End user engagement with domestic hot water heating systems: Design implications for future thermal storage technologies

The strategies used by householders for heating and using hot water heating have a signi ﬁ cant impact on energy consumption in domestic buildings. A better understanding of the interaction between occupants and hot water heating systems can improve the energy e ﬃ ciency of a building. This paper maps the interaction between occupants and their current domestic hot water heating systems to provide insights for the design of future thermal energy storage systems. A total of 35 householders from the Midlands region of the UK took part in semi- structured contextual interviews about their current strategies for the provision of hot water and the way they engage with their heating systems. Using the DNAs framework as an analysis lens, drivers, needs and actions relating to the provision of hot water were evaluated and four distinct hot water heating types are presented: On Demand, For All Eventualities, Just Enough and Sunny Days . Findings provide insights into occupants ’ behaviour in relation to hot water heating usage and design implications for thermal energy storage technologies.


Introduction
In the UK, 29% of the energy use is attributed to the domestic buildings [1]. One of the contributors to this energy consumption is the demand for domestic hot water, making up 20% of the domestic consumption; approximately 6% of the total UK energy use. Considering the UK's target of reducing the greenhouse gas emissions by at least 80% by 2050 [1], innovation in the design and deployment of future energy technologies is essential. These technologies include thermal energy storage systems. Whilst phase change or thermochemical storage may be a technology for the future, many homes have a current thermal energy store in the form of a hot water tank to supply domestic hot water. However, there is a lack of understanding about the interactions between occupants and these systems and limited knowledge on potential barriers to adopting future thermal energy stores, which will be needed in order to design future socially, technically and economically viable systems. This paper aims to fill this gap by identifying occupants' engagement with current domestic hot water systems in order to determine user insights for the design of future thermal energy stores.
Currently, a number of technologies are proposed to help reduce emissions from the domestic sector in the UK, with a focus on electrification of heating including the deployment of electric heat pumps within homes [2]. This will increase the demands placed on the production and supply of electricity, with the additional complication of significant variances in demand across seasons to match the temperate maritime climate in the UK. This will inevitably require demand side response (shifting) and local generation of electricity. To support these two developments, energy storage will become an important feature [3]. Electrical and thermal energy storage could mitigate the need for new energy plants and their associated costs. Distributed thermal stores are already present in 13.7 m UK households [4] as hot water storage cylinders; a 100 L cylinder, with water heated to 50°C, can store about 6 kW h. Measurements have found average hot water use to be 122 litres/day with an energy content of 4.7 kW h which, when aggregated across the UK, equates to about 65 GW h/day [4]. Thermal stores offer a means of decoupling supply from demand and differ from traditional hot water cylinders as they present a means of storing energy from multiple energy generating sources and could enable householders to store heat for later use over a period of hours, days, weeks or even months [5]. Thermal energy stores enable householders to take advantage of variable pricing of electricity during peak and off-peak hours where available and, for occupants with home energy generation technologies (which provide intermittent energy generation at less controlled times) such as solar thermal and photovoltaic (PV) systems and/or heat pumps, thermal storage systems can improve the overall efficiency [6]. Interactions with energy systems depend heavily on occupants' perceived comfort [7], however studies often focus on the average occupant behaviour, neglecting individual variability [8]. Misconceptions regarding domestic heating systems and their contribution to global warming [9], the required energy to heat water [10] and the benefits of using different heating systems in context [11] can affect the energy consumption [12,13]. It has been argued that occupants focus on those energy-related behaviours that impact positively or negatively on their personally held values, including motivational values relating to self-enhancement and self-transcendence [14,15]. Self-enhancement values may drive an individual to gain personal pleasure and excitement (hedonism) or to safeguard an individual's resources (egoism). Self-transcendence values relate to ensuring the welfare of others (altruism) or the environment (biospherism). The values reflect occupants' inner motivations that lead to subsequent judgements and actions [11]. The use of a hot water heating system within a home is influenced by these values through the provision of hot water for oneself or others in the household. People's subsequent choices and actions are also impacted by the available technology [16]. Therefore, occupants' heating strategies may counteract the efficient use of a thermal energy storage system, if they are motivated by personally held values. In order for a technology to be utilised effectively within real-world contexts, developers need to ensure that the drivers and motivations behind end user behaviour are considered in the design.
