Most of us have heard of or used rubber hot water bottles. They are most common in the West! Have you ever heard of Japanese hot water bottles though?
If you haven't, we hadn't either, until yesterday.
Fascinated by the advantage they bring, we decided to write an article presenting the Japanese hot water bottle, also known as "Yutanpo".
The main source for the advice and descriptions below is Iwatani, Japanese household items and Yutanpo manufacturer since 1954.
What is a Japanese hot water bottle?
Japanese hot water bottles, also called "Yutanpo" in Japanese, are hard hot water bottles made of polyethylene or galvanized steel. In Japan, they are commonly used as a bed or futon warmer. Compared to rubber hot water bottles, Japanese hot water bottles are rigid, have a handle, a larger capacity (up to 3.4L) and stay warm much longer (up to 12 hours). Just like rubber hot water bottles, they need to be wrapped in cloth or a cover to prevent direct contact with your skin.
What are Japanese hot water bottles made of?
The most commonly used Japanese hot water bottles are made of plastic (polyethylene). These are generally easier to handle and safer than steel ones, having a less hot surface when filled.
They are to be used with a cover to prevent direct skin contact. Yutanpos made of polyethylene last between 2 and 3 years, depending on how well you store them. The cap should be replaced every year as the rubber seal deteriorates over time.
Japanese hot water bottles should be stored the same way as rubber hot water bottles. Read our article on how to properly store a hot water bottle to keep your item as long as possible!
Some Japanese hot water bottles are made of galvanized steel. Their service life is considerably shorter than those made of polyethylene, lasting 1 year on average. Make sure to change your steel Yutanpo after a year to prevent accidents.
Just like polyethylene (plastic), steel is a great thermal insulator and will trap heat in the bottle for hours. It should also be wrapped in a thick cloth or tailored cover to prevent direct contact with the skin.
How long does a Japanese hot water bottle stay warm?
The larger your Yutanpo's capacity, the longer it will stay warm. For instance, a Japanese hot water bottle containing 0.8L of water can stay warm for up to 6 hours while one with a 3.4L capacity can keep you warm for up to 12 hours.
The table below provides an overview of how long your bottle will stay warm by capacity.
Using a cover for warmth and safety
To keep your Yutanpo warm for longer, we advise you to wrap it in thick cloth or other insulating material such as wool. A well-filled and well-wrapped Yutanpo can stay warm for up to 12 hours in the case of 3,4L bottles.
Wrapping or using a cover for your Yutanpo has another important function: safety. This is especially important if your item is made of galvanized steel, which will be burning hot on the outside.
In addition, using a cover will also decrease the chances of you getting a "low-temperature burn", which typically occurs when your body is in contact with a heat source for too long. This also happens with rubber hot water bottles, which is why it is recommended to avoid body contact for more than 15 minutes.
How to fill a Japanese hot water bottle (Yutanpo)
You should exercise the same precaution when filling a Yutanpo as when filling a rubber hot water bottle.
Here are a few simple steps to follow to safely fill your Japanese hot water bottle:
Check for leaks and damage before each use
Do not fill with boiling water, as this may boil over and cause burns: let the water cool down for about 5-10 minutes before pouring it into your Yutanpo
Fill your Japanese hot water bottle up to the top: if you only fill the bottle up partially, the pressure inside the Yutanpo will change, which may cause the hot water bottle to dent and accelerate its deterioration. (this rule does not apply to rubber hot water bottles, which you should only fill up to two-thirds).
Make sure the cap is tightly screwed on: check for leaks by gently turning the Yutanpo upside down over your sink
Wrap it in a thick cloth or use a dedicated cover: this will prevent burns and keep it warm for longer
Japanese hot water bottles vs rubber hot water bottles
Japanese hot water bottle
Made of plastic (polyethylene) or steel
No rubber smell
Larger capacity (up to 3,4L)
Can't be used directly on the body
Stay warm longer (up to 12 hours)
Rubber hot water bottle (British hot water bottle)
Made of rubber, PVC, or silicone
Rubber smell (for rubber hot water bottles)
Easily transportable within your home, and at work
Smaller capacity (up to 2L for classic hot water bottles)
Can be used directly on the body if wrapped
Stays warm for shorter periods (2-3 hours)
How can Japanese hot water bottles be used?
Historically and to this date, Japanese hot water bottles were and are used as bed warmers. Come winter time or on cold nights in general, they provide a cheap alternative heat source for those who cannot afford it or don't wish to turn the central heating up.
A Yutanpo is usually placed at the end of the bed (futon in Japan) to keep your feet warm (cold feet in the winter are the worst!). Keeping your body's extremities warm will help keep your whole body warm, hence the effectiveness of Yutanpos for most people.
Japanese hot water bottles are great for keeping your bed warm and can also be great companions in the office or when you're working from home.
Given their flat surface, stability, and rigidity, they can be used as a warm footrest as you're sitting at your desk or on the couch. Just make sure not to actually stand on it as exercising too much pressure on the bottle may lead it to burst. Lay a cloth or cover on the Yutanpo or wear thick socks before resting your feet on it.
A few precautions when it comes to using Japanese hot water bottles
Do not heat it in a microwave: read why you shouldn't microwave a hot water bottle.
Do not sit or step on it
Do not sleep with it: (use it to warm your bed and then remove it before sleeping)
Do not add anything other than water to it
Do not drink water from it
Keep away from unsupervised children: make sure your Yutanpo is safely wrapped to prevent them from accidentally touching it.
Check your Yutanpo has the SG Mark certification: this guarantees its compliance with official safety standards (STC, 2022)
We hope you read this article about Japanese hot water bottles with as much curiosity and passion as we wrote it!
While not very common in the West compared to rubber hot water bottles, Japanese hot water bottles offer many benefits, notably when it comes to how long they stay warm, and how they can complement your home heating system.
Turning your heating down by a couple of degrees during the night and using a Yutanpo instead will be easier on your wallet. And the environment.