Diverse heating demands of a household based on occupant control behavior of individual heating equipment

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Diverse heating demands of a household based on occupant control behavior of individual heating equipment Rongjiang Ma , Chunliu Mao , Xingli Ding , Mengsi Deng , Jill Baumgartner , Xianlin Wang , Xicheng Wang , Wei Yang , Huaican Liu , Ming Shan , Xudong Yang PII: DOI: Reference:

S0378-7788(19)31374-X https://doi.org/10.1016/j.enbuild.2019.109612 ENB 109612

To appear in:

Energy & Buildings

Received date: Revised date: Accepted date:

5 May 2019 19 October 2019 16 November 2019

Please cite this article as: Rongjiang Ma , Chunliu Mao , Xingli Ding , Mengsi Deng , Jill Baumgartner , Xianlin Wang , Xicheng Wang , Wei Yang , Huaican Liu , Ming Shan , Xudong Yang , Diverse heating demands of a household based on occupant control behavior of individual heating equipment, Energy & Buildings (2019), doi: https://doi.org/10.1016/j.enbuild.2019.109612

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Highlights 

Six LTAAHPs are installed in six rooms as the sole heating equipment for a family.



Usage of remote control for 75 days is considered as occupant control behavior.



Difference in heating control behavior of occupants in six rooms is studied.



Only power on/off, temperature set, and fan speed are frequently used by occupants.



The proposed method helps to further understand real-life heating demand.

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Diverse heating demands of a household based on occupant control behavior of individual heating equipment Rongjiang Ma a,b, Chunliu Mao a, Xingli Ding a, Mengsi Deng a, Jill Baumgartner b, Xianlin Wang a,c,d, Xicheng Wang c,d, Wei Yang c,d, Huaican Liu c,d, Ming Shan a,*, Xudong Yang a a

Department of Building Science, Tsinghua University, Beijing 100084, China

b

Institute for Health and Social Policy and Department of Epidemiology, Biostatistics and Occupational Health, McGill University, Montréal, Québec H3A 1A3, Canada

c

State Key Laboratory of Air-conditioning Equipment and System Energy Conservation, Zhuhai, Guangdong 519070, China

d

Gree Electric Appliances, Inc. of Zhuhai, Guangdong 519070, China

Abstract The traditional design principle of household heating is based on full-space and continuous heating, which does not match the true heating demands of different functional spaces and thus leads to energy wastage. In this study, we used low-temperature air-to-air heat pumps (LTAAHPs) for space heating and studied the heating demands of six different rooms by testing the control behavior of occupants using heating equipment in a rural house in Beijing, China. The diverse heating demands of the six rooms were observed for 75 days (from January 12 to March 27, 2017) and verified using field measurements: (1) the utilization rates of the LTAAHPs differ; the mean rates range from 6.0% to 58.7%, indicating that heating is not required 41.3%–94.0% of the time; (2) the desired temperature settings differ, with average temperatures ranging from 20.3 to 25.7 ℃; (3) the power inputs per unit heating area differ, with average inputs ranging from 34.2 to 97.4 W/m2 ; and (4) the energy consumption per unit heating area differs, ranging from 6.7 to 37.3 kWh/m2. These results indicate that different rooms have different heating demands, although they belong to the same household. This study helps to understand actual heating demands while saving energy based on the proper control of individual heating equipment.

Keywords Heating demand; occupant control behavior; demand setting; low-temperature air-to-air heat pump; rural household

*

Corresponding author. E-mail address: [email protected] (M. Shan). 2

1. Introduction The building sector consumes more than 30% of the total energy globally [1]. A large portion of this energy is used for space heating in residential or commercial buildings. For instance, in Organization for Economic Co-operation and Development countries, space heating accounts for 45% of the total energy consumption in buildings [2]. In China, which has made considerable progress in enforcing building energy regulations and improving the efficiency of existing buildings and the district heating network, the building sector accounts for ~20% of the total final energy consumption [3]. Furthermore, space heating alone accounts for ~35% of the total energy consumption in northern urban buildings [4] and 53.6% of the total energy consumption in northern rural residential buildings [5]. In the building heating sector, various attempts have been made to reduce the energy consumption, which can be categorized into two main aspects: 1) the reduction of the heating load through building energy-saving design such as passive solar and insulation enhancement, and 2) the improvement of the performance of heating equipment and systems, which includes the use of more efficient heating equipment, such as heat pumps with high coefficients of performance (COPs), and the reduction of the heat loss of the heating system. These measures mainly consider the technical aspects. Less attention is paid to the users’ actual heating demand level because current space heating design standards mainly focus on centralized heating and stipulate the heating mode and indoor air design conditions. For instance, the heating system of a residential building in China should be designed with the ―full-time, full-space‖ continuous heating mode and the indoor air design temperature should not be lower than 18 ℃ in severely cold and cold zones based on thermal comfort analysis [6]. Based on the strict implementation of this standard, the actual indoor temperatures in urban households with centralized heating are distinctly higher than 18 ℃ during the heating season [7]. Thus, continuous heating and uniform temperature requirements for different rooms have inevitably caused the waste of energy in rarely used rooms or in rooms with a fixed service time [8]. Therefore, it is of great significance

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and practical value to study the actual heating demand of occupants in existing decentralized heating buildings and prepare a subsequent design and operation to optimize the heating system according to the actual demand. As typical representatives of decentralized heating, rural residential buildings in northern China are ideal for studying the users’ real heating demands because of the scattered distribution and independent heating control of rural households. Several studies showed that the average indoor temperature ranges from 5 to 13 ℃ during the heating season in rural houses [9] depending on different living [10] and dressing habits [11], which confirms that it significantly differs from the centralized heating sector. However, relevant data were mainly obtained through field surveys in existing studies. In addition, these studies proposed the suitable indoor temperature ranges for the heating season in this region based on these relevant data. However, different studies yielded different results (e.g., 10–18 ℃ [10], 13–16 ℃ [9]). Furthermore, the actual heating demands of rooms with different functions were not distinguished in these studies and the original data were not based on field tests carried out during the heating season; thus, it is unclear if the results differ from the actual situation and trying to correct them is even more difficult. In recent decades, many studies have been conducted to better understand how the occupant behavior influences the energy consumption in buildings and to determine the relationship between the occupant behavior and related drivers [12]. For example, Kempton et al. [13] conducted an on-site investigation of the air-conditioning usage in eight households in the same apartment building in the United States and found that the energy consumptions of the air-conditioning units in these households significantly differ depending on the usage behavior, that is, they range from 1.2 to 1048 kWh (within 115 days). Based on a similar survey, Li et al. [14] reported that the electricity consumption per unit area of split air conditioners in different households in a residential building in Beijing varies greatly based on different control behaviors; it ranges from ~0 to 14.2 kWh/m2, with an average of 2.3 kWh/m2 in summer.