Previous research has focused on simulating the impact of occupant preferences on building energy consumption using either building simulation tools or occupant modelling [17][18][19]. However, a common finding across energy simulation studies is the discrepancy between the predicted and actual energy consumption of the building [20][21][22]. This discrepancy is explained by the fact that extant knowledge on the effectiveness of the implementation of thermal energy storage technologies is based on statistical analyses and modelling [23], lacking insights regarding occupants' behaviour when planning new domestic buildings as well as when retrofitting [8]. A recent review that synthesised existing evidence on the influence of occupants' behaviour on building energy consumption highlighted the gap of knowledge in the field and the need for more relevant research to be undertaken [24] and others have identified that human behaviour and occupant preferences are the most overlooked factors when it comes to energy consumption in domestic buildings [25]. To address this gap, this paper takes a user centred design approach to ensure that context of use is considered and focuses on occupants' behaviour and their engagement with their hot water heating systems [26]. This approach is in contrast to a technology-led design process which focuses on the development of the capabilities of the technology itself as an isolated object; such a process often results in products, services and systems that require users to adapt their attitudes and behaviours in order to learn and use them effectively [27]. In this research, in order to explore occupant interactions with the domestic hot water system, both semi-structured interviews and use of behavioural frameworks are employed to identify occupants' needs and requirements.
A number of theoretical cognitive-behavioural frameworks 1 have attempted to categorise the different types of occupant-energy interactions with a focus on the role of social human behaviour in energy consumption. However, these frameworks are mainly concerned with the stochastic and reactive nature of human behaviour, in a specific space as a function of time. One recent framework has attempted to capture key elements influencing occupants' energy behaviour: the DNAs occupant behaviour framework [28]. The DNAs framework suggests that four main components underpin the relationship between occupants' behaviour and energy in buildings: the drivers of behaviour, the needs of the occupants, the actions carried out by the occupants and the systems acted upon by the occupants. The framework was developed to implement fast and accurate occupant behaviour models into building simulation tools and reduce the gap between the predicted and measured energy performance of buildings. Drivers provoke energyrelated occupant behaviour; occupants' needs for feeling satisfied with the environment should be met and, if not, certain actions should be taken to satisfy their expectations by interacting with the building's systems (e.g. equipment, mechanisms or measures). The framework was developed with the purpose of use in building performance simulation programs to improve simulation assumptions on occupancy presence and adaptive interactions [29], but it provides a lens through which to consider occupants' direct interaction with hot water heating systems. This paper draws upon the DNAs occupant behaviour framework [28] to understand the interaction of occupants with their hot water heating systems in the UK. This framework takes the cognitive processes of users' energy-related behaviour and structures actions and thoughts into a format that can be of value to the technology designers. This bridging role has been identified in literature as an important step to understanding occupant behaviour in the context of engineering solutions [30,31].
This paper adopts a novel, multimethod approach, combining insights gained from semi-structured interviews with 35 householders in the East Midlands area of the UK and the use of DNAs occupant behaviour framework [32]. The study makes an original contribution to knowledge in three important ways. First, it introduces an innovative methodology to explore occupant interaction with hot water heating systems using a bespoke hot water timeline tool that encourages participants to talk about hot water usage. Second, a typology of hot water heating strategies is developed based on reported hot water usage and occupants' drivers, needs and actions underpinning the interaction between the occupant and the hot water heating system. Finally, it provides insights on user requirements for future thermal energy stores and adds to the canon of knowledge by exploring how different heating strategies might affect the design of future thermal energy stores.

