Imagine a personal heating system that works indoors as well as outdoors, can be taken anywhere, requires little energy, and is independent of any infrastructure. It exists – and is hundreds of years old. The hot water bottle could save a great deal of energy and money without sacrificing thermal comfort.
Hot water bottles work both indoors and outdoors. Illustration: Marie Verdeil.
A hot water bottle is a sealable container filled with hot water, often enclosed in a textile cover, which is directly placed against a part of the body for thermal comfort. The hot water bottle is still a common household item in some places – such as the UK and Japan – but it is largely forgotten or disregarded in most of the industrialised world. If people know of it, they usually associate it with pain relief rather than thermal comfort, or they consider its use an outdated practice for the poor and the elderly.
Nevertheless, when I sent two dozen hot water bottles to friends and family as a Christmas present, the reactions were almost unanimously enthusiastic. People show themselves very much surprised that such a humble object can provide so much comfort. Because I don’t have the time nor the budget to send hot water bottles to everyone, I have written this article. It’s largely based on my personal experience – I have been using hot water bottles for many years and they are the only heat source in my apartment.
The history of the hot water bottle
Croation inventor Eduard Penkala patented the rubber hot water bottle – which he dubbed the “Termofor” – in 1903. However, it did not come out of nowhere. In fact, the history of the hot water bottle goes back thousands of years, albeit in different guises.
The first “hot water bottles” – quite literally – were other people and animals. Since time immemorial, people have warmed themselves by huddling together. For example, it was common for the whole family to sleep together in the same bed – and this included potential visitors.  People also took advantage of the heat from animals – “hot water bottles” with a standard fur cover.
They snuggled up against cows and pigs, which were either sharing the living space or lived in the stables below it. In the eighteenth century, wealthy women kept specially bred “hand dogs” – toy poodles – around to keep their lap and hands warm.  Personal heating devices also took the form of objects – stones, bricks, potatoes – that were heated in or near the fire, wrapped in cloth or paper, and kept in people’s laps, in pockets, or in the bed.
As early as the 1500s, people started to use all kinds of portable containers filled with hot coals from the fire. These were used as foot warmers, hand warmers, and bed warmers.  Most were made of metal, either brass or copper, and placed inside wooden or ceramic enclosures to prevent skin burns. Over time, hot coals were replaced by hot water, which is a cleaner and safer heat storage medium.
Initially, these first “real” hot water bottles were made from hard materials such as glass, metal, or stoneware. It was only with the invention of vulcanised rubber in the nineteenth century that more comfortable lightweight and flexible hot water bottles became an option. Spanish friends told me that hot water bottles used to be made from animal skins, but I could not verify this. It may well be true, because all over the world there’s a long tradition of using “water skins” for storing liquids.
A foment can is filled with hot water and used very much like a hot-water bottle to apply warmth to the body. Fomentation actually means “to apply warm liquids to treat the skin.” This oval-shaped can is curved to fit the body. Maker: Kenworthy Son and Company. Place made: Southport, Sefton, Merseyside, England, United Kingdom. Source: Science Museum, London. (CC BY 4.0). https://wellcomecollection.org/works/gf42542b
Hexagonal hot-water bottle, Austria, 1791-1798. This hexagonal hot-water bottle is made of pewter and is engraved with a forest scene. Source: Science Museum, London. (CC BY 4.0). https://wellcomecollection.org/works/b452vwjm
This foot warmer was used to give warmth and comfort to patients who were resting in the hospital wards. Made from tinned iron, the warmer would have been filled with hot water and secured with a cork. This copy was made in 1927 to commemorate one hundred years since Joseph Lister’s birth. Place made: Glasgow, Glasgow, Scotland, United Kingdom. Source: Science Museum, London. (CC BY 4.0). https://wellcomecollection.org/works/mfjujndv
Hot water bottles today
The classical hot water bottle for sale today is either made from rubber or PVC plastic. The latter material has few advantages. It’s often a bit cheaper and can be made transparant, but unlike rubber it contains toxic chemicals (which make the plastic flexible). A third option – a bit harder to find – are plastic hot water bottles without chemical softeners, which are rigid instead of flexible.
The distinctly shaped Japanese hot water bottle – the “yutampo” – is usually of that type. Its use dates back to the fifteenth century when it was made from metal or stoneware. Of course any sealable container can function as a hot water bottle. I have successfully used metal drinking bottles and even plastic PET-bottles – more about those later.
