district heating system was applied to theOndol to keep indoor temperatures within comfortable ranges.

Sensible and total heat exchange types of heat recovery venti-lators were installed in each of the identical units in the buildings.The heat recovery ventilator installed in building A was capable ofexchanging sensible and latent heat. The mean exchange efficiencyof sensible and latent heat was 39.3% and 62.5%, respectively. Forbuilding B, a heat recovery ventilator that was able to exchangesensible heat was installed. The mean exchange efficiency of sensi-ble heat was 55%. In order to supply air from the outdoors to eachroom using the ventilator, air supply diffusers were installed in thebedrooms and living rooms. Diffusers for returning air were placedin the living rooms, kitchens, and dining rooms of both residentialunits. The diffusers were connected to the heat recovery ventila-tors through ducts, so that heat exchange occurred between the airexhausted from inside and the air supplied from outside.

The schematic layout of diffusers connected to the ventilatorsis shown in Figs. 1 and 2. The heat recovery ventilators were regu-lated to supply recommended ventilation rates for residential use.The control settings assigned to each of the ventilators installedin the units are summarized in Table 1. In order to investigatethe effects of heat recovery ventilators on energy savings, and todetermine optimum operation schedules in residential buildings,the heat recovery ventilators placed in each of the units in build-ings A and B were operated according to the operation conditions

summarized in Table 1.

In Case 1, both supplied air indoors and returned air out-doors, while the indoor and outdoor air passed through the heatrecovery ventilator and exchanged heat. When this study’s field

Building ‘A’).

S.-M. Kim et al. / Energy and Buildings 46 (2012) 3–13 5

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Fig. 2. Floor

easurements were performed, the ventilation rate was set at 0.5ir change rate per hour (ACH) in order for the heat recovery ven-ilators to meet the national building codes of Korea effective in003 [15]. The set up of this study’s ventilation rates generally methe mandatory requirements for residential buildings. The venti-ation guidelines currently in effect require 0.7 ACH for residentialuildings only by mechanical ventilation [16]. Since this study waserformed in 2003, the ventilation rate established for the fieldeasurements was 0.5 ACH, based on the ventilation guidelines

hen in effect.For Case 2, the heat recovery ventilator was operated without

he core part where heat exchange occurs. Accordingly, outdoornd indoor air passed through the heat recovery ventilator withoutxchanging heat. The ventilation rate was set at 0.5 ACH. In Case 3,he heat recovery ventilator was shut completely off for 24 h, ando indoor or outdoor air passed through the ventilator. Accord-

ngly, infiltration through building envelopes was the only sourcef ventilation, and no heat exchange occurred in the heat recoveryentilator. For each of these three cases, the indoor temperatureas kept at 26 ◦C during the entire period of data monitoring.

The heat recovery ventilators used for Cases 4–6 in building Bere operated with the same control settings used for Cases 1–3

n building A. For Cases 4–6, however, the indoor temperature wasept at 23 ◦C during the monitoring period.

To determine ventilation rates for Cases 4–6, two methodsere utilized. First, the ventilation rate was determined using the

otal air volume supplied to each space through the heat recov-ry ventilator when the heat recovery ventilator was in use. The

able 1ontrol Settings for heat recovery ventilators (HRVs).

Operation conditions Unit no. Bldg

Fan Heat exchanger

Case 1 On On 3903 A (Mokdong building)Case 2 On Off 4003Case 3 Off Off 4003Case 4 On On 1003 B (Seocho building)Case 5 On Off 1103Case 6 Off Off 1103

Building ‘B’).

determination of ventilation rates was based on the volume of airmonitored at the diffusers for air supply. Second, the gas concen-tration decay method was used to determine ventilation rates byinfiltration and mechanical systems in room 3 and the living roomsof each unit. The gas concentration decay method is known to bean effective method to determine ventilation rates by infiltrationand mechanical systems in buildings [17].