Materials and methods
To understand how current experiences with hot water heating systems can inform the design of future thermal energy stores, a series of in-depth interviews were undertaken, aided by a bespoke and engaging timeline tool. Then, insights from the interviews were used to develop a typology of hot water heating strategies leveraging the DNAs occupant behaviour framework [28].

Data collection and analysis
A series of contextual semi-structured interviews with a purposive sample drawn from 35 households was undertaken. Inclusion criteria included: 1) owner-occupiers in the Midlands region of the UK; 2) having a gas central heating system to heat their home and hot water; 3) at least two people living permanently in the dwelling. Participants were selected into quotas based on the type of hot water heating system (combination gas boiler to provide instant hot water or standard gas boiler with a separate hot water cylinder. Some participants in each group also had photovoltaic panels for the generation of electricity and one participant with a hot water tank also had a solar thermal system to heat hot water). For those with a hot water cylinder, the mean capacity was 149 litres (range 117-180 L), with water heating set to a mean temperature of 60°C (range 45-70°C). Within the quotas, participants were purposively selected to represent a wide range of household types, family structures, incomes and educational background to provide a sample of the population which allowed for different domestic situations to be explored; it was not intended to be representative but exploratory, as is common in rich qualitative studies with small samples 1 For example, Perceptual control theory [18], Human Operator Simulator [19], Cognitive Complex Theory [20], Executive Process Interactive Control [21], State, Operator and Result [22], Adaptive Control of Thought [23], Cognitive as a Network of Task [24], Architecture for Procedure Execution [25] and Business Redesign Agent-Based Holistic Modelling System [26]. [33]. All participants gave written consent prior to data collection, as part of the approval process by the local University Ethics Committee.
Interviews were conducted with one adult member from each household who actively engaged with the heating and hot water system but in some cases additional family members contributed to the interviews with their comments. Interviews took place in the context of participants' own homes, in a room of their choice, often around the dining table. Interviews ranged in duration from two to two and a half hours and were recorded using a digital voice recorder. Data collection started with questions about householders' characteristics (e.g. age, gender, education level etc.), home property characteristics (e.g. age of property, type of property etc.) and hot water and heating systems. Participants were then encouraged to talk about the hot water usage in their home during a typical day when the home is occupied e.g. a typical Saturday, using a bespoke timeline tool constructed for the purposes of the study. The timeline tool ( Fig. 1) was developed to prompt recollection about the sequencing of hot water event, activities and hot water heating times and formed a framework for discussion in relation to interactions with participants' hot water systems, whilst enabling participants a degree of control over the data collection process [34]. Previous studies that have used a similar approach to the timeline tool [34][35][36] argue that the visual and tactile engagement of the board enhances the recall of routine activities and helps with the flow of the conversation. Participants also discussed potential variations in the use of their hot water heating system from the typical day.
For the semi-structured interviews, the questions were predetermined, however the question order and wording were flexible and where necessary, further explanations were given. Where appropriate, additional questions were asked to follow avenues of discussion raised by participants that were considered as potentially relevant. Figure 1 shows a magnetic board with a timeline running from 1am to 12 midnight, with prompt cards depicting a range of tasks that require hot water. Participants were instructed to reconstruct a typical hot water daily routine for the household by adding cards to the timeline Audio recordings of the interviews were transcribed in full. The transcribed interviews contained rich, descriptive data which were entered into NVivo 8 software (QSR International, Pty Ltd., Doncaster, Victoria, Australia). NVivo was used for data retrieval and thematic analysis [37], which used inductive coding to unveil concepts, categories and subcategories that emerged from the data [38].

Development of hot water heating types
Insights identified from the thematic analysis were used to develop a set of hot water heating types, to account for the interaction between occupants and hot water heating systems. Patterns and distinctions between the hot water heating types were aided by the DNAs framework by assigning characteristics from the data, including: a. The drivers underpinning the choice of hot water heating usage and engagement. b. Interviewees' needs relating to the heating system, often expressed in terms of dissatisfaction to their current system. c. The actions undertaken by the interviewees to satisfy their needs. d. Characteristics of their hot water heating system that impacted on their usage.
Finally, a narrative for each hot water heating type was developed with reference to the interview transcripts and the timeline. In cases of uncertainty, the researchers consulted the transcripts for evidence to ensure appropriate categorisation. Quotes from the primary data were used to illustrate the hot water heating types to give them ecological validity. The resulting hot water heating types were reviewed by the research team, referring back to the original data, to increase the reliability of the analysis [23]. The approach followed is schematically represented in Fig. 2.

Householder characteristics
Participants were drawn from a sample with a range of family types, household income, social statuses and hot water systems. The interviewees were part of a family with children (80%, n = 28) or a couple (20%, n = 7) of which three were retired couples. Families comprised of two to five permanent occupants (median = 3.3). There were 14 families with their youngest child under five years of age and 14 families with older children still living at home. Mean household annual income band was £40,000-50,000 (range £15,000-£149,000), and properties ranged from pre-1850 to post 2002, with property types including detached, semi-detached and terraced house and bungalow (single storey).