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Walker and Meier [15] observed that occupants in the same household generally prefer different temperature settings. For instance, the temperatures desired by different family members in Florida had a relatively large range (5 °C) [16]. In addition, many studies focused on the relationship between the occupant behavior and related driving factors (e.g., light switching/dimming and outdoor temperature, solar radiation [17]; light switching/dimming and schedule, daily length of use [18]; window state and indoor/outdoor air temperature, indoor/outdoor air relative humidity, CO2 concentration, solar radiation [19]; switching on of the air-conditioning device and indoor air temperature [20]; switching off of the air-conditioning device and leaving room [21]; and set point temperature adjustments for cooling and outdoor air temperature [22]). In the field of residential building heating, thermostat adjustments are affected by environmental factors such as the outdoor air temperature, indoor relative humidity, and solar radiation [23]. While these studies provided useful references for heating, they focused on air conditioning, which is very different from heating. Limited studies focusing on residential heating were carried out in buildings in which the heating system had been deployed before the study. Thus, it remains unclear whether the usually centralized heating system had a flexible and convenient control function for the users and whether the heating capacity and equipment design could meet the actual heating demand, especially in cases of intermittent instead of continuous space heating. Therefore, to increase the level of knowledge about the real-life heating demand, the occupant behavior with respect to the control of heating equipment was studied in this work and field tests were conducted in different rooms of a residence in suburban Beijing, China, to determine the occupants’ actual heating demands during the heating season based on the comparison of the utilization rates of heating equipment, occupant control settings, power inputs, and energy consumptions.

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2. Methods and materials 2.1. Test household A typical Chinese rural single-family home in the outskirts of Beijing was selected as the test household in this study. It was a one-story, detached house with a total land area of 339.9 m2. Fig. 1 shows the floor plan of the test household and the wall materials. Similar to the layout of traditional Chinese residences, such as the Beijing quadrangle [24], the main rooms of the test household are arranged around the courtyard. In addition to the courtyard, the test household has three bedrooms, one living room, one bathroom, one sun lounge, one tool room, one garage, two storage rooms, and one kitchen and dining room, with a total building floor area of 267.2 m2. The wall layers include solid brick and insulation materials with various thicknesses. The outer walls of five commonly used rooms on the northern side of the household consist of 370 mm solid brick walls and 70 mm thick exterior polystyrene granules were used as the insulation material on the northern and western outer walls. Except for the 120 mm thick brick wall in the south of the courtyard, all other interior and exterior walls are 240 mm thick. The house has a flat roof made of 100 mm thick plaster concrete and an interior polystyrene granule package with a thickness of 120 mm. All windows and the sun lounge have double panes and the floors are made of ceramic tiles and plaster concrete (40 mm thickness).

6

370 mm solid brick with 70mm exterior adhesive polystyrene granule layer 370 mm solid brick 240 mm solid brick 120 mm solid brick

Low temperature air-to-air heat pump (indoor unit)

6.35 m

HP

HP

4#

2#

2.00 m

HP

Bedroom M

2.74 m

Living room

Bedroom E

3#

1#

2.00 m

Bathroom Bedroom W

Sun lounge

6.10 m

Storeroom E Tool room

Storeroom W Courtyard

Kitchen and dining

2.90 m

HP

4.00 m

6#

6.00 m

5#

3.00 m

5.60 m

2.77 m

HP

5.60 m

2.26 m

3.00 m

HP

HP

LEGEND

Garage

6.00 m

7.67 m

4.20 m

Fig. 1. Floor plan of the test household.

2.2. Heating scheme and implementation for the test household The heating scheme of the test household should satisfy the inhabitants’ heating requirements in a flexible and easy-to-implement way. It includes the collection of the room occupancy and subjective heating needs, calculation of the heating load, and selection of appropriate heating equipment.

(1) Collection of the room occupancy and subjective heating needs The test household has many independent spaces; thus, the determination of the estimated occupancy and the inhabitant’s subjective heating requirements for each room in winter is useful for the development of a targeted heating solution. Table 1 summarizes the occupancies estimated for each room during the heating season based on detailed responses of the occupants. We identified if the room requires heating when occupied. We found that a total of six rooms must be heated when they are occupied. Bedroom E and living room are occupied every day and have notable heating requirements. Bedrooms M 7

and W, which are guest rooms, are not inhabited every day and heating is only required when guests are present. The three bedrooms and living room are mainly occupied during the nightand daytime, respectively. In addition, the bathroom requires heating every morning, noon, and evening and the kitchen and dining room is heated daily, during cooking. The remaining five rooms do not require active heating. As shown in Fig. 1, the sun lounge is the only connection between several rooms; however, this space does not require heating because the occupants do not stay in this area for a long time and it is heated by the sun during the daytime. The garage also does not require heating because it is used for a very short time per day. The tool room, storeroom W, and storeroom E are not used every day. Their usage is irregular and the duration of their use is short; thus, these rooms do not require heating. This summarized information was recognized by all residents in the test household who decided to set up active heating facilities only in six rooms, that is, bedroom E, living room, bedroom M, bedroom W, bathroom, and kitchen and dining room.

Table 1 Estimated room occupancy and heating requirements of the test household during the heating season. Room

Area (m2)

Occupied every day?

Primary occupation period

Requires heating when occupied?

Bedroom E

15.3

Yes

Nighttime

Yes

Living room

35.6

Yes

Daytime

Yes

Bedroom M

16.8

No

Nighttime

Yes

Bedroom W

15.5

No

Nighttime

Yes

Bathroom

12.7

Yes

Morning, noon, and evening

Yes

25.2

Yes

Cooking time

Yes

Sun lounge

36.7

Yes

Daytime

No

Garage

17.4

Almost

Morning and evening

No

Tool room

10.7

No

Uncertain

No

Storeroom W

24.4

No

Uncertain

No

Storeroom E

12.6

No

Uncertain

No

Kitchen and dining

(2) Calculating the heating load The basic heating load of each of the six rooms that require active heating was first calculated 8

using a continuous heating mode [6,7], as shown in the ―basic load‖ column in Table 2. Based on the communication with the occupants and their needs, they prefer an intermittent mode for heating operation rather than the continuous mode. Therefore, according to the design code recommendations [6], an additional load of 20% or 30% was added to the ―basic load,‖ yielding the ―total load.‖

Table 2 Room heating load calculated for the test household. Room

Heating load (W) Basic load

Additional load for intermittent heating

Total load

Bedroom E

1492.6

298.5

1791.1

Living room

2750.8

550.2

3301.0

Bedroom M

1298.1

259.6

1557.8

Bedroom W

1197.7

239.5

1437.2

Bathroom

1153.1

345.9

1499.0

Kitchen and dining

2969.6

890.9

3860.4

(3) Selection of appropriate heating equipment The selection of appropriate heating equipment is critical to meet the occupants’ diverse heating needs. This process is based on the following principles:  The heating capacity of the equipment meets the room heating load requirements of each room.  The air temperature of each room can be controlled independently. The users can adjust the temperature according to actual requirements.  The heating equipment has no antifreeze demand and thus an intermittent heating mode can be achieved.  The usage and adjustment information can be automatically recorded. Based on the above-mentioned principles and careful deliberation, the advanced split-type, low-temperature air-to-air heat pump (LTAAHP), which is a heating device that is commonly used and fulfills various heating requirements of rural residences in northern China [25–28],

9

was selected as the heating equipment for this household. Furthermore, to avoid problems with respect to the adaption and control of different types and/or different specifications of heating equipment, the same LTAAHP model was installed in each of the six rooms as the sole space heating equipment. Table 3 presents the main specifications of the device. The inverter LTAAHP has a wide nominal heating capacity range of 400–7500 W. At an outdoor temperature of -20 ℃, the heat output of the LTAAHP is 4000 W. Compared with the heating loads listed in Table 2, the heating equipment is oversized, which allows the users to run the LTAAHP without worrying about the heating capacity. The minimum heating capacity was set to 400 W to ensure that the equipment operates normally, even at low load rates. Each LTAAHP is equipped with an individual remote control with an on/off switch and temperature controls.