To investigate the consumption of energy by cooling and heatingwhen the heat recovery ventilators were applied to the buildings,the total amount of electricity consumed by controllers of the heatrecovery ventilators, air-conditioning systems, and fans was moni-tored. The heating energy consumed by the Ondol was determinedaccording to the input calories of district hot water used for districtheating in each building. Data monitoring in building A was per-formed from June 1 to August 30, 2003, and monitoring in buildingB was performed from January 1 to February 28, 2004.

2.2. Computer simulation

The data collected via field measurements is limited becausethe measurements were only performed in summer and winter.The field data do not cover the remainder of the year. Hence, thisstudy employs computer simulations to validate the results of thefield measurements, and to predict energy consumption by heatrecovery ventilators during the remaining seasons when field mea-surements were not conducted.

The software TRACE 700 was utilized as the primary simulationtool to determine energy consumption under a variety of operationschedules for heat recovery ventilators. The algorithm of the TRACE700 analyzes dynamic load calculations to predict building energyloads under various design alternatives, equipments, and systemcomponents. TRACE 700 is pre-programmed with common designparameters for construction materials, equipments, base utilities,weather conditions, and scheduling [18].

TRACE 700 has been effectively applied to energy analysis for

various buildings since building loads were calculated using theresponse factor method that explains the effects of heat stor-age on building envelopes [19,20]. In addition, various buildingenergy loads, such as natural infiltration rates, global and diffuse

6 S.-M. Kim et al. / Energy and Buildings 46 (2012) 3–13

Table 2Operation schedules for heat recovery ventilators (HRVs).

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Where O.S, operation schedule; O.H, operation hour; S, sensible heat excha

rradiance, and heat gain by lighting systems and occupants, arelso considered in the computation algorithms.

When this study utilized TRACE 700, the input data for simula-ions was identical to the boundary conditions for the two buildingshat were used for the field measurements. As usual, the generalimensions of each unit, heat transfer coefficients, and thermalesistances of materials applied to the envelopes, as well as theffects of lighting on energy loads were taken into account for dataomputation. Standard weather data for Seoul, Korea was also useds input data to consider real world situations in simulations [21].

Setting conditions identical to the settings used for the heatecovery ventilators during the field data monitoring periods werepplied to the boundary conditions for the simulations. Variousperation schedules were established and used for the computer

imulations in order to examine the contribution of heat recoveryentilators to energy savings, and to determine optimum opera-ion schedules. The operation schedules established for this studyre summarized in Table 2.

pe of HRV; T, total heat exchange type of HRV.

For the first step, schedules of 6, 9, and 12 h of operation wereset up based on the periods of time when heating and coolingwere most necessary to maintain comfortable temperature rangesindoors (schedules A, B, C, D, E, and F). For the second step, schedulesof 6, 9, and 12 h of operation were set up based on the preferences ofresidents for typical activities (schedules G, H, I, J, L, and K). Thesehours of operation were determined based on previous researchexamining the preferences people have for operation of heat recov-ery ventilators according to their residential activities [18].

For the third step, the study assumed that heat recovery ven-tilators were operated for 1 h and subsequently shut off for thenext 1 h. This schedule was repeated for 24 h (schedules M andN). Fourth, continuous operation for 24 h was established to exam-ine the effects of continuous operation of heat recovery ventilators

(schedules O and P). Finally, the non-operation of heat recoveryventilators (schedule Q) was established as the base case in orderto compare energy savings by the various operation schedules ofheat recovery ventilators.

S.-M. Kim et al. / Energy and Buildings 46 (2012) 3–13 7

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living room varied from 0.01 to 0.04 ACH. This implies that recom-mended ventilation rates were not met by the measured infiltrationrates, and that additional ventilation rates using heat recovery ven-tilators should be added to the space in order to meet the standards.

Fig. 3. Amount of supply air to space.