The typical hot water bottle has a rectangular shape and holds up to two litres of water. However, in spite of its dull image, the hot water bottle has seen some interesting innovations lately. A first novelty are much smaller rectangular bottles, which hold between 0.2 and 0.8 litres of water. Judging by their covers, these are mostly aimed at children, but they can be just as useful for adults who can carry them in pockets or put them inside clothing.
There are now also larger hot water bottles available, which hold up to three litres of water or more. Finally, the most successful novelty has the form of a hot dog: it’s a hot water bottle 80 centimetres long. It can be tied around the waist but is just as practical as a companion on the couch or in the bed. It can easily be shared by two people and its shape makes it luxuriously comfortable. It holds up to two litres of water.
Rubber and PVC hot water bottles. Image by Marie Verdeil.
Rubber hot water bottles. Image by Marie Verdeil.
How to use hot water bottles?
People who know hot water bottles usually think of them as bed companions. However, they can keep you warm wherever you are, throughout the day. This includes the sofa, of course, but you can also surround yourself with one or more hot water bottles when seated at a desk or a table. I use one, two, or exceptionally three hot water bottles simultaneously, depending on the indoor temperature. They usually end up in my lap, behind my lower back, and/or under my feet. Although only some body parts are directly heated, the warmth from the bottle(s) is distributed throughout the body by skin blood flow.
Hot water bottles can be combined with a blanket, which further increases thermal comfort. If I put a blanket over the lower part of my body when seated at my desk, it traps the heat from the bottles and keeps them warm for longer. Even better is a blanket with a hole in the middle to stick your head through – a basic poncho – or a blanket with sleeves. If it’s large enough, it creates a tent-like structure that puts your whole body in the warm microclimate created by the water bottles. Draping long clothes over a personal heat source was a common comfort strategy in earlier times.
A blanket traps the heat of hot water bottles. Illustration by Marie Verdeil.
You can go one more step further and put a large blanket over the desk or table and then put your legs underneath it. Such heating arrangements have been used in different parts of the world, usually with hot coals as the heat storage medium. Examples are the Japanese “kotatsu”, the Middle-Eastern “korsi”, and the Spanish “brasero de picon”. The first two are rather low to the ground – people sit on the floor – while the latter fits the common seat height in the Western world. It’s easy to build such a heating arrangement – and a few hot water bottles are the ultimate heat source for it.
Hot water bottles outdoors & on the move
The arrangements described above only work for people who stay in one place. The need for an external heat source decreases when we move around and are physically active, because our body produces more heat. Nevertheless, hot water bottles can also keep you warm when you are standing up doing things or when you are moving through a space or a building. They can be worn underneath clothing or even put in specially designed pockets or backpacks. A small backpack holding a hot water bottle – positioned between the shoulder blades – also works great while sitting on a chair.
Hot water bottles provide thermal comfort with all the windows open. Illustration by Marie Verdeil.
Hot water bottles work both indoors and outdoors – provided that the body is protected from wind and rain – or indoors with the all the windows open. Modern central heating systems provide thermal comfort mainly by heating the air in a space, an approach that obviously won’t work well outdoors or in a well-ventilated indoor space. In contrast, hot water bottles transfer heat directly to people through physical contact (a heat transfer method called “conduction”). They heat people, not spaces. This makes hot water bottles a safe and sustainable alternative for terrace heaters in bars and restaurants. The investment is minimal: a collection of hot water bottles and a kettle – the water can be re-used over and over again. Alternatively, everyone could bring their own hot water bottle and fill it up on the terrace.
One could take this idea even further and envision a public infrastructure for refilling hot water bottles, not just on bar terraces but in multiple locations such as schools, offices, and public buildings.  People could gather around the hot water dispenser just like they gather around the water cooler. Historically, hot water bottles – and their predecessors using hot coals – were also taken out of the house. Their use was common in coaches and trains, as well as in churches, which were unheated. Smaller hot water containers with carrying strings and fabric covers were put into fur muffs or pockets. Nowadays, you could also store hot water in a vacuum flask and then pour it into a hot water bottle hours later.