Hot water and heating systems characteristics
All participants lived in a property with a gas boiler for the primary delivery of heat. Of the 35 properties, 14 had a combination boiler and 21 had a system with a hot water cylinder. All the households with hot water cylinders were able to heat their hot water using a gas boiler and 10 cylinders were additionally fitted with an electrical immersion heater (although eight of these were used either rarely or never). Of the 14 properties with combination boilers, four also had solar photovoltaic (PV) systems. Of the 21 properties with hot water cylinders, six also had solar systems (five had PV panels fitted, which used excess electricity to run the immersion heater contributing to the heating of the hot water, and one had solar thermal panels which also contributed to heating the hot water in the cylinder). Table 1 shows the hot water systems in the sample group. Hot water heating types The analysis identified that occupants had certain needs from their current heating system regarding the provision of hot water which related to hot water temperature, delivery time and quantity of hot water. Failure to meet occupants' needs triggered a number of system and behaviour-related actions in order to restore their personal and their family's comfort, expressed by either employing avoidance behaviours or by taking actions such as using overrides or alternative heat sources to satisfy their hot water needs. Drivers were also linked to systemrelated factors and personally held values. Four distinct hot water heating types emerged from the analysis with commonalities in the users' engagement with the heating system. These are referred to here as On Demand, For All Eventualities, Just Enough and Sunny Days and are described in summary in Table 2 and in more detail in the following paragraphs.
The On Demand hot water heating type was employed by 14 of the 35 participants and included all those with combination boilers, where hot water was heated instantly. The behaviour of occupants in the On Demand hot water heating type was substantially influenced by the contextual domain including the physical infrastructure in their home which, in this instance, was the combination boiler [16]. The presence of a combination boiler meant that participants did not need to plan their use of hot water as it was always available on demand; one participant commented "It's instant…you don't have to wait." The predominant drivers for the On Demand hot water heating type were the convenience of the system, perceived efficiency and not having to interact with the system. The system factors (the combination boiler itself) had a significant influence; occupants had little choice on the interaction with the hot water heating system as it was not required. Participants employing the On Demand hot water heating type were sometimes dissatisfied with the time taken to achieve the required hot water temperature which is consistent with evidence suggesting that combination boiler owners experience lower hot water temperatures than standard boiler users [39]. The On Demand users were driven by hedonic values and took actions to improve their personal pleasure by using tactile checks to ensure that hot water was at their desired temperature and drawing off excess water until they were confident that it was hot enough.
The For All Eventualities hot water heating type was employed by five participants in the sample with a hot water tank or combination of hot water tank with PV at their households. Participants in this hot water heating type set their hot water heating times to either continuous (throughout the daytime) or used programmes with hot water heating times of prolonged duration to "avoid hassle" and know that "there's always hot water". Participants in the For All Eventualities hot water heating type would frequently heat and store excess hot water to meet unexpected demands, "just in case." Householders also employed pre-emptive overrides to avoid insufficient hot water being available. One participant recognised that they could use the system more efficiently, but "couldn't be bothered with that". The predominant drivers for this approach were the convenience as well as the immediacy of hot water. Participants in the On Demand and For All Eventualities hot water heating types considered altruistic values (the well-being of other people) by placing value on the household harmony through their use of the hot water heating system, ensuring there was sufficient hot water for all needs.
The Just Enough hot water heating type was employed by 13 of the 35 participants who typically used two pre-programmed activation times that were short in duration. Participants set their heating times to provide sufficient hot water only for their predicted daily tasks, with heating times that matched routine times of peak use, such that the timer was set so the hot water was heated just before and during routine periods of use. One participant talked about experimenting with programme times: "I had it on for longer and dropped it down to about an hour and it still seems to have enough hot water for us." A number of participants in this type reported taking comfort from having instances where they did not have sufficient hot water for some tasks as evidence that their pre-programmed times were sufficient yet not excessive,  suggesting a type of egoistic value, with a focus on their finances. Participants displaying the Just Enough hot water heating type were driven by the perceived efficiency, cost of their system and lifestyle choices. For the Just Enough users, it was evident from the interviews that their systems covered the majority of their hot water needs with reported instances of dissatisfaction being rare. Where there was insufficient hot water, participants were willing to use reactive overrides and wait or use an alternative hot water supply, such as boiling a kettle, to meet their needs. The Sunny Days hot water heating type was employed by just three participants in the sample. Participants in this heating type had either solar PV systems (n = 2) or a solar thermal system (n = 1). This hot water heating type was predominately employed in the summer when the need for space heating is minimal. These participants did not have any set preprogramed times for the boiler to heat the hot water, instead they used only their PV or solar thermal systems to heat the hot water. Participants in the Sunny Days hot water heating type had no control over timing or duration of hot water heating; this was determined by the presence or absence of sunlight. All three participants commented that the utilisation of the "free" energy was appealing to them with one participant reporting using hot water "without feeling guilty" with it providing "an endless supply". Participants in the Sunny Days hot water heating type were partly motivated by the egoistic financial benefit of the system and partly by biospheric values. These users were dissatisfied with the insufficient hot water in the months outside summer and often struggled with the insufficient PV energy to heat hot water at these times. However, these users tolerated a little wait to satisfy their needs (e.g. postponing tasks) or used limited overrides from their central heating system to cover hot water needs.