. Results

.1. Outdoor temperature, ventilation rates and efficiency of heatecovery ventilators

The outdoor dry bulb temperature, which was measured dur-ng the data monitoring periods, changed showing typical rangesf temperature variation for summer and winter. Overall, solarltitude effectively impacted the temperature in both seasons.emperatures remained high during daytime with the existence ofolar irradiance, but began to decrease as the sun set every evening.he outdoor dry bulb temperature in summer ranged from 23.4 ◦Co 31.5 ◦C. In the absence of solar irradiance at night, tempera-ures still remained higher than 26 ◦C, and in some cases, exceeded9.6 ◦C. This variation means that cooling systems should be oper-ted in summer to cool down the outdoor air and supply the coolerir to indoor spaces for ventilation.

The outdoor dry bulb temperature in winter ranged from12.5 ◦C to 4.4 ◦C, and remained below 0 ◦C for the majority of

ime. This variation is typical for winter days in Korea. The range ofhange still indicates that any heating system should be operatedor most of the day to keep indoor temperatures comfortable. Theemperature difference between indoors and outdoors was greatern winter than the temperature difference in summer. This differ-ntial was relevant to the energy consumption required to keepndoor spaces comfortable for occupants in both seasons.

When a heat recovery ventilator is installed and operated in aesidential unit, the exchange of heat between the outdoor air andhe air that is exhausted from indoors is a critical factor in energyonsumption. In this study, indoor air temperatures controlledrimarily by heat recovery ventilators were monitored at rangeshat varied from 25.3 ◦C to 26.4 ◦C in summer, and from 19.6 ◦C to3.6 ◦C in winter, respectively. The ranges of indoor temperatureset recommended temperature guidelines for summer and win-

er. When indoor temperatures are kept within comfortable ranges,he effects of energy savings by heat recovery ventilators dependrimarily on outdoor temperatures, which vary by season. In thistudy, the outdoor temperature in winter was significantly lowerhan the outdoor temperature in summer. It follows that the con-ribution of heat recovery ventilators to energy savings was moreffective in winter than in summer.

Ventilation rates in each residential unit were measured usinghe amount of air supplied to the space per hour while heat recovery

entilators were in use. Fig. 3 shows the amount of air measured athe diffusers in each space. The amount of air ranged from 17.1 m3/ho 38.2 m3/h in each space. The range of variation in the amount ofir depended on the volume of each space. In order to examine

Fig. 4. Air change rate per hour in space.

whether ventilation rates satisfied the recommended standards,the absolute amount of air supplied to the space per hour was con-verted into air change rate per hour (ACH) to represent practicalventilation rates.

Fig. 4 shows the ventilation rates as measured, using the amountof air supplied to the space of each unit when the heat recoveryventilators were in use. Overall, ventilation rates for each roomranged from 0.42 to 0.73 ACH. The differences in ventilation rates ofeach unit ranged from 0.13 to 0.25 ACH, depending on the locationof adjacent rooms. Specifically, the mean ventilation rate of room4 in Cases 1, 2, 4, and 5 was 0.502, 0.512, 0.562, and 0.514 ACH,respectively. These rates generally satisfied the ventilation ratesrequired by guidelines. In addition, the ventilation rates in the livingroom approximately satisfied required rates, although the rates inCases 4 and 5 were slightly lower than required rates by 0.06 ACH.

Ventilation rates were measured using the gas concentrationdecay method for all cases, in order to examine the ventilationrates by natural infiltration and the effects of heat recovery ven-tilators on ventilation rates. Fig. 5 shows the measured ventilationrates in room 3 and the living room when the heat recovery ven-tilators were shut off, and air was blocked from passing throughthe heat recovery ventilators. Data shows that the infiltration ratesvaried from 0.19 to 0.32 ACH for unit numbers 3903, 4003, 1003,and 1103. Differences in infiltration rates between room 3 and the

Fig. 5. Natural infiltration rates (gas concentration decay method).