Table 3 Main specifications of the LTAAHP. Item

Parameter

Model

GN-40DZW/(40549)FNhAa-1 H(－12 ℃)

W

4000(－12 ℃)

H(7 ℃)

W

4000(7 ℃) (400–7500)

H(－20 ℃)

W

4000(－20 ℃)

Ph/V/Hz

1/200-240/50

H(－12 ℃)

W

1818(－12 ℃)

H(7 ℃)

W

1111(7 ℃) (90–2680)

H(－20 ℃)

W

2000(－20 ℃)

Maximum power input

W

2680

COP(7 ℃)/COP(－12 ℃)/COP(－20 ℃)

W/W

3.60/2.20/2.00

Sound pressure level (indoor/outdoor)

dB(A)

42/52

Heating capacity

Power supply

Heating power input

3

Airflow volume (indoor unit)

m /h

650

Refrigerant and charge quantity

kg

R32 1.25

Maximum discharge pressure

MPa

4.3

Maximum suction pressure

MPa

2.5

Maximum allowable pressure of high/low pressure side

MPa

4.3

Note: The above-mentioned product specifications were tested according to the Chinese Standard JB/T 13573-2018 [29].

After the successful commissioning of the LTAAHPs, we trained the residents with respect 10

to the equipment use in accordance with the instruction manual. At the same time, for this study to truly reflect the actual needs of room heating, the residents were instructed to follow the guidelines below when using the LTAAHPs:  As the only space heating device, the regulation of the LTAAHPs must be the only means of heating in each of the six rooms.  Each LTAAHP must be turned on/off and the settings must be regulated according to the actual needs of the occupants, that is, they must not be influenced by any other factors.  The occupants pay the electricity bill on their own. No subsidies are received from the research team, except for the upfront equipment cost. After confirming that the occupants can successfully operate the LTAAHP without sacrificing comfort [30], the research team handed over all equipment.

2.3. Data collection method Each LTAAHP was equipped with a hand-held remote control. As an integral part of the LTAAHP, the remote control was the only component regulated by the users to operate the LTAAHP. Fig. 2 shows the remote control of the LTAAHP. The remote control consists of a system monitor and control buttons. The system monitor displays the current state of the LTAAHP and the user settings. The control buttons include common control options, such as power on/off, temperature setting, and fan speed, and several advanced options such as thermal comfort and sleep modes. All current state and user setting information is collectively called operation data. With the residents’ consent, the operation data of the six LTAAHPs were collected in this study by using the data collection system shown in Fig. 3. The control signals from the user’s remote control were received by the signal receiver on the indoor unit of LTAAHP and processed. The data converter collected and converted the control and real-time status data of the LTAAHP according to the conventional format, which were transmitted via the RS-485 interface and finally stored in the data logger. The indoor temperatures of the six

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rooms and outdoor air temperatures were also recorded using two types of temperature self-recording instruments (WZY-1A and WZY-1B) and a deployment method specified the literature [26]. Furthermore, all acquisition time steps were set to 1 min.

Fig. 2. Remote control and function specifications of the LTAAHP.

Remote control Signal receiver

Data converter

Indoor unit of LTAAHP Data logger

Fig. 3. Data collection system of the LTAAHP.

2.4. Data analysis method As discussed in Section 2.2, not every room was regularly or continuously occupied. Therefore, the concept of active heating was introduced, which is defined as the time when the heating facility actually supplies heat to the space [31]. Based on the method used for the 12

identification of heating periods proposed in [32], active heating has been identified in several studies by determining if the increasing room temperature reaches a predetermined threshold over a period of time such as 0.3 ℃ in 30 min [33]. This method is mainly applied because of the lack of direct measurements of active heating in these studies. However, in this study, the switching on behavior of the user actively switching on the LTAAHP was observed using the data acquisition system. Thus, active heating is defined as the period of used of the LTAAHP in this study. The active heating input required to maintain the intended thermal comfort level in a space is defined as the heating demand, which includes the time points, duration, intensity, and quantity of active heating as well as the power input and energy consumption of the active heating system. To visually reflect the heating demands of the six rooms in the test household, we analyzed the respective situations from multiple perspectives. First, we selected the data obtained during active heating periods and analyzed and calculated the active heating periods along with the mean active heating probabilities. Furthermore, we statistically analyzed the occupant control setting behaviors and their effects during active heating periods. Subsequently, as the corresponding energy demand for heating, the real-time power input and energy consumption of each LTAAHP during active heating were analyzed using the data collected for the six LTAAHPs. In addition, analysis of variance test was adopted in this study to assess the differences in the heating demands of the six rooms. All raw data collected in this study and their primary uses are summarized in Table 4.

Table 4 Data collected in this study and their main uses. No. Item

Unit

Main use in this study

YYYY/MM/DD hh:mm

■

1

Data and time

2

Power on/off control

N/A

▲

3

Desired temperature setting

N/A

▲

4

Fan speed (including quiet mode and rapid heating mode)

rpm

▲

5

Operation mode change control

N/A

▲

6

Timer control

min

▲

13

7

Sleep mode

N/A

▲

8

Thermal comfort control

N/A

▲

9

Horizontal swing angle control

N/A

▲

10 Vertical swing angle control

N/A

▲

11 Light control

N/A

▲

12 Drying the indoor coils control

N/A

▲

13 Indoor air temperature of the six rooms

℃

△

14 Outdoor air temperature

℃

△ □

15 Power input

W

○

kWh

○

16 Energy consumption

Note: ■ represents the data used for time-related analysis; ▲ represents the data used to analyze the occupant control behavior; △ represents the data used to evaluate the actual heating effect; □ represents the data that served as references for the screening of the study period; and ○ represents the data used to analyze the energy demand and consumption.

3. Results and discussion 3.1. Test period and control behavior data The data were collected in the test household from January 11 to April 3, 2017. The outdoor air temperature during this period is shown in Fig. 4. The outdoor temperature ranged from -10.5 to 26.3 ℃ and the daily mean temperature ranged from -5.4 to 17.2 ℃. In this study, the daily mean outdoor air temperature of 10 ℃ was used as the threshold temperature of the heating season. The data remaining (consecutive data from January 12 to March 27, 2017) after the elimination of the daily mean temperature data above 10 ℃ were used for the analyses.

