Thermos Not Keeping Drink Cold or Hot? Here’s Why

Thermos flasks are great at keeping drinks hot or cold for an extended period of time. However, if you've noticed that your Thermos is no longer working like it used to and drinks go to room temperature rather quickly then there are a few problems that could be occuring.

If your Thermos is not keeping your drink hot or cold, this is likely caused by the vacuum insulation being compromised and filling up with air. When this happened the Thermos loses it's insulation layer and unfortunately there is no way to fix it.

However, Thermos does have a generous 5 year warranty on their vacuum insulated stainless steel bottles and a 1 year warranty on their glass vacuum insulated products so it may be covered under warranty and you may be eligible for a replacement.

There are a couple of other reasons your drinks might not be holding their temperature in your Thermos but loss of vacuum insulation is definitely the most common one.

Why Isn’t My Thermos Bottle Working Properly 

The Thermos bottles are made from double-walled stainless steel with vacuum insulation. This means that there is an inner and an outer wall within the bottle and between these walls they suck out all the air to create a vacuum which stops heat transfer. 

But when your Thermos bottle is no longer keeping water cold, this usually means that the insulation has somehow gotten damaged and air has been able to fill the cavity. Once the vacuum is compromised and filled with air the heat is able to pass through the walls of the bottle and either cool down or heat up your drink.

How Long Should Water Stay Cold or Hot In Your Thermos

Before jumping to conclusions it's important to assess how long drinks should stay hot or cold in your Thermos.

This depends of a variety of factors such as size of your Thermos (larger bottles keep drinks cold or hot for longer), starting temperature of the drink, outside temperature, how full the bottle is etc.

But here are some rough estimates on how long your Thermos should keep drinks hot or cold for.

  • Boiling water – Hot for 8-12 hours before turning lukewarm
  • Ready to drink coffee (starting temp around 60ºC/140ºF) – 2-5 hours before turning lukewarm
  • Thermos full of ice (no water) – 24-48 hours
  • Half ice/half cold water – 16-24 hours
  • Cold water (no ice) – 4-6 hours

How To Check If The Insulation in The Bottle Has Been Compromised 

If there are no obvious signs of damage on the outside of your bottle and tumbler, like dents or cracks, there are ways to test the inside of the bottle. 

Test Your Bottle or Tumbler With Boiling Water

  • Fill the bottle or tumbler with boiling water 
  • Leave the lid off 
  • Touch the outside of the Thermos bottle below the neck 

If you can feel a lot of warmth on the outside of your bottle or tumbler, this means that the insulation layer has been broken and your Thermos needs to be replaced.

If the outside is still cool to touch or only slightly warm (hardly noticeable) then your insulation is likely still intact and you have a different issue.

Once this layer of insulation has been compromised, there is no way of fixing this, and I suggest buying a new bottle or contacting Thermos to see if this damage is covered under warranty. 

Test Your Bottle or Tumbler By Submerging It In Water

This method will work with vacuum-insulated bottles and is quite an easy method to use. You can use a sink, bucket, or even your bathtub!  

To start, you will need to fill the bottle with water until it is full and place the bottle in a sink of water. You should see that the bottle sinks to the bottom. 

If you can see that the bottle is floating, even slightly, in the water, this means that the vacuum insulation has been broken. The cavity between the 2 walls is full of air and this is why the bottle is now floating.

To check out some of my other tricks to test if an insulated bottle is broken, read my helpful guide

Other Reasons Why Your Thermos Bottles/Tumblers Are Not Keeping Drinks Cold or Hot

Sometimes it’s not always broken insulation that is to blame. There are some other reasons why your bottle or tumbler is not working. 

You’re Not Using Enough Ice 

When it comes to keeping drinks cold, ice is your best friend. If you’re only using a little bit of ice in your bottle or tumbler (or just using cold drinks) your drink will warm up rather quickly and there is nothing to keep it cold.

Say for example you're using milk in your Thermos and you want it to stay cold. You may want to avoid using ice because it dilutes the milk so you just put cold milk in your Thermos and nothing else. On a hot day this milk will warm to room temperature in just a few hours. The same goes for cold water without ice.

I like to fill my bottle with at least 50% of ice so that it stays as cold for as long as possible, especially if the bottle is going to be sitting in my backpack all day. 

You’re Using Warm Tap Water (For Cold Drinks)

Filling your bottle or tumbler with ice only to pour room temperature tap water over it will melt the ice rather quickly and your drink won't stay cold for as long.

I fill my bottle with ice and then pour in pre-chilled water from the fridge, so the ice doesn’t melt too fast. 

Your Thermos Is Only Half Full

This one can cause major issues especially if you're using a large thermos and only filling it up partially.

The more empty space inside your thermos the quicker your drink will cool down or warm up.

For example, 20 oz of boiling water placed in a 40 oz Thermos will cool down much faster than that same 20 oz of boiling water placed in a smaller 20 oz thermos. The same works for cold drinks as well.

Try to fill your Thermos up all the way to keep your drinks cold or hot for as long as possible.

You're Leaving The Lid Open

If you leave the lid open of your thermos, especially when it's filled with hot drinks, then a lot of heat wall escape from your drink in the form of steam.

As steam rises it actually takes with it a lot of heat and your drink will cool down faster. Leaving your lid on when not drinking will keep your drink hot or cold for longer in your Thermos.

You Didn't Preheat The Thermos

This is a small tip and doesn't have a major difference on how long your drinks stay hot for in your Thermos but for maximum heat retention you need all the help you can get.

If you're filling your Thermos without preheating it some of the heat from your drink will be lost to the cold walls of the Thermos.

Pre-heating your Thermos by filling it with boiling water first and leaving it to sit for 30-60 seconds. Then discard the boiling water before filling up your Thermos with your hot beverage.