Curved rectangular hot-water bottle, France, 1751-1810. Made of pewter, an alloy of tin and lead, this hot-water bottle is engraved with birds and plants and has a curved shape to fit close against the body. Source: Science Museum, London. (CC BY 4.0). https://wellcomecollection.org/works/g5ufhayn
900 hot water bottles per day: energy savings
Unsurprisingly, there’s little – or actually no – academic research into the energy savings potential of hot water bottles. Instead, in recent years scientists have investigated more sophisticated personal heating devices such as electrically heated desks and seats, radiant heat bulbs, or battery-powered heat pillows. [5-7] These alternatives look needlessly complex in comparison to the hot water bottle. Water can be heated in many ways both high-tech and low-tech, and containers can be made from locally available materials.
Nevertheless, these studies show that personal heating sources with similar effects as hot water bottles could save a great deal of energy while maintaining and often even improving thermal comfort. For example, one study revealed that lowering the air temperature in an office from 20.5 to 18.8 C and giving employees a heated chair to compensate for the discomfort leads to 35% less energy use and consistently higher scores for thermal comfort. There are few interventions in the building envelope that can achieve such large energy savings for such a small investment, and yet the decrease in air temperature was far from radical in this experiment. If personal heating devices would be combined with a change in clothing insulation and/or blankets the energy savings could become much larger still.
Another way to investigate the energy savings potential of the hot water bottle is to calculate how much energy it takes to prepare one and compare that to the energy use of a central heating system. Because rubber or PVC bottles can only be filled up to two-thirds for safe and comfortable use, I assume a somewhat larger model – 3 L – which can hold two litres of water in practice. This makes the calculation also valid for containers that can be filled completely, such as the Japanese yutampo. It takes 4,200 joule to raise the temperature of 1 litre of water by 1°C, meaning that heating two litres of water from 10°C to 60°C requires 420 kilojoule or 116.7 watt-hours.
In comparison, the average household energy use for gas heating in Belgium – which has a moderate climate – is 20,000 kWh per year. Assuming that the average Belgian heating system is used for six months per year, daily energy use corresponds to 109.6 kWh per day. This energy could heat roughly 900 water bottles per day – enough to keep the whole neighbourhood comfortable. Imagine that four household members each use two hot water bottles simultaneously and reheat them every two hours throughout their waking hours (16 hours). Total energy use is then below 4 kilowatt-hours, almost 30 times less than the heating energy consumed by the average Belgian household.
This is not to suggest that hot water bottles need to replace a central heating system. The rather short and mild winters here in Barcelona allow me to use hot water bottles as the only heating system because it rarely gets colder than 12°C in my unheated apartment. In less hospitable climates, hot water bottles can be combined with a central heating system. The hot water bottles create islands of thermal comfort for low metabolism activities while the rest of the indoor space is comfortable to move through or be physically active in.
Hot water is a safer heat storage medium than hot coals, but it is not without its risks and hot water bottles need to be used carefully. They carry the instruction not to use boiling water, which is very sound advice, but hot water doesn’t need to boil to be dangerous. Water above a temperature of 60°C can scald you and lead to very serious injuries. Therefore, it’s recommended to use only hot tap water, or any other hot water source below 60°C. This temperature is sufficiently high to make you comfortable and the only advantage of using hotter water is that you need to reheat it less often.
Too hot water can hurt you in several ways. First, there’s always a chance you spill water on your hands while filling the bottle. Second, a rubber or plastic hot water bottle can start leaking, either through the cap or through the seams. Third, and this is the worst-case scenario, a hot water bottle can burst and release two liters of hot water on your body. Such accidents are rare, because nowadays hot water bottles are made according to quality standards. However, they do occur, usually because the bottle has worn out.
To safely use rubber or PVC hot water bottles at higher water temperatures, it’s important to replace them after a few years of use, and to store them properly. If you really want to use higher water temperatures, metal hot water bottles – inside a cover to prevent skin burns – are the safest option. However, if you keep the temperature below 60°C, the worst-case scenario is just getting wet. If you use PET-bottles, you should surely stick to this maximum temperature, because at higher temperatures they could melt. Furthermore, a PET-bottle should not be used for drinking after it has been used for heating, because the higher temperatures may release chemicals in the water.
Water use & infrastructure
Hot water bottles also require a source of water. It’s possible to reheat the same water over and over again, thus limiting the water use to a few litres during the lifetime of the bottle. However, that’s not always the most practical solution. In modern households, hot water can be sourced from an electric kettle, a pot on the cooking stove, or the hot water tap. Although hot tap water is the safest source of water for a hot water bottle, once the water has cooled down there’s no way to get it back into the pipes for reheating. Furthermore, it takes time before the water comes up to temperature, meaning that more than two litres of water will be consumed.