14

30

Temperature (℃)

20 15

2017/01/12 00:00

25

Outdoor air temperature Daily mean outdoor air temperature

10

10

5

2017/03/27 24:00

0 -5 -10

-15 2017/01/11 2017/01/25 2017/02/08 2017/02/22 2017/03/08 2017/03/22 2017/04/05

Date

Fig. 4. Outdoor temperatures during the test period.

The cumulative number of uses of various control commands during this period is listed in Table 5. In each room, the power on/off, temperature setting, and fan speed controls were most frequently used, which is common with respect to the operation of LTAAHPs. The other commands were rarely or never used during the test period.

Table 5 Cumulative count of different control items used in different rooms. Control item

Bedroom E

Living room

Bedroom M

Bedroom W

Bathroom

Kitchen and dining

Power on/off

130

78

29

40

106

75

Temperature setting

245

90

45

39

42

33

Fan speed a

356

505

131

120

39

4

1

0

0

0

0

0

0

0

0

0

0

0

Timer Others a

b

The quiet and rapid heating modes were included in the fan speed data.

b

―Others‖ includes the operation mode change, thermal comfort, horizontal swing angle, vertical swing angle, light

control, drying the indoor coils, and sleep mode.

3.2. Heating periods and durations The usage of the LTAAHPs in the six rooms during the 75-day period can be determined by analyzing the power on/off control data. The LTAAHPs in the bedroom E and living room were used for active heating every day. The numbers of days of LTAAPHP use in bedroom M,

15

bedroom W, bathroom, and kitchen and dining are 25, 28, 66, and 51, respectively. Fig. 5 shows the cumulative active heating distribution of each LTAAHP during the test period. Bedroom E, living room, and bedrooms M and W have different cumulative utilization rates at different times; however, they generally have a high usage rate in the nighttime and a low usage rate in the daytime. For example, the utilization rates of bedroom E from 10 pm to 8 am are basically maintained at a level ranging from 80% to 94.7%. In contrast, the utilization rate between 10 am and 8 pm is less than 25.0% (the lowest rate is 8.0%), with an average rate of 16.5%. The usage rates of the living room between 8 pm and 7 am are basically maintained at a level ranging from 80% to 90.7%, while the usage rates between 10 am and 6 pm are relatively lower than that during nighttime, with an average of 27.8%. The rates of the guest bedrooms M and W are much lower than that of the master bedroom (bedroom E). The maximum utilization rates are only 17.3% and 29.3%, respectively, during the nighttime and the average usage rates during the daytime between 10 am and 8 pm are as low as 4.7% and 3.7%, respectively. Completely different from the above-mentioned usage modes, the bathroom and kitchen and dining show a very similar usage trend, especially after 4 pm. The two rooms exhibited their usage peak hours and reached maximum usage rates of 46.7% and 49.3% at ~ 9 and 8 pm, respectively. Considering the functions of the two rooms, the two periods correspond to the dinner and wash times. In addition, the kitchen and dining rates obtained during the lunchtime period (10 am to 2 pm) are not as high as that of the dinner time period; however, compared with other time periods with the rates close to 0%, the usage rate is relatively high. In terms of the mean usage rate, the two most frequently used LTAAHPs are in the bedroom E and living room, with mean active heating probabilities of less than 60%, that is, 51.7% and 58.7%, respectively. Furthermore, the mean active heating probabilities of the two guest bedrooms are only 8.8% and 12.3%, respectively. That of the bathroom and kitchen and dining are as low as 6.0% and 7.0%, respectively. This means that for these six rooms, the remaining probabilities of 48.3%, 41.3%, 91.2%, 87.7%, 94.0%, and 93.0% do not require

16

active heating, respectively. Note that the occupation time (also known as occupancy or occupant presence) affects the heating device utilization. This factor was not explicitly considered in the above-mentioned analysis. However, it is implicitly considered as an outcome of the occupants’ utilization of the LTAAHP because exceptions could occur, for example, that the usage of the heating equipment does not always correspond to the occupation time. Examples of such cases include: 1) the room temperature is satisfactory and the occupants do not need to turn on the heating equipment or the stay in the room is too short (e.g., bathroom use), and 2) the preheating period is not long enough to overcome the thermal inertia of the room such that the temperature increase does not meet the expected demand of the room in which case the occupants would not bother to turn on the LTAAHP. Hence, to meet the objectives of this paper, the actual utilization rate of the LTAAHP was selected as one of the indicators of the heating demand.

17

00:00

04:00

08:00

12:00

16:00

20:00

24:00

100 80 51.7%

60 40 20

Bedroom E Mean utilization rate

0

100 80

58.7%

60 40 Living room Mean utilization rate

20 0

20

Utilization rate (%)

15 8.8%

10 5

Bedroom M Mean utilization rate

0

40 30 20

12.3%

10

Bedroom W Mean utilization rate

60

0

40 Bathroom Mean utilization rate

20

6.0%

0

60 40 Kitchen & dining Mean utilization rate

20 7.0% 0

00:00

04:00

08:00

12:00

16:00

20:00

24:00

Time of the day

Fig. 5. Cumulative distribution of the heating period in the six rooms.

Fig. 6 shows the frequency distribution of the duration of LTAAHP operation and cumulative percentage growth curve in each room during the 75-day period. On average, bedroom E is actively heated 1.7 times a day for an average of 435.8 min (~7.3 h); more than half (66 times) of the heating operation duration is less than 300 min or 5 h. More than 40% of the heating operation lasts more than 10 h (49 times = 10–15 h, 5 times = 15–24 h, 2 times >24 h), and the longest operation equals 2252 min (~37.5 h). The average active heating operation in the living room is once a day for an average of 816.0 min (13.6 h). Moreover, 28 times the duration is less than 5 h, accounting for 35.9% of the total operation. A total of 27 operation 18

durations range from 10 to 15 h, accounting for 34.6% of the total operation. The above-mentioned two parts account for a total of more than 70%. In addition, the number of times exceeding 24 h reaches 10 and the longest operation equals 5572 min (~92.9 h). Bedroom M is, on average, actively heated 0.4 times a day for an average of 328.6 min (~5.5 h); in addition, more than half of the heating periods (15 times) last less than 2 h. A total of eight operations last from 5 to 10 h, accounting for 27.6% of the total operation. Furthermore, the longest operation equals 1367 min (~22.8 h). Bedroom W is actively heated 0.5 times a day for an average of 504.7 min (~8.4 h) and 19 heating periods (47.5%) last less than 5 h; 13 periods last from 5 to 10 h, accounting for 32.5% of the total operation. The above-mentioned two parts account for a total of 80%. The number of operations over 24 h is 2 and the longest operation equals 4279 min (~71.3 h). The average active heating operation in the bathroom is 1.4 times a day for an average of 63.3 min (~1.1 h). 69 times of operation duration are less than 1 h, accounting for 65.1% of the total operation. In total, 21 heating periods range from 1 to 2 h, accounting for 19.8% of the operation, and 14 periods range from 2 to 5 h, accounting for 13.2% of the total operation. The total operation of the above-mentioned three parts reaches up to 98.1%. Furthermore, the longest operation equals 468 min (7.8 h). The kitchen and dining areas are actively heated once a day for an average of 101.3 min (~1.7 h) and 28 periods (37.3%) last less than 1 h. 25 heating periods range from 1 to 2 h, accounting for 33.3% of total heating operation, and 19 periods range from 2 to 5 h, accounting for 25.3% of the operation. The sum proportion of the above-mentioned three parts reaches up to 96.0%. Furthermore, the longest

