A Thermos flask is a great way to keep drinks cold for hours or even days at a time. But can you put boiling water in a Thermos?
Whether you want to make a hot coffee, a tea or you want to store hot water for later a Thermos has great insulating properties and can be a great option to keep water or drinks hot.
If you're looking to store boiling water there's some good news:
You can safely put boiling water in a Thermos without any issues. You do need to be careful with glass lined Thermoses as the quick change in temperature can cause it to shatter, but stainless steel Thermoses are fine. The boiling water will stay hot for 6-12 hours.
So if you want to put some boiling water in your Thermos then by all means do it. They are designed to easily handle boiling water without breaking and they will keep it hot all day long.
With a glass lined thermos you may want to preheat it with warm or hot water first before pouring boiling water straight in. Otherwise the quick temperature change could potentially shatter the glass.
Will Putting Boiling Water In a Thermos Damage It?
No, putting boiling water in a stainless steel thermos will not damage it. The stainless steel can easily handle the heat. Putting boiling water in a glass thermos may cause it to shatter. To avoid this preheat the glass with warm water before adding boiling water.
This article outlined that stainless steel can handle extremely high temperatures. In fact, temperatures need to get as high as 1,598°F (870°C) intermittently before causing the stainless steel any damage.
Also glass can handle similar temperatures to stainless steel before it starts to melt.
I think it safe to say you're unlikely to be putting anything that hot on your Thermos and boiling water is nowhere near that temperature.
However, completely submerging your Thermos in boiling water is not advise. Boiling water can affect the outside paint of your thermos and could cause it damage.
Exposing the outside of your Thermos to extreme heats does have the potential to melt and break the vacuum seal. If this happens the vacuum will fill up with air and your Thermos will no longer insulate properly.
How Long Will Boiling Water Stay Hot In a Thermos?
If you put boiling wait straight into a Thermos it can say hot for quite a long period of time. Boiling water in the morning can often stay hot enough to make tea, coffee or even a hot cup of soup or noodles by lunchtime.
If you put boiling water in a preheated thermos and keep the lid closed it should remain hot for at least 6 hours, cooling down to a lukewarm temperature in about 12 hours.
The larger the thermos the longer your water will stay hot for. A small 16 oz thermos won't keep water as hot as a larger 1.2L thermos.
Below is a great video where someone talks about their experiments with a 1.2 L thermos and how long it kept water hot for.
The results were staggering as after 6 hours the water was still so hot it could burn you and after 24 hours it was still hot enough to make a tea or coffee:
Results on how long a 1.2L thermos will keep water hot for.
Can You Keep Water Boiling in a Thermos?
While a Thermos is great at keeping water hot it is unable to keep water at boiling temperature of 212ºF (100ºC) or above. This is because there is no heating element so as soon as your pour boiling water in it stops boiling.
You can however keep water very close to boiling temperature for hours at a time and if you pre-heat the inside of your Thermos the water will stay hotter for longer.
To pre-heat your thermos boil some water and fill up your thermos with the boiling water. Leave for 2-5 minutes so the stainless steel gets hot then discard.
Quickly refill your Thermos with more boiling water and close the lid.
The hot walls of your Thermos now won't strip away any heat from your fresh boiling water and it'll stay hotter for that little bit longer.
What Happens When You Put Boiling Water In a Thermos?
When you put boiling water in a Thermos the water will instantly stop boiling and start to cool down. It will stay extremely hot (too hot to drink) for 4-6+ hours due to it's vacuum insulation.
If you pour boiling water into a glass Thermos that has been sitting at room temperature or had cold water in it then there is a chance that the sudden change in temperature can break the glass.
When this happens the pressure from the vacuum will shatter the glass lined thermos into lots of little pieces. This will ruin your thermos and fill up your drink with glass.
This is why it's important to pre-heat glass vacuum thermos flasks with warm water before pouring boiling water into them.
How To Keep Water Boiling For Longer In a Thermos Flask
If you want to keep water as close to boiling temperature for as long as possible in your Thermos there are a few different things you can do so the water stays hot longer.
I wrote an entire article on how to keep drinks hot in a vacuum flask so you can read that for full details but below is a shortlist of helpful tip.
- Pre-heat the inside of the thermos before adding your boiling water
- Larger amounts of water will stay hotter for longer
- Pour water straight off the boil into your Thermos and close the lid instantly, don't let it sit and cool down
- Use a towel or insulated pouch/sling
- Heat your water hotter than 212ºF (100ºC) if possible
- Keep your Thermos out of cold environments.
Do the above tips and your boiling water will stay closer to boiling temperature for longer in your Thermos.
Can You Boil Water Inside a Hydro Flask?
It's not recommended to try and boil water inside a Thermos and you definitely should NOT put your thermos on a hot store, fire or hot element in order to try and boil the water inside.
The vacuum insulation will stop most of the heat getting through the thermos into your water, meaning it will take an extremely long time to boil.
Also the extreme heat of the stove would likely ruin the vacuum seal which will cause your Thermos to no longer insulate properly.
Heating the outside will likely cause damage to the paint of your Thermos, discoloring it and maybe even chipping or burning it off.
Below you can see a video of someone trying to boil water inside a vacuum insulated mug and it does NOT go well for mug. It takes ages to boil and even causes the mug to completely deform.
If you desperately need to boil water inside your thermos you're going to be better off using a hot element that you can place inside the flask to boil the water from the inside.
But really it's better to boil the water separately from your Thermos and then pour it into the Thermos.