8 S.-M. Kim et al. / Energy and Buildings 46 (2012) 3–13

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ture

[ºC

]

-20

-10

0

10

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30

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50

1 3 5 7 9 11 13 15 17 19 21 23

Time [h]0

10

20

30

40

50

Exc

hang

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ficie

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[%]

OA dry bulbSensible heatSensible & Latent heat

Fig. 6. Ventilation rate by HRVs (gas concentration decay method).

he data further suggests that infiltration rates from outside wereot equal for all of the units located on different floors. Differences

n outdoor air pressure appear to have caused the slight differencesn each unit.

Fig. 6 shows the contributions to ventilation rates by heatecovery ventilators in room 3 and the living rooms in each unit.ifferences in ventilation rates in each space ranged from 0.07 to.25 ACH. For unit numbers 3903 and 4003, the ventilation ratesy heat recovery ventilators in the living rooms were greater thanhe rates in room 3 of each unit. For unit numbers 1003 and 1103,he rates by heat recovery ventilators in room 3 were greater thanhe rates in the living rooms. This means that the heat recoveryentilators did not maintain equal ventilation rates in each space.lthough the rates were slightly different in each space, overall ven-

ilation rates satisfied the rates recommended by Korean nationaluidelines in 2003 [15]. When the heat recovery ventilators wereperated continuously for 24 h, the rates changed from 0.44 to 0.58CH.

Greater heat exchange efficiency of the heat recovery ventilatorsffectively influenced energy savings when the ventilators wereperated according to operation schedules. Figs. 7 and 8 show thefficiencies for both types of sensible and total heat exchanges inhe heat recovery ventilators on a particular day during the periodsf field measurements in summer and winter. Overall, the effi-

iencies varied according to changes in outdoor temperatures. Onhe sunny summer day August 21, when the temperature differ-nce between indoor and outdoor was great, the mean exchange

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OA drybulbSensible heatSensible & Latent heat

Fig. 7. Efficiency of heat exchange (August/21).

Fig. 8. Efficiency of heat exchange (January/21).

efficiency for sensible and latent heat was 33.9% and 32.6%, respec-tively. A maximum efficiency of 41.4% occurred at noon due thegreatest temperature difference at that point in the day. On a sunnywinter day, as shown in Fig. 8, the mean exchange efficiency ofsensible heat was 37.3%, and the efficiency for total heat, includingsensible and latent heat, was 35.3%.

Efficiencies of heat exchange in heat recovery ventilators var-ied according to changes of outdoor temperatures and humidityin each season. In particular, heat exchange efficiency shows clearvariations in regions where the temperature profile shows cleardifferences season by season. Accordingly, operation schedulesfor heat recovery ventilators should be commensurately deter-mined to increase the effects of heat recovery ventilators on energysavings in Korea, where there are four discrete seasons everyyear.

3.2. Measurement of energy consumption and validation forsimulations

The energy consumption in the high-rise residential units wasreduced when the heat recovery ventilators were operated inaccordance with recommended ventilation rates and comfortabletemperatures. Figs. 9 and 10 show the variations of energy con-

sumption for selected periods of three continuous days in summerand winter to maintain indoor temperatures at recommendedranges for thermal comfort.

Fig. 9. Cooling energy consumption (August/2–4).

S.-M. Kim et al. / Energy and Buildings 46 (2012) 3–13 9

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Measured-Case 1Measured-Case 3Simulated-Case 1Simulated-Case 3

Fig. 10. Heating energy consumption (January/21–24).

Regardless of the use of heat recovery ventilators, the higherhe difference in temperature between indoors and outdoors, the

ore energy was consumed in both summer and winter. Overall,ess energy was consumed when heat recovery ventilators wereperated continuously in both seasons. This is because the airxhausted from indoors and the supply air from outdoors effec-ively exchanged heat while passing through the heat recoveryentilators.