100

4

80

Frequency (Bedroom E) Cumulative percentage (Bedroom E)

3

60

2

40

1

20

0 0

300

600

900

1200

1500

1800

2100

5

100

4

80

Frequency (Living room) Cumulative percentage (Living room)

3 2

40

1

20

0

0 2400

0 0

Operation duration (min)

600

1200

1800

2400

3000

3600

Operation duration (min)

19

60

4200

4800

5400

Cumulative percentage (%)

Frequency (times)

5

Cumulative percentage (%) Frequency (times)

operation equals 421 min (~7.0 h).

80

Frequency (Bedroom M) Cumulative percentage (Bedroom M)

60

2

40

1

20

0 0

120

240

360

480

600

720

840

960

1080

1200

1320

0 1440

100

4 3

40

1

20

0

0 0

600

1200

60

2

40

1

20

0 60

120

180

240

300

2400

3000

3600

4200

360

420

5

Frequency (times)

80

Frequency (Bathroom) Cumulative percentage (Bathroom)

Cumulative percentage (%)

Frequency (times)

100

0

1800

Operation duration (min)

5

3

60

2

Operation duration (min)

4

80

Frequency (Bedroom W) Cumulative percentage (Bedroom W)

0 480

100

4

80

Frequency (Kitchen & dining) 3

Cumulative percentage (Kitchen & dining)

2

40

1

20

0

0 0

Operation duration (min)

60

Cumulative percentage (%)

3

5

Cumulative percentage (%)

100

4

Cumulative percentage (%) Frequency (times)

Frequency (times)

5

60

120

180

240

300

360

420

Operation duration (min)

Fig. 6. Frequency distribution of the heating operation duration and cumulative percentage growth curve for each of the six rooms during the 75-day period.

3.3. Occupant control settings and effects In this section, we analyze how the occupants control the LTAAHPs during active heating sessions. As discussed in Section 3.1, except for the power on/off command, the temperature setting and fan speed are the most frequently used remote control commands in the test household. The temperature settings include 15 optional values ranging from 16 to 30 ℃. Fig. 7 shows the temperature settings in each room during the active heating period. The temperatures in bedroom E mainly range from 19 to 25 ℃ (94%, the percentage in parentheses represents the proportion of the mentioned setting of all settings). The temperature distribution from 20 to 24 ℃ (84%) is well-proportioned; the ratio of each temperature ranges between 14% and 19%. The average temperature of all settings is 22.0 ℃. The desired temperatures in the living room mainly range from 18 to 20 ℃ (72%) and from 22 to 24 ℃ (23%); the total proportion of the two ranges is 95%. The proportion of the temperature of 20 ℃ is as high as 55%. The average temperature of all settings is 20.3 ℃. The main temperature in bedroom M is 20 ℃ (28%), with a range from 22 to 25 ℃ (66%); the total proportion of the two settings is 94%. The proportion of the temperature of 23 ℃ is as high as 49%. The average temperature of all settings is 22.0 ℃.

20

The desired temperatures in bedroom W are 20 ℃ (31%), 22 ℃ (40%), and 24 to 26 ℃ (24%), with a total proportion of 95%. The average temperature of all settings is 22.1 ℃. The main temperatures in the bathroom are 17 ℃ (8%), 20 ℃ (3%), and 24 to 30 ℃ (85%), accounting for a total proportion of 96%. The average temperature of all settings is 25.7 ℃. The temperatures in the kitchen and dining are 20 ℃ (8%), 23 to 28 ℃ (74%), and 30 ℃ (17%); the total proportion of the three parts reaches up to 99%. The average temperature of all settings is 25.7 ℃. Considering the average temperature settings, the six rooms can be divided into three levels: the first level consists of the living room, with the lowest average temperature of 20.3 ℃; the second level consists of the three bedrooms, with a moderate average temperature of ~22.0 ℃; the bathroom and kitchen and dining, with a relatively low usage rate, belong to the third level with the highest average temperature of 25.7 ℃.

100

24℃ 3% 23℃ 8%

Temperature setting and proportion

25℃ 5% 90 24℃ 14%

22℃ 12%

80 70

25℃ 5% 24℃ 4%

23℃ 18%

26℃ 5%

30℃ 7%

25℃ 9%

29℃ 11%

30℃ 17%

24℃ 10%

28℃ 13%

23℃ 49%

28℃ 18% 27℃ 3%

60 50

27℃ 22% 26℃ 17%

22℃ 40%

22℃ 19%

20℃ 55% 26℃ 10%

40

25℃ 3%

22℃ 8% 21℃ 17%

30 20

25℃ 19% 19℃ 4%

20℃ 16% 10

20℃ 28%

20℃ 31%

18℃ 13% 19℃ 5%

0

roo

m

E

m

oo gr

om

M

dro Be

in Liv

d Be

o dro Be

m

W

24℃ 21%

24℃ 3% 20℃ 3%

23℃ 12%

17℃ 8%

20℃ 8%

m ng oo ini thr &d Ba n e ch Kit

Fig. 7. Distribution of the active heating duration at different temperature settings in the six rooms. In the figure, 100

Fan speed grade and proportion (%)

the label is hidden if the column height is below 3%.H 90

H 25%

5%

M 22%

80

H 33%

H 52% H 56% of the indoor unit M 5% During active heating, fan speed of the LTAAHP should operate 70 the H 67%

L 4% 60

SL 29%

SL 10%

M 16%

strictly according to the desired fan speed set by the user. However, when the indoor 50 ML 9% 40

M 7%

MH 26% temperature reaches (or30slightly exceeds) the temperature set by the user, the fan in the indoor L 19% L 14%

Off 52%

MH 12%

Off 43%

20

unit stops running and enters the standby SL 11% mode (speed of 0 rpm). When the LTAAHP detects Off 22% L 7%

10

Off 13%

Off 9%

SL 13%

0

that the indoor temperature is lower than the desired temperature, the fan starts to operate again roo

d Be

m

E

m

oo gr

in Liv

om

dro Be

M

roo ed

m

W

thr Ba

m oo

& en

i din

ng

based on the speed set by the user. Table 6 listsB the distribution ch of the active heating durations Kit 21

at different fan speeds for the six rooms. During the 75-day period, the fans were not operated at a superhigh speed. The different rooms show different fan speed distributions. The specific distribution of each room is not presented in detail because of the limited space.