Using an electric kettle – or a pot on the cookstove – makes it easy to reuse the same water over and over again, but it faces some problems too. First, if your electric kettle does not come with a programmable water temperature, you need to make sure the water does not get too hot. I solve this by dipping the probe of a digital thermometer in the kettle while warming the water. Second, if you reheat the water from rubber bottles, the kettle (or pot) can no longer be used to heat water for human consumption because it will taste bad. So, either you use a separate kettle for use with hot water bottles, or you warm the water in the only household kettle and discard it after use.
Even if the water is not reused for other purposes (such as watering the plants) the waste is quite limited. The average shower consumes enough water to fill 37 hot water bottles. Likewise, the energy use of the average shower corresponds to the energy use for heating 17 hot water bottles (which use water with a higher temperature than a shower). Consequently, even a slightly lower shower frequency easily provides you with the water and energy for continuous hot water bottle use.
The use of metal and ceramic hand warmers has a long tradition in China and Japan. The Japanese offered guests a small roundish ceramic pot with fuel inside, called a “te-aburi”. Copper or bronze box-shaped hand warmers a few inches across, often with perforations and carrying handles, were called “shou lu” in China. Image in the public domain. Read more: https://homethingspast.com/2011/11/26/hand-warmers-muff-warmer/
Cold water bottles
Hot water bottles can be used for cooling as well. In this case, they are filled with cold water or put in the freezer. Cooling people is much more energy efficient than cooling spaces. I don’t have air conditioning and rely entirely on fans and cold water bottles in summer, when temperatures are usually above 30°C. I use “cold water bottles” in a similar fashion to hot water bottles – they go into the bed, under my feet, or behind my back. For cooling I use plastic PET-bottles and metal drinking containers, not rubber water bottles as they get hard and brittle. Keep in mind not to fill the bottle completely – water expands when it’s frozen – and to put the bottle inside a protective cover to prevent iceburn. Also keep in mind that they will get a bit wet on the outside as the ice melts – although this effect only enhances the cooling. Like hot water bottles, cold water bottles work outdoors as well as indoors.
Kris De Decker
Proofread by Alice Essam
Notes & references:
 This custom was accompanied by strict rules. For example, male visitors ended up sleeping on one side of the bed, while the family’s daughters were on the other side. Source: Ekirch, A. Roger. At day’s close: night in times past. WW Norton & Company, 2006.
 The “warming pan” or “bed warmer” was a metal container filled with hot coals and fitted with a long handle. It was slid between the bedsheets and then moved across the bed to warm all corners before someone got into it. Yet another solution to warm the bed was the so-called “bed wagon”: a wooden frame or sledge designed to hold a pot of hot coals, which was slid below the bed and covered with a metal sheet. Unlike a warming pan, the bed wagon provided warmth throughout the night. See: http://www.oldandinteresting.com/warming-the-bed.aspx
 Some cities had public hot water supply systems. For example, in the first half of the twentieth century, the Dutch city of Rotterdam counted hundreds of “water distilleries” where people came to fill buckets with hot water for domestic use. China has a long and continuing tradition of providing its citizens with hot water everywhere they go – mainly for drinking. By the 1830s, hot water stores – known as “laohuzao” or “tiger stoves” – popped up in major cities all over the Yangtze river delta. Today, almost every government body, business and school administrative office in China has hot water dispensers – even high speed trains have them. Read more: https://www.sixthtone.com/news/1000919/the-history-behind-chinas-obsession-with-hot-water
 Verhaart, Jacob, Michal Veselý, and Wim Zeiler. “Personal heating: effectiveness and energy use.” Building Research & Information 43.3 (2015): 346-354. https://www.tandfonline.com/doi/abs/10.1080/09613218.2015.1001606
 Deng, Qihong, et al. “Human thermal sensation and comfort in a non-uniform environment with personalized heating.” Science of the total environment 578 (2017): 242-248.
 Mishra, A. K., M. G. L. C. Loomans, and Jan LM Hensen. “Thermal comfort of heterogeneous and dynamic indoor conditions—An overview.” Building and Environment 109 (2016): 82-100. https://www.sciencedirect.com/science/article/pii/S0360132316303560