The total energy consumption in Cases 1 and 3 in summer condi-ions was 466.5 and 536.6 kWh, respectively. In winter conditions,he energy consumption for Cases 4 and 6 was 471.1 and 597 kWh,espectively. During the periods of continuous operation over threeays, the continuous operation of heat recovery ventilators in sum-er and winter reduced cooling and heating energy by 13.06% and

1.08%, respectively.This study conducted data monitoring of energy consumption

n two residential buildings for a total period of 5 months dur-ng summer and winter seasons. To examine the contribution ofeat recovery ventilators on energy savings over an entire year,omputer simulations were conducted using TRACE 700. The dataollected from field measurements was put into the simulations toetermine the energy consumption during the periods in which theata monitoring was conducted. Standard weather data for Koreaas used for the computer simulations to establish exact boundary

onditions.

Figs. 11 and 12 show variations in measured and predicted

nergy consumption for a particular day representing typical con-itions in summer and winter. Overall, less energy was consumed

0

5

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242220181614121086420

Time [h]

Hea

ting

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Measured-Case 4 Measured-Case 6Simulated-Case 4 Simulated-Case 6

Fig. 11. Measured and simulated energy consumption (January/23).

Time [h]

Fig. 12. Measured and simulated energy consumption (August/3).

when heat recovery ventilators were operated in both summer andwinter. The amount of simulated energy consumption was largerthan the amount of measured energy consumption for all cases. Inwinter, energy for heating was mainly consumed for 12 h in theperiods of time from 4:00 to 11:00 and from 19:00 to 23:00, whenthe outdoor air temperatures were lowest. In contrast, the primaryconsumption of energy for cooling occurred from 10:00 to 24:00,when outdoor air temperatures were higher. These results indicatethat the heat recovery ventilators were most effectively utilizedwhen the differences between indoor and outdoor air temperatureswere greatest.

Data about both the measured and simulated monthly energyconsumption required to keep indoor air temperatures within tar-get ranges is shown in Figs. 13 and 14. Overall, less energy wasconsumed in Cases 1 and 4 than in Cases 3 and 6, because in Cases1 and 4 more effective heat exchange occurred between the airexhausted from indoors and the air supplied from outdoors whilethe indoor and outdoor air passed through the heat recovery venti-lators. Results of the field data indicated that the amount of energyconsumed in August was greater than the amount consumed inJune by 40.54% and 39.96% in Cases 1 and 3, respectively. Likelythis occurred in accordance with temperature profiles showing thatoutdoor temperatures and humidity are higher in August than Junein Korea.

In general, the heat exchange in heat recovery ventilators ismeaningful for energy savings when the temperature differencebetween the air exhausted from inside and the air supplied from

Fig. 13. Monthly energy consumption (Bldg. A).

10 S.-M. Kim et al. / Energy and Buildings 46 (2012) 3–13

oeftifGeihvcwr

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3

ot

0 5 10 15 20 25Measured energy [kwh]

5

10

15

20

25

Sim

ulat

ed e

nerg

y [k

wh]

_

____

_

_

_

___

_

__

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_

_

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_

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_

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Simulated = -0.67 + 1.12 * MeasuredR-Square = 0.91

Fig. 15. Relationship between measured and simulated energy consumption.

Fig. 16. Monthly energy consumption under schedule F, C, Q (Bldg. A).

TA

Fig. 14. Monthly energy consumption (Bldg. B).

utside is large. The heat recovery ventilators achieved greaternergy savings during the winter because the temperature dif-erence between the exhausted and supplied air was greater thanhe temperature difference during summer conditions. The targetndoor temperature was 26 ◦C in summer, and the temperature dif-erence between exhausted and supplied air was less than 7 ◦C.iven the weather conditions in Korea, it does not appear that theffects of latent heat recovery were significant for energy savingsn winter. These results are inconsistent with previous studies thatave been performed to investigate the effects of heat recoveryentilators on energy savings in high-rise buildings in a variety oflimatic conditions [7,9]. In order to increase the savings of energyith heat recovery ventilators, the focus should be on latent heat

ecovery by heat recovery ventilators.The method of linear regression analysis was utilized in this

tudy to examine the results of the field experiments and computerimulations. The simulated energy consumption was validatedgainst the field data to perform further computer simulationso determine the amounts of energy consumption under variousperation schedules for heat recovery ventilators. The linear rela-ionship between the data is described in Fig. 15. ANOVA testsere conducted to determine if the relationships were acceptable.

