Preheating and pre-cooling a thermos bottle: step by step
You pour piping hot tea into the bottle, and two hours later it is only warm? Often it is not the bottle’s fault but a missing step: if you preheat your thermos bottle before the drink goes in, you get much more out of the insulation. The same applies the other way round to pre-cooling in summer. In this guide we show you step by step how both work, why they help and what to watch out for.
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Why you should preheat your thermos bottle
An insulated bottle holds on to heat – but it does not produce any. When you pour hot tea into a bottle that has been standing in the cupboard at room temperature, something invisible happens first: the cold stainless steel inner wall absorbs heat from the drink until it is warm itself. That heat is then missing from the tea. So it starts the day at a lower temperature than you think.
If the inner wall has already been preheated, on the other hand, it hardly needs to draw any energy from the drink. The tea keeps its starting temperature, and from there the insulation can do its job. How strong the effect is depends on the bottle, the amount and the surroundings – but it is almost always noticeable, especially on cold days.
In standardised tests of insulated bottles according to the EN 12546-1 standard, the bottle is also preheated with hot water before filling, as a guide by Galaxus on heat retention describes. This shows that preheating is no insider tip but the usual basis for good results.
Preheating your thermos bottle: step by step
Preheating only takes a few minutes and can easily be combined with making tea. Here is how to do it:
- Heat water: Boil a little more water than you need – some for the tea, some for preheating.
- Pour in hot water: Pour hot water into the empty bottle, about halfway or more. Ideally use a funnel or pour slowly so that nothing splashes.
- Close and wait: Screw the lid on loosely so that the closure also gets some heat, and leave the bottle to stand for about two to five minutes.
- Pour out: Open the lid carefully and pour out the water. Caution, steam can escape.
- Fill immediately: Pour in the hot drink straight afterwards so that the preheated inner wall does not cool down again.
- Close tightly: Screw the lid on firmly – done.
By the way, you do not have to throw away the water from the bottle: you can use it to warm a cup or for rinsing.
While the tea is steeping
The best moment for this step is the steeping time. Brew the tea in a teapot and at the same time pour hot water into the bottle. When the tea is ready, pour out the preheating water and fill the bottle with tea. That way the step does not cost you an extra minute.
Pre-cooling for cold drinks
What applies to preheating for hot drinks works just the same in the other direction. A bottle at room temperature gives off heat to the cold water. If you cool it down beforehand, your drink stays cold for longer.
- Cold water and ice cubes: Fill the bottle with very cold tap water and, if they fit through the opening, a few ice cubes.
- Close briefly: Put the lid on and leave it to stand for about five to ten minutes.
- Pour out and fill: Empty the water and ice and immediately pour in the chilled drink.
Alternatively, you can put the empty, open bottle in the fridge for a while. The freezer is not meant for this – that is a separate topic. You will find more tips for hot days in our article Drinking bottle in summer.
Without a kettle: in the office or on the go
A kettle is not always at hand. In the office, the hot water function of the coffee machine or a water dispenser with hot water often helps. At home, very hot tap water will do at a pinch – it is less effective than boiling water, but much better than a cold bottle. In a hotel or holiday flat, you can swirl the bottle out with hot water from the coffee machine before pouring in your coffee.
The same applies to coffee as to tea: a warm inner wall ensures that it does not cool down as soon as you pour it in. You notice the difference especially with small amounts, which quickly lose temperature in a large cold bottle.
Preheating and pre-cooling compared
| Step | Preheating (hot drinks) | Pre-cooling (cold drinks) |
|---|---|---|
| With what | hot water from the kettle or tap | cold water, possibly ice cubes |
| How long | about 2–5 minutes | about 5–10 minutes |
| Lid | screw on loosely | screw on |
| Afterwards | fill hot immediately | fill cold immediately |
| Typical drinks | tea, coffee, hot water | water, iced tea, spritzer |
Safety when handling hot water
Care is important when handling hot water. A few points you should bear in mind:
- Do not shake: You should not shake a closed bottle of very hot water vigorously. Steam can build up pressure and shoot out when opened.
- Open carefully: Keep your face away from the opening when unscrewing.
- Work on a stable surface: Place the bottle on a stable surface for filling instead of holding it in your hand.
- Keep children away: Hot water and children’s hands do not go together.
- Check the drinking temperature: A preheated bottle keeps the drink particularly hot. Take the first sip carefully.
Test it yourself
If you want to see how much the trick does for your bottle, try a simple experiment: one day, pour hot water into the bottle without preheating it, and the next day with a preheated bottle – with the same amount and the same water temperature. After a few hours, measure with a kitchen thermometer. That way you see for your own bottle and your everyday life what difference the step makes.
Which bottle is worth it?
The step helps with any insulated bottle, but most with bottles that have a good double wall and a tight closure. According to the manufacturer, our two IZY bottles are made of double-walled, rustproof stainless steel and keep drinks hot for up to 12 hours and cold for up to 24 hours. The leak-proof screw cap ensures that nothing spills. The IZY Bottles Van Gogh Flowers thermos bottle features a floral design inspired by Vincent van Gogh, the IZY x Pip Studio “La Dolce Vita” thermos bottle the playful Pip Studio pattern. You can find both in our IZY collection.
After use, it is best to rinse the bottle by hand with warm water and mild washing-up liquid. According to the manufacturer, the IZY bottles do not belong in the dishwasher.
Related reading: Filling a thermos bottle properly: step by step and without scalding · How long does a thermos bottle keep drinks warm? Claims and practice · Coffee in a thermos bottle: how it stays hot and aromatic
Conclusion
Preheating your thermos bottle is one of the simplest tricks to keep hot drinks hot for longer: pour in hot water, wait a few minutes, pour it out and fill immediately. For cold drinks the same works with cold water and ice. If you combine the step with the tea’s steeping time, it does not even take extra time. Just be careful when handling hot water – then you will enjoy your drink on the go for longer.
Frequently asked questions
How long should I preheat the thermos bottle?
Usually about two to five minutes with hot water is enough. Then pour it out and immediately fill in the drink.
Does preheating really make a difference?
Yes, because otherwise the cold inner wall absorbs heat from the drink. How big the difference is depends on the bottle – your own test with a kitchen thermometer will show you.
Do I have to use boiling water for preheating?
No. Very hot water from the kettle works best; hot tap water is better than nothing. What matters is careful handling so that you do not scald yourself.
Can I pre-cool the bottle as well?
Yes. With cold water and, if they fit, a few ice cubes for about five to ten minutes. Then empty it and pour in the cold drink.
Should I put the lid on when preheating?
Yes, screwed on loosely. That way the closure warms up too and the heat stays inside. Open it carefully afterwards, as steam can escape.
Personally written by Martin – founder of BeachAndSteam and sauna operator at Lake Kulkwitz for more than 18 years.