Switching hot water heating type
Additional insights gained from the interviews indicate that some participants switched hot water heating types regularly while others never switched. A total of 12 participants reported actions that demonstrated they changed hot water heating types; nine switched from Just Enough to For All Eventualities, two switched from Sunny Days to For All Eventualities and one switched from Sunny Days to Just Enough. Switching hot water heating type was triggered by consistent drivers: to accommodate less routine events (e.g. guests) and to match the sleep/ lifestyle patterns, e.g. varying between weekdays and weekends. Some participants, in particular those in the On Demand and For All Eventualities types, never switched heating type, suggesting that their current provision matched their needs. The Just Enough users switched to For All Eventualities by extending heating times through the use of the continuous setting (n = 4), using the boost (n = 2) or advance options (n = 3). This was driven by a need for hot water at unexpected or additional times (to match changes in routine) rather than the limited capacity of their hot water cylinder. A larger store would be seen as an unnecessary investment to this waste-sensitive group. The Sunny Days users switched types during the majority of the winter, when there was the insufficient hot water from the solar PV during less sunny days.

Discussion
The derived heating types in this research demonstrate a diversity amongst occupants in relation to hot water usage and engagement with their current heating systems. Participants were found to employ one predominant heating strategy, often influenced heavily by the system they had or their personal values, or switch from one approach to another in particular circumstances. The drivers for each hot water heating type were centred around safeguarding personal interests, be it convenience, lifestyle or environmental awareness, which were proven to be strong motivational factors that influenced subsequent judgements and actions. Switching hot water heating type, whether regularly or rarely, was triggered by the consistent drivers. The On Demand and For All Eventualities users were motivated by convenience, pleasure and comforts for themselves and their household, without necessarily considering the environmental consequences. This contrasts with the Just Enough and Sunny Days users who were driven by egoistic and biospheric values. An understanding of these drivers, and the actions taken as a result, can help identify user requirements for thermal storage as part of future heating systems. These future energy systems are likely to include heat pumps, sensible, phase-change and thermochemical energy storage, and increased use of photovoltaics and community district heating, combined with variable pricing tariffs.
On Demand users were influenced by the contextual domain in their household (primarily the presence of a combination boiler), which allowed for lifestyle flexibility which was highly valued. As a consequence, On Demand users are likely to be resistant to a change in service delivery and would only be interested in a future heating system if it provides hot water on demand, to maintain their household harmony. The On Demand users avoided advance planning, so would be favourable towards heating systems that requires limited interaction. A common dissatisfaction reported among On Demand users was the time taken to reach an adequate hot water temperature; thermal storage could provide a suitable buffer to avoid these delays. Simultaneous usage of hot water without experiencing instances of inadequate hot water through loss of pressure was also important to these users and so a thermal store that can maintain rate of hot water delivery and meet demand would be favourably received.
For All Eventualities users desire a system that provides sufficient hot water to meet all their needs, whether planned or not. These users have little tolerance to instances with insufficient hot water and so a thermal energy storage system could store sufficient hot water to cover 'all eventualities'. The majority of the For All Eventualities users in this study considered their current hot water heating strategy to have minimal cost implications, as they heated water only to replenish hot water that had been used. Although the extended heating periods will inevitably result in a higher energy consumption, these users perceived their use to be cost effective and efficient. These users chose these prolonged heating times to avoid having to think, plan ahead and interact with the system; their needs may be met by a future system that does not require much interaction, but must deliver adequate hot water whenever needed.
The Just Enough users chose shorter specified heating times that were closely matched to routines. These users may be positive in the adoption of a future heating system if it provides hot water at times that match their daily routine and sleep patterns. As these users take comfort from instances where they did not have sufficient hot water for some tasks (suggesting carefully planned efficiency), they are likely to tolerate instances of insufficient hot water as long as the system is not seen as wasteful. The Just Enough users had experience with unsatisfactory water temperatures and may be interested in a system that provides feedback about hot water temperature and volume to enable them to plan their use and so heat sufficient hot water only for their daily routines without waste.
As the Sunny Days users had experience with intermittent energy generation, they are more likely to embrace future systems that are weather dependent. The Sunny Days users in this study were environmentally aware and thus these users may particularly appreciate the long-term environmental benefits of using efficient heating systems such as heat pumps with thermal stores. These users have tolerance to extended periods of unplanned hot water provision, but a system with a thermal energy store that could mitigate the variations in the hot water generation would provide an enhanced service. The Sunny Days users may make an investment to a system that is energy efficient and has environmental benefits, especially if this allowed some mitigation against volatile energy prices. Future systems that combine fast-response gas heating with solar PV and thermal storage are likely to meet the biospheric values of the Sunny Days user, whilst still delivering the household's hot water demands throughout the year. A summary of the user requirements for thermal storage by hot water heating type is presented in Table 3.
The developed hot water heating types describe individuals' usage and engagement with hot water heating systems within the UK households and align with current literature suggesting that occupants' decisions to engage with an energy efficient system depends heavily on their knowledge, motivation to do so, contextual factors, values, trust in involved parties, potential costs and benefits and public involvement [31]. These factors may influence both use and adoption of energyconsuming technologies and renewable energy sources.
Importantly, the data from the interviews indicate that some participants switched hot water heating type as circumstances in their household changed. This indicates the need for a future system to be adaptable, to account for occupants' changing behaviour, lifestyle, needs and preferences. It has been argued that occupants more readily adopt energy policies when they allow for flexibility and do not have negative consequences [40]. For the successful adoption and impact of future domestic heating systems, occupants' drivers, needs and actions can highlight user requirements to be considered in the design [29]. Energy policies and the design of new energy efficient technologies will be more successful if they target important antecedents of behaviour, removing significant barriers to change [11].