Table 6 Distribution of the heating duration at different fan speeds for the six rooms. Fan speed

Bedroom

Living

Bedroom

Bedroom

(rpm)

E

room

M

W

750

0.0%

0.0%

0.0%

0.0%

0.0%

0.0%

High

650

25.4%

56.3%

5.1%

33.2%

52.4%

67.4%

Medium–high

600

0.7%

2.0%

0.0%

0.0%

26.3%

11.9%

Medium

550

5.1%

6.7%

22.5%

16.3%

0.7%

2.7%

Medium–low

500

2.6%

0.9%

0.0%

8.8%

0.0%

0.0%

Low

450

4.2%

14.2%

0.4%

18.6%

7.2%

2.2%

Superlow (quiet mode)

350

9.8%

11.2%

28.8%

1.2%

0.0%

13.0%

0

52.3%

8.6%

43.3%

21.9%

13.3%

2.8%

Fan running grade Superhigh (rapid heating mode)

Off (automatic standby mode)

Bathroom

Kitchen and dining

Based on the above-mentioned analysis, the usage period, duration, and control settings of each room differ. Even in the same room, the setting varied and users often used different combinations based on their needs, leading to different effects. Fig. 8 shows the settings and operation states of the LTAAHPs in the living room and kitchen and dining are between 5:30 and 9:30 pm on January 18, 2017. The fan speeds of both LTAAHPs are high (650 rpm) during the operation; however, the temperatures desired in the living room and kitchen and dining area are 20 and 26 ℃, respectively. The indoor temperature in the living room is 17 ℃ at the start of the heating and reaches 20 ℃ after 65 min of heating. The average temperature change rate is 2.8 ℃/h. In contrast, within 123 min of heating in the kitchen and dining area, the indoor temperature increases from 3.4 to 17.6 ℃; the average temperature change rate is 6.9 ℃/h. Fig. 9 shows the settings and operation states of the LTAAHPs in the bedrooms E and M between 7 pm and midnight on February 17, 2017. During the heating operation, the fan speeds in the bedrooms E and M are 650 and 350 rpm, 22

respectively. At the same desired temperature of 23 ℃, the performances of the LTAAHPs in these two rooms differ. Within 27 min of heating, the indoor temperature in bedroom E increases from 18.2 to 23.0 ℃, with an average temperature change rate of 10.7 ℃/h. In contrast, the indoor temperature in bedroom M increases from 14.5 to 21.4 ℃ within 70 min, with an average temperature change rate of 5.9 ℃/h. The above-mentioned two examples show that different settings have different effects on the heating performance. It can be concluded that the higher the desired temperature/fan speed is, the more rapid is the heating.

30

Temperature (℃)

25

26 (2017/01/18 18:01, 17.0℃)

(2017/01/18 19:06, 20.0℃)

20

20

15

Desired (Living room) Indoor/OFF (Living room) Indoor/ON (Living room)

10 5

(2017/01/18 20:55, 17.6℃)

(2017/01/18 18:52, 3.4℃)

Desired (Kitchen & dinning) Indoor/OFF (Kitchen & dinning) Indoor/ON (Kitchen & dinning) Outdoor air

Fan speed (rpm)

0 -5 17:30 800

18:00

18:30

19:00

19:30

20:00

20:30

21:00

21:30 650

600 400 200 0 17:30

Living room Kitchen & dinning

18:00

18:30

19:00

19:30

20:00

20:30

21:00

21:30

Time

Fig. 8. Settings and operation states of the LTAAHPs in the living room and kitchen and dining are between 5:30 and 9:30 pm on January 18, 2017.

23

30 (2017/02/17 21:00, 21.4℃)

(2017/02/17 21:25, 23.0℃)

Temperature (℃)

25 23 20 (2017/02/17 20:58, 18.2℃)

15 (2017/02/17 19:50, 14.5℃) 10 5

Desired (Bedroom E) Indoor/OFF (Bedroom E) Indoor/ON (Bedroom E)

Desired (Bedroom M) Indoor/OFF (Bedroom M) Indoor/ON (Bedroom M) Outdoor air

Fan speed (rpm)

0 19:00 19:30 20:00 20:30 21:00 21:30 22:00 22:30 23:00 23:30 00:00 800 650

600 400

350

200

Bedroom E Bedroom M

0 19:00 19:30 20:00 20:30 21:00 21:30 22:00 22:30 23:00 23:30 00:00

Time

Fig. 9. Settings and operation states of the LTAAHPs in the bedrooms E and M between 7 pm and midnight on February 17, 2017.

Table 7 statistically summarizes the average desired temperature and the corresponding room temperatures in the six rooms during the active heating period. The average desired temperatures and average indoor temperatures in the beginning of active heating differ. However, the temperature differences gradually decrease with the heating. During the entire active heating period, the average indoor temperatures in bedroom E, living room, bedroom M, and bedroom W reach the respective average desired temperatures. In the bathroom and kitchen and dining area, the average indoor temperatures and desired temperatures remain different because of the short heating duration; however, in contrast to the initial values, the increase in the average indoor temperature varies. As confirmed by the occupants, the thermal demands were met in all six rooms during the investigation period, including the bathroom and kitchen and dining area. The occupants explained that they turned on the heating equipment in advance, extended the heating duration, or increased the desired values if they were dissatisfied. At the same time, they also mentioned that they felt warm enough during the active heating period, which caused them to adjust the temperature to a lower desired value during the process. This explains why the average desired values of the six rooms during the active heating period decreased by 0.1–1.0 ℃ compared with

24

the initial average values (Table 7). Therefore, it can be concluded that the occupants used different settings to reach different heating effects in the six rooms and thus successfully achieved their thermal goals.

Table 7 Comparison of the desired and indoor temperatures for each of the six rooms. Periods of Average desired temperature (℃) Room

Average indoor temperature (℃)

active

Start

heating

heating

heating

heating

heating

Bedroom E

130

22.7

22.0

19.1

22.3

Living room

78

21.3

20.3

14.9

19.9

Bedroom M

29

22.2

22.0

17.3

22.2

Bedroom W

40

22.9

22.1

14.2

21.7

Bathroom

106

26.2

25.7

15.8

20.5

25.8

25.7

6.6

13.2

Kitchen and dining 75

of

active During

active Start

of

active During

active

3.4. Power input and energy consumption of the LTAAHPs The control of the heating equipment by the occupants is not only reflected by the heating performance [34] but also by the power input and energy consumption [35]. Fig. 10 shows the power input per unit area for each room for the 75-day period and respective LTAAHP. The thick black curves represent the average power input for each room at different times. The power input of bedroom E ranges from 14.9 to 79.8 W/m2, with an average of ~44.3 W/m2. The power input of the living room ranges from 23.1 to 45.9 W/m2, with an average of ~34.2 W/m2. The power input of bedroom M ranges from 0.7 to 81.6 W/m2, with an average of ~34.3 W/m2. The power input of bedroom W ranges from 1.0 to 129.3 W/m2, with an average of ~56.6 W/m2. The power input of bathroom ranges from 0.3 to 214.2 W/m2, with an average of ~97.4 W/m2. The power input of the kitchen and dining area ranges from 0.5 to 107.0 W/m2, with an average of ~80.8 W/m2.

