Why Your Stanley Thermos Isn’t Keeping Food Hot Anymore

Using a thermos is the best solution to maintain the temperature of your food and drinks. However it might happen that your Stanley thermos isn’t keeping your food hot anymore.

It is also possible that a new Stanley thermos you bought is not working properly. So what exactly influences the heat retaining capacity of a thermos and how can you identify if your thermos is broken or faulty?

If your Stanley thermos isn’t keeping things hot anymore it is likely because air has entered the vacuum insulation layer which is responsible for retaining heat. Once the vacuum is gone there is no other option other than to replace your Stanley thermos.

There are certain tests that you can perform to check whether your Stanley thermos is working properly or not.

This article will describe all these tests and what you should do if your Stanley thermos has broken or damaged.

Why Isn’t My Stanley Thermos Working Properly

All Stanley thermoses contain a double-layer construction on the inside. A vacuum is created between the outer and inner walls which acts as an insulator. This vacuum doesn’t allow the exchange of heat between the two walls.

Did you know: A vacuum is the most effective form of insulation. It works better than any foam or other material for keeping things hot and it’s why thermoses work so well.

If for some reason this vacuum layer breaks, air fills up this space depriving the thermos of its insulating property. Air allows much more heat to pass through the layers so the drinks and food inside the thermos will quickly get to room temperature.

The same thing makes other insulated products like Yeti Tumbler cups or Hydro Flask Bottles stop working properly.

In a properly working Stanley thermos, the only place where the heat can escape is the lid or opening of the thermos where there is no vacuum. So the heat escapes very slowly keeping the inside food hot for long.

How Can My Stanley Thermos Lose Its Vacuum?

The most common way a thermos’ vacuum layer breaks is from physical damage. If you accidentally drop your Stanley thermos or it receives a heavy blow, the vacuum layer can open up allowing the air to enter inside. 

While not a “Stanley” branded product my Hydro Flask bottle (imaged below) lost it’s vacuum when my kids somehow damaged the bottom of the bottle. It stopped keeping drinks cold or hot and they would return to room temperature very quickly.

It can be difficult to identify what caused your Stanley thermos to lose its vacuum because only a little hole is enough for the air to fill the vacuum chamber completely. 

Sometimes there is no physical sign of damage at all to your Stanley bottle and the only way of knowing the vacuum insulation is compromised is through the performance of the product. If it’s not keeping food hot anymore then chances are the vacuum has filled up with air (whether you can see damage or not).

The performance of such a thermos drops significantly because the vacuum is the only thing stopping the heat flow. Because this is not filled with air molecules heat can be transferred through the air and out of the thermos, so your food will cool down faster.

It will still work better than a non-insulated bottle but the purpose of using a bulkier thermos would be lost.

How To Check If The Vacuum Seal Has Been Compromised?

If there are no signs of physical damage on the outside of your Stanley thermos, the vacuum seal must be broken from the inside.

Before you go ahead and contact customer support or go out to buy a new thermos (if your warranty is void) you’ll want to double check and make sure the vacuum seal is in fact compromised.

Following are some ways to ensure if your Stanley Thermos has lost its vacuum.

1. Filling Your Stanley Thermos With Boiling Water

This is the quickest and easiest way to check whether your Stanley thermos is working properly or not. Pour some boiling water into your Stanley thermos without putting the lid on. 

After a while touch the thermos below the neck. If you feel warm patches all around it means that the vacuum insulation has broken. 

A proper functioning thermos should feel cold to touch on the outside as it does not radiate much heat after adding hot water.

In the video above you can see me pouring boiling water into thermoses. Most of them work and one of them is broken.

All thermoses started with an outside temperature of around 80ºF (26ºC) which was room temperature on the day. After pouring boiling water in them the working thermoses maintained an outside temperature of around 80ºF (26ºC)

The broken thermos on the other hand had an outside temperature of around 110ºF (43ºC) within just a few seconds of the boiling water being in there.

2. Compare your Stanley Thermos With Another Thermos

You can easily know whether your thermos is working well or not by comparing it with another similar Stanley thermos. If you don’t have another one lying around you can borrow it from a friend. 

It doesn’t have to be a Stanley brand either. Any vacuum insulated thermos will do.

Fill both the thermoses with the same temperature warm water and notice the initial temperature. Use boiling water like mentioned above to see faster results or you can also used ice water but this will take longer to notice the difference.

Using a thermometer will yield accurate results but you can also get an idea of the temperature by touching the water inside thermoses. Obviously be careful here that you don’t get burned by the boiling water.

Over time you should notice the water in your broken thermos return to room temperature significantly faster than the other working thermos.

3. Submerging The Thermos In Water

If you completely fill a thermos with water whose vacuum insulation is still intact and submerge it in water, it will sink to the bottom.

This is because there is no air inside it and the metal is denser than the water so it sinks. Above you can see my white thermos BEFORE it was broken and I can confirm that it sunk. However, after it became broken it then floated slightly when filled with water.

However, if air has taken over the vacuum chamber the thermos will float even if you fill it with water completely.

Start by filling a deep container like your sink or bathtub with water. Now slide your Stanley thermos in and let it fill up completely. If the thermos floats when you release it, air must have gotten inside your vacuum layer.

What Should You Do With A Broken Stanley Thermos

Once your Stanley thermos loses its vacuum insulation it’s pretty much an impossible task to recreate the vacuum again. You would need access to a manufacturing facility and these only really exist overseas (mainly in China).

Your best bet would be to do a warranty claim for a replacement of your product. 

Fortunately, Stanley thermoses come with a lifetime warranty and you just need to fill out their warranty claim form to request an exchange.

However, the thermos must not be heavily damaged or dented otherwise you’ll have to buy a new one yourself. 

If you’ve just purchased a new thermos and it’s not functioning properly Stanley provides a 30 days cancellation and return policy for such products as well.

Can A Damaged Stanley Thermos Still Work?

Your Stanley thermos will keep working fine (even for years) after it has been dented or damaged if the vacuum seal is still intact. As long as the vacuum layer stays fine there’s no need to worry.

Sometimes the thermos can get a deep dent or a heavy blow without damaging its vacuum layer. In that case it should work as well as it came new. 

Pass your Stanley thermos through the tests mentioned above to see if the damage dealt has compromised its vacuum insulation. If you find it’s still working well there’s no need to replace it.

In fact you can use dry ice to get dents out of some thermoses. It’s not a guaranteed process but it has worked for a lot of people and can reduce the damage and keep your Stanley in good working order.

The Bottom Line

A Stanley thermos can work like new for a long time. It will only lose its insulation properties if the vacuum seal is broken and air has entered the vacuum chamber. A damaged thermos can still work fine if the vacuum layer inside the thermos is still intact. 

There are some tests you can perform to check whether your thermos is still working properly or not.

  • Compare your Stanley thermos side-by-side with another similar thermos and notice the time it takes to gain room temperature.
  • Submerge your Stanley thermos in water completely. If it floats, the vacuum layer has been filled with air.
  • Fill your Stanley thermos with boiling water. The vacuum seal might be compromised if you feel any hot spots on the thermos.

If your Stanley thermos has lost its vacuum insulation you should ask the manufacturer for a replacement.