summary of the tests is shown in Table 3.The ANOVA results implied that the relationships from the linear

rediction model were acceptable at a very low significance level of.01 (F(1,71) = 714.16, p < 0.01), and the coefficient of determinationr2) was 0.9094. This implies that the error variance in simulatednergy consumption was reduced by 90.94% when field data wassed to predict energy consumption. Based on acceptable valida-ion, this study performed further computer simulations to predictnergy consumption for the balance of the year under a variety ofperation schedules for heat recovery ventilators.

.3. Determination of operation schedules for energy savings

The predicted monthly energy consumption under particularperation schedules is shown in Figs. 16–19 and Table 4. Posi-ive values indicate energy consumption for heating, and negative

Fig. 17. Monthly energy consumption under schedule F, C, Q (Bldg. B).

able 3NOVA test results for validation.

Variable Unstandardized coefficients t Sig. ANOVA test

B Std. error ‘F’ test Sig.

(Constant) −0.665 0.430 −1.546 0.12 F(1,71) = 714.16 0.00Slope 1.122 0.042 26.72 0.00

S.-M. Kim et al. / Energy and Buildings 46 (2012) 3–13 11

Fig. 18. Monthly energy consumption under schedule P, O, Q (Bldg. A).

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type of heat recovery ventilators were operated for 6, 9, and 12 h,

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Fig. 19. Monthly energy consumption under schedule P, O, Q (Bldg. B).

alues indicate energy consumption for cooling in the figures. Ineneral, compared with the cases in which the heat recovery ven-ilators were in use, more monthly energy was consumed underchedule Q, in which the heat recovery ventilators were shut off

nd no air passed through them.

When both types of sensible and total heat exchanges occurredn heat recovery ventilators that were operated continuously for

able 4onthly energy consumption under schedules (unit: kWh/m2).

Bldg. O.S forHRV

Month and energy (H: heating energy, C: cooling energy), unit

1 2 3 4 5 5 6 7

H H H H H C C C

A F 10.89 10.80 7.35 1.94 0.09 2.23 2.84 3.8C 10.90 10.82 7.36 1.94 0.09 2.23 2.84 3.9L 10.90 10.83 7.37 1.95 0.09 2.23 2.84 3.9I 10.91 10.83 7.38 1.95 0.09 2.24 2.85 3.9N 10.93 10.84 7.40 1.96 0.09 2.24 2.85 3.9M 10.93 10.85 7.41 1.96 0.09 2.24 2.86 3.9P 10.56 10.37 6.74 1.82 0.09 2.04 2.55 3.7O 10.61 10.43 6.82 1.83 0.09 2.04 2.27 3.8Q 11.30 11.28 7.82 2.04 0.11 2.35 2.98 4.0

B F 9.46 9.22 6.50 1.84 0.23 2.64 3.25 4.2C 9.46 9.22 6.50 1.84 0.23 2.64 3.25 4.2L 9.49 9.22 6.51 1.85 0.23 2.65 3.25 4.2I 9.49 9.22 6.52 1.85 0.23 2.68 3.25 4.2N 9.50 9.24 6.53 1.85 0.23 2.68 3.26 4.2M 9.50 9.24 6.54 1.85 0.23 2.68 3.26 4.2P 9.22 8.93 6.11 1.64 0.23 2.54 3.07 4.1O 9.22 8.93 6.15 1.65 0.23 2.54 3.07 4.1Q 9.80 9.51 6.81 1.93 0.23 2.76 3.31 4.2

Fig. 20. Annual energy consumption under schedules (Bldg. A).