25

00:00

04:00

08:00

12:00

16:00

20:00

24:00

200 Bedroom E 100

0

100 Living room

Power input per unit heating area (W/m2)

50

0 200 Bedroom M 100

0

200 Bedroom W 100

0 200

Bathroom

100 0 Kitchen & dining

100 50 0

00:00

04:00

08:00

12:00

16:00

20:00

24:00

Time of the day

Fig. 10. Power input per unit heating area for the active heating period. In the figure, the light color curves represent the real-time power input per unit area for each day and the black curves represent the average power input at each time.

The energy consumption in the six rooms during the 75-day period was calculated, as shown in Fig. 11a. The total energy consumption of the test household was 2679.2 kWh. The energy consumption in bedroom E was 553.2 kWh, accounting for 20.6% of the total consumption. The energy consumption in the living room was the highest (1328.3 kWh), accounting for 49.6% of the total consumption. The smallest energy consumption was determined for bedroom M (consumed 112.3 kWh), accounting for 4.2% of the total amount. The energy consumption in

26

bedroom W was 235.2 kWh, accounting for 8.8% of the total amount. The energy consumption in the bathroom was 195.1 kWh, accounting for 7.3% of the total consumption. The energy consumption in the kitchen and dining area was 255.1 kWh, accounting for 9.5% of the total heating consumption. To eliminate the impact of the room size on the consumption, we plotted the energy consumption per unit area for the six rooms in Fig. 11b. Among the six rooms, bedroom E and the living room consumed more energy (36.2 and 37.3 kWh/m2, respectively) than the other four rooms because of the higher utilization than in the other four rooms. The energy consumption in bedroom M, bedroom W, bathroom, and kitchen and dining area was 6.7, 15.2, 15.4, and 10.1 kWh/m2, respectively, within the 75-day period. Furthermore, by dividing the cumulative energy consumption per unit area into a 24 h period (Fig. 11c), it can be seen that the six rooms generally consumed more heat at night compared with the daytime and there are different degrees of consumption differences among the six rooms at different times. These differences can be explained based on the utilization rates (Fig. 5) and power inputs (Fig.

1600

Energy consumption per unit heating area (kWh/m2)

Energy consumption of LTAAHP in each room (kWh)

10) of the corresponding time periods.

Proportion of the energy consumption of the LTAAHP in each room

1400

1328.3

Bathroom 7.3%

Kitchen & dining 9.5%

Bedroom E 20.6%

1200 Bedroom W 8.8% Bedroom M 4.2%

1000 800 600

Living room 49.6%

553.2

400 235.2 200

255.1 195.1

112.3

0

om dro Be

E

m oo

L

gr ivin

om dro Be

M

om dro Be

W

g m oo inin thr &d Ba en h c Kit

40 37.3

36.2 35 30 25 20

15.4

15.2

15

10.1

10 6.7 5 0

om dro Be

E ing Liv

m roo

om dro Be

M

om dro Be

W

m oo

thr Ba

K

& en itch

(a)

(b)

27

ing

din

Hourly energy consumption of the LTAAHP per unit heating area (kWh/m2) 4

Bedroom E Living room Bedroom M Bedroom W Bathroom Kitchen & dining

3

2

1

0 0

2

4

6

8

10

12

14

16

18

20

22

Hour of the day

(c) Fig. 11. Energy consumption during the 75-day period. Cumulative energy consumption of the LTAAHP (a) in each room and (b) per unit heating area of each room, and (c) cumulative hourly energy consumption of the LTAAHP per unit heating area for the six rooms. Each point in the graph represents the cumulative energy consumption for one hour. For example, the number 0 below the abscissa represents the cumulative energy consumption in the time period of [00:00, 01:00).

3.5. Discussion and limitations The above-mentioned results demonstrate the different occupant behavior with respect to the control of the LTAAHPs in six rooms, yielding different heating performances, power inputs, and energy consumptions. To assess these differences, an analysis of variance (ANOVA) test was performed to compare the heating demand data of six rooms with the initial hypothesis testing of equal variances of the usage rates, power input, and energy consumption, respectively. Based on the use of the Levene’s test, these homogenous assumptions of the variances were rejected at the 0.05 level. In addition, the Welch test was used to examine the differences in the cumulative active heating rates, power inputs per unit area, and hourly energy consumptions per unit heating area. The results show that the population means of the three tests significantly differ at the 0.05 level. This means that at least two of the six rooms have different mean usage rates, power inputs, or energy consumptions. To determine which differences are significant, the Games–Howell test, a post-hoc test (that does not assume equal variance) was employed to compare the means of the six rooms. Furthermore, the test was repeated using cumulative active heating rate data, power input per unit area data, and hourly energy consumption per unit 28

heating area data, respectively. Fig. 12 illustrates the heating demand including the standard error for the six rooms and Fig. 13 shows the pairs for which the difference of the means are insignificant in the three tests, where the p-values are greater than the significance level of 0.05. With respect to the cumulative active heating rates (Figs 12a and 13), only the bathroom and kitchen and dining areas are paired and have the same usage rate level (~55.2%). The remaining four rooms have statistically different usage rates, ranging from ~6.0% to 12.3%. With respect to the power input per unit area (Figs 12b and 13), only the living room and bedroom M are paired and have the same power input level (~34.3 W/m2). The remaining four rooms have statistically different power inputs, ranging from 44.3 to 97.4 W/m2. The last test of the hourly energy consumption per unit heating area (Figs 12c and 13) yielded similar interpretations. Note that, statistically, the utilization rate, power input, and energy consumption levels of all rooms differ. This means that from a statistical point of view, the heating demands of each room significantly differ, although the rooms are in the same household.

65

120

Power input per unit heating area (W/m2)

Utilization rate (%) 60

Hourly energy consumption per unit heating area (kWh/m2)

2.0

100

55

80

50 20

60

15

40

1.5

1.0

10

0.5 20

5 0

0

d

m roo

Be

E in

Liv

oo gr

m Be

om dro

M dro

om

Be

W

m ng oo ini thr &d Ba en h c Ki t

0.0

m roo

E in

d

Be

Liv

oo gr

m

om

M

dro

dro

Be

Be

27

(a)

om

W

m ng oo ini thr &d Ba en h c Ki t

om

dro

Be

E

m

Liv

ing

roo

om

dro

Be

(b)

M

om

dro

Be

om

W

o thr

Ba

n&

i din

ng

e ch Ki t

(c)

26

Desired temperature (℃)

Fig. 12. Heating demand including the25standard error for the six rooms. (a) Mean utilization rate, (b) mean power 24 input per unit heating area, and (c) hourly mean energy consumption per unit heating area. 23 22 21 20 1 0

d Be

roo

m

E ing

Liv

roo

m d Be

r oo

m

M d Be

roo

m

29

W th Ba

r oo

ing

m

e ch Kit

n&

din

Fig. 13. Pairs with insignificantly differing mean heating demands among the six rooms. The red line represents the mean utilization rate pair; the yellow line represents the mean power input per unit heating area pair; and the green lines represent the hourly mean energy consumption per unit heating area pairs. The means of rooms at the ends of a line are insignificantly different. The thicker the line is, the larger is the p-value and vice versa.