Conclusion
This research has explored hot water usage and user engagement with hot water heating systems in order to understand how current experiences with hot water heating systems can inform the design of future systems and the role of thermal energy storage. Given the gap in knowledge around occupant interaction with hot water heating, the paper provides valuable insights on end user engagement with domestic hot water heating systems and identifies user requirements for future thermal storage technologies. This research explored occupants' engagement, usage strategies and experiences with hot water heating systems through a series of interviews in 35 households with gas-fired central heating systems, typical of those used in the UK. The timeline tool developed specifically for this study enabled householders to talk about their hot water use in an engaging and informative way and offers an approach for further research where the underlying drivers for behaviour are sought. A typology of hot water usage types was developed from the collected data by considering the drivers, needs and actions that underpin occupants' decisions to interact with their heating system. Four hot water heating types, including On Demand, For All Eventualities, Just Enough, Sunny Days, and associated user requirements for the design of future thermal energy storage systems for each type have been determined. The identified hot water heating types depict the heterogeneity of occupants' behaviour patterns. User requirements for each type show differences, with a focus on convenience and immediate provision of hot water for some, and a tolerance to compromise or wait for others. Thermal storage can provide a viable solution to all these users by offering a buffer between supply and demand. In some cases, this buffer could be at a community level, in others it could be seasonal. Without storage, a move to more efficient systems that do not provide such fast-response heating is likely to leave users dissatisfied. For those that have become used to hot water on demand, any delay will be seen as a reduction in service. If prices increase significantly, those users that are currently heating hot water for all eventualities may need to change their approach as prolonged periods of heating becomes prohibitively expensive. Thermal storage also offers the opportunity to smooth intermittent supply. In order to facilitate the successful deployment of future energy efficient solutions to meet the UK's targets for the greenhouse gas reductions by 2050, researchers should consider occupants' energy-related behaviour alongside technological advancements to ensure successful adoption and impact of thermal energy stores at domestic and policy level. It is imperative that technology developers and engineers incorporate user insights into their design to ensure that products meet end user requirements. Improved understanding of occupant behaviour can inform the design of energy efficiency technologies leading to increased market uptake of thermal energy stores.

Funding
This work was undertaken as part of the End User Energy Demand Centre, i-STUTE (interdisciplinary centre for Storage, Transformation and Upgrading of Thermal Energy supported by the UK's Engineering and Physical Sciences Research Council (grant number: EP/K011847/ 1). Access to the underlying data is available via the Loughborough University data repository, where participant consent allows.

Conflicts of interest
None.