24 h, as in schedules P and O in this study, the annual energyconsumption of building A was reduced by 9.16% and 9.05%, respec-tively. Annual energy savings in building B ranged from 7.13% to7.29%. Total energy savings obtained in buildings A and B were4.35% and 3.38%, respectively, under schedule F, in which theheat recovery ventilators were operated for 12 h per day and theyexchanged sensible and latent heat simultaneously. Energy savingsachieved by the sensible exchange type of heat recovery ventilatorwere 4.24% and 3.38%, respectively. These findings imply that theexchange of sensible heat is primarily accountable for the mainportion of energy savings.

Annual energy consumption by the heat recovery ventila-tors showed clear differences under various operation schedules.Amounts of energy consumption under each operation scheduleare shown in Figs. 20 and 21. Overall, more energy was saved asthe hours of operation of the heat recovery ventilators increased.The total heat exchange type of heat recovery ventilators achievedmore energy savings than the sensible heat exchange type of heatrecovery ventilators.

Under schedules A, B, and C, where the sensible heat exchange

respectively, from midnight until morning in building A, achievedenergy savings were 1.53%, 2.50%, and 4.59%, respectively. Whenthe total heat exchange type of heat recovery ventilators were

: [kWh/m2] Total

8 9 9 10 11 12C H C H H H

9 5.06 0.02 2.40 2.71 6.74 10.45 67.420 5.08 0.02 2.40 2.71 6.75 10.45 67.492 5.08 0.02 2.40 2.72 6.76 10.46 67.573 5.10 0.02 2.40 2.72 6.77 10.46 67.663 5.10 0.02 2.40 2.74 6.77 10.48 67.753 5.12 0.02 2.40 2.75 6.78 10.49 67.828 4.93 0.02 2.24 2.40 6.36 10.13 64.020 4.95 0.02 2.24 2.40 6.42 10.19 64.104 5.26 0.03 2.50 2.88 7.05 10.86 70.49

0 5.73 0.02 3.36 2.04 5.48 8.87 62.861 5.73 0.02 3.37 2.05 5.48 8.87 62.881 5.73 0.02 3.37 2.06 5.49 8.88 62.961 5.73 0.02 3.37 2.07 5.49 8.88 63.022 5.74 0.02 3.37 2.07 5.49 8.90 63.112 5.74 0.02 3.37 2.08 5.50 8.90 63.157 5.67 0.00 3.23 1.77 5.15 8.58 60.327 5.68 0.00 3.23 1.79 5.17 8.58 60.428 5.81 0.03 3.51 2.19 5.71 9.17 65.06

12 S.-M. Kim et al. / Energy and B

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Fig. 21. Annual energy consumption under schedules (Bldg. B).

perated under schedules D, E, and F, slightly less energy wasonsumed by schedule F than schedules A, B, and C. Very sim-lar patterns of energy savings occurred in building B when theeat recovery ventilators were operated under the same schedulespplied to building A.

It appears that the savings of energy by heat recovery ventila-ors was influenced by changes in outside temperatures. The totalperation hours of heat recovery ventilators for schedules G, H,nd I were equal to the hours of operation for schedules A, B, and. However, less energy savings was achieved by schedules G, H,nd I since the particular periods of time during which the ventila-ors were in use were determined according to the preferences ofesidents [18].