Notably, different heating demands were met by different control settings such as different temperature settings. Based on the above-mentioned results, not only the temperature settings for each room were different but also the usage probability and fan speed settings. At the same time, manually varied settings according to the diverse demands were used in all six rooms. The research goals of this study were mainly achieved because all controls can be flexibly and independently regulated by the occupants. Considering the area, the heating using the LTAAHP can be regarded as a constant pressure process and the supplied heat can be expressed using the following equation: ∫ (

(

))

,

(1)

where Q is the supplied heat in J, ρ is the density of the indoor air in kg/m3, cp is the specific heat capacity at constant pressure in J/(kg·℃), G is the indoor air volume flow rate in m3/s, T2 is the air temperature at the outlet of the indoor unit of LTAAHP in ℃, T1 is the air temperature at the inlet of the indoor unit of the LTAAHP in ℃, and τ is the time of the heating operation in s. The density and specific heat capacity are inherent properties of air and their changes during the heating season are negligible. The remaining parameters directly or indirectly affect the amount of supplied heat. Fortunately, these parameters are directly or indirectly controlled by the LTAAHP. First, the fan speed controls the amount of air that flows through the heat 30

exchanger in the indoor unit of the LTAAHP. The higher the fan speed is, the more air moves through the exchanger and the more heat is pumped out of the space. This means that the fan speed setting directly controls the indoor air volume flow rate G. Second, the desired temperature setting of the LTAAHP affects the speed of the compressor. The higher the desired temperature is, the faster the compressor turns and the more heat is pumped out based on the increase in the temperature of the supplied air. This means that the desired temperature setting directly affects T2. Third, the time of the heating operation, τ, depends on the power on/off control of the LTAAHP. Finally, the last parameter, T1, is equivalent to the indoor air temperature, which is affected by the three above-mentioned controls during the active heating period. Overall, the three types of separate controls, that is, power on/off control, desired temperature setting, and fan speed setting, control all manageable parameters during the active heating by the LTAAHP. Moreover, the three types of controls are the controls that are most commonly used by the residents in each room. Consequently, the method proposed as well as the heating equipment employed in this study expand our understanding of the heating demands of different rooms used by a family. The heating behavior in rural areas is governed by many factors such as the layout and insulation of residential buildings, local meteorological conditions, energy type for heating, performance of the heating equipment, and living and dressing habits and income of the occupants. Based on the combination of our results with those of previous research [5,11,25,26,27], the heating mode in rural areas is the ―part-time, part-space‖ intermittent heating mode, which differs from the ―full-time, full-space‖ continuous heating mode in urban areas. Hence, special attention should be paid to the actual heating demand and control under ―part-time, part-space‖ intermittent heating conditions in future studies. The housing design as well as heating equipment selection should also be considered to save energy and understand and meet the real needs of rural occupants. This pilot study has the following limitations:

31

(1) As mentioned earlier, the LTAAHP is operated via a remote control with more than ten control functions. However, in this study, only three controls were frequently used by the occupants; other controls were rarely or never used. Based on a survey conducted after the field measurement period, the residents appreciated the heating performance of the LTAAHP. However, they complained about the remote control. For example, the buttons and fonts were too small to be quickly and easily identified and manipulated; some abbreviations, terminology, and symbols were difficult to understand and too complicated to use. Furthermore, it lacked feedback regarding the controls such as the display of the real-time indoor temperature on the screen. These issues did not directly affect the research findings and conclusions of this study; however, they should be addressed in future research. (2) Because of continuous improvements in the heating performance [5,26,27], the indoor temperature could quickly be reached and continuously maintained by using a LTAAHP. Therefore, the users could enjoy the warm room environment during the cold winter. To eliminate problems related to users adapting to different types of equipment, the same type of LTAAHP was used in different rooms in this study. The residents were very satisfied with the general heating performance of the LTAAHP. However, detailed thermal comfort studies were not conducted during the data collection period. It was assumed that the thermal comfort of the occupants was ensured because they were in control. In addition, the effectiveness of each active heating period was not analyzed in this study and unnecessary heating periods were not screened out because of the lack of real-time occupancies of each room. However, the diverse heating demands for different rooms were demonstrated based on the manual adjustments made by the occupants using the remote control.

4. Conclusions In this study, we employed LTAAHPs as the sole means of space heating of six rooms in a selected rural residential household in Beijing. Detailed field measurements of the control

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behavior of the occupants using the LTAAHPs were performed to investigate the heating demand of the six rooms. The monitoring data obtained during 75 days from January 12 to March 27, 2017, verify the diverse heating demands of different rooms. The following conclusions can be drawn:  The utilization rates of the LTAAHPs in the six rooms differ. The mean rates vary from 6.0% to 58.7%, which indicates that heating was not required in 41.3%–94.0% of the time.  The desired temperature settings of the six rooms differ. However, they can be divided into three levels. The first level consists the living room, with an average temperature of 20.3 ℃; the second level consists of the three bedrooms, with a moderate average temperature of ~22.0 ℃; and the third level consists of the bathroom and kitchen and dining area, with a relatively low usage rate and the highest average temperature of 25.7 ℃.  The power inputs per unit heating area in the six rooms differ; the average input ranges from 34.2 to 97.4 W/m2.  The energy consumption per unit heating area in the six rooms differ; it ranges from 6.7 to 37.3 kWh/m2.  Diversity exists in the different operation durations, different fan speeds, and different indoor temperatures during the active heating period. Irrespective of the small sample size of this study, the detailed descriptions of the heating demands in different rooms indicate that different rooms have different heating demands, although the rooms belong to the same household. The proposed method helps to understand the real-life heating demand while saving energy based on the efficient control of individual heating equipment.

Conflicts of Interest The authors declare no conflict of interest.

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Acknowledgements This study was supported by the State Key Laboratory of Air-conditioning Equipment and System Energy Conservation (Grant No. ACSKL2018KT1201); Beijing Science & Technology Program (Grant No. Z181100005418001 and Z181100005418005); National Science and Technology Pillar Program during the Thirteenth Five-year Plan Period (Grant No. 2018YFD1100702); and Innovative Research Groups of the National Natural Science Foundation of China (Grant No. 51521005). The authors are grateful to all occupants of the test household.

AUTHOR DECLARATION We wish to draw the attention of the Editor to the following facts which may be considered as potential conflicts of interest and to significant financial contributions to this work. We confirm that the manuscript has been read and approved by all named authors and that there are no other persons who satisfied the criteria for authorship but are not listed. We further confirm that the order of authors listed in the manuscript has been approved by all of us. We confirm that we have given due consideration to the protection of intellectual property associated with this work and that there are no impediments to publication, including the timing of publication, with respect to intellectual property. In so doing we confirm that we have followed the regulations of our institutions concerning intellectual property.

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