Presumably, the time periods for the operation of heat recoveryentilators were preferred by residents for the sake of satisfyingeeds such as ventilation for cooking and dinning. During the peri-ds of time for the operation of heat recovery ventilators that theesidents preferred, the outside air temperatures were warmer inchedules G, H, and I than the outside air temperatures under sched-les A, B, and C. This tendency led to less energy savings underchedules G, H, and I, even though the heat recovery ventilatorsere operated for the same total amount of hours under all the

chedules.To regulate these matters of inconsistency, the heat recovery

entilators needed to be operated continuously for an entire day.his study established the continuous operation of heat recoveryentilators for periods of 12 and 24 h per day. Effective energyavings were achieved under these two operation schedules. In par-icular, 9.84% and 7.09% of total energy savings were achieved bychedule P in buildings A and B, respectively. These results implyhat the continuous operation of heat recovery ventilators wasffective for reducing energy consumption for heating and cooling,n comparison to the situation in which the heat recovery ven-ilators were not operated at all. The operation of heat recoveryentilators also maintained the recommended ventilation rates inigh-rise residential buildings.

In summary, the heat recovery ventilators effectively con-ributed to the savings of heating energy due to the effective heatxchange occurring between the air exhausted from indoors andhe air supplied from outdoors while the inside and outside airassed through the heat recovery ventilators. The heat recoveryentilators accomplished meaningful energy savings, as well asaintained recommended ventilation rates in the high-rise res-

dential buildings where natural infiltration rarely occurs due tohe tightly sealed building envelopes. The influence of heat recov-ry ventilators on the overall improvement of indoor air qualityeeds to be examined simultaneously with energy savings from

uildings 46 (2012) 3–13

heat recovery ventilators for a more complete picture. The resultsof this study do not address the indoor air quality, but rather focusprimarily on the effects of energy savings by heat recovery ventila-tors in residential buildings. It follows logically, however, that heatrecovery ventilators improve indoor air quality given that recom-mended ventilation rates were proved to be maintained by the heatrecovery ventilators.

4. Conclusions and future studies

This study was conducted to investigate the contribution ofheat recovery ventilators to energy savings in high-rise residentialbuildings. The summary of findings is as follows.

First, total energy consumption was reduced in each unit ofthe high-rise residential buildings when the heat recovery venti-lators were operated to maintain recommended ventilation ratesand comfortable temperature ranges. One important finding is thatthe heat exchange by heat recovery ventilators is most meaningfulfor energy savings when the temperature differences between theair exhausted from indoors and the air supplied from outdoors isgreat. Accordingly, the heat recovery ventilators achieved higherenergy savings during the winter, when the temperature differ-ence between the exhausted and supplied air was greater, than inthe summer.

Second, the linear prediction models developed to compare theresults of this study’s simulations and field measurements wereacceptable under a meaningful significance level. The predictionresults showed that the heat recovery ventilators contributed toannual savings of heating and cooling energy by 9.45% and 8.8%,respectively, when the ventilators were operated continuously for24 h in the high-rise residential buildings.

Third, more overall energy was saved as the operation hoursof heat recovery ventilators increased. Further, the total heatexchange type of heat recovery ventilators achieved more energysavings than the sensible heat exchange type of heat recovery ven-tilators. This finding implies that the continuous operation of heatrecovery ventilators is effective for energy savings, as well as formaintaining necessary ventilation rates since the continuous oper-ation of heat recovery ventilators met recommended standards forventilation rates in high-rise residential buildings.

Finally, the heat recovery ventilators achieved effective energysavings and maintained appropriate ventilation rates in the high-rise residential buildings where natural infiltration rarely occursbecause of tightly sealed building envelopes. The influence of heatrecovery ventilators on improving the quality of indoor air needsto be examined in conjunction with energy savings by the heatrecovery ventilators. Based on the current research, it is logicallyassumed that heat recovery ventilators improve indoor air qualitygiven that recommended ventilation rates were regularly main-tained by the heat recovery ventilators throughout this study.

The results of this study are based on field measurementsperformed within limited periods of time. Long-term field mea-surements are necessary in future research to achieve more reliableanalysis results. Although the simulated data was validated againstfield measurement data, the predicted data is limited because thesimulations contain their own algorithms for computation. Furthercomputer simulations, using various software, would be useful infuture research.

Acknowledgement

This work was supported by the National Research Foundationof Korea (NRF) grant funded by the Korea government (MEST) (No.2011-0001031).

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