How Does a Thermos Work? (The Secret to Warm Coffee!)

Can you imagine human life without a thermos flask? It is a magical device that can keep cold beverages cold and hot beverages hot. This is the reason why you have one in every home. But how does a thermos flask work to keep your coffee hot and, at another time, keep your drink cold?

A thermos is a bottle that is protected with a double-walled silver-coated container. The contents’ temperature is kept constant by the vacuum between the double walls. The reflective layer of the inner wall constantly bounces back the radiation to keep the contents’ temperature constant.

How Does a Thermos Work

How Does a Thermos Flask Work?

A vacuum is a space that is devoid of air. It acts as an excellent insulator. The Thermos or Vacuum Flask is made to prevent the flow of heat from and to the flask. It consists of two inner chambers that are placed together and joined at both ends.

The air between the gap of the two inner chambers is partially emptied to create a partial vacuum. The vacuum reduces heat flow via conduction, convection, and radiation.

Invented in 1892 by Sir James Dewar, a thermos Vacuum flask is an insulating storage device that will extend the time its contents maintain its hotness or coolness.

Thermos vacuum flask is usually made of glass, plastic, foam, borosilicate, and metal because they are bad heat conductors. Hence, heat is not lost or gained.

Heat transfer often takes place via the opening at the neck of the flask where no vacuum exists. The openings are made with stoppers like polyethylene plastic or natural cork. Many manufacturers are now using expansion stoppers to keep the stopper tight with the top cup.

Vacuum flask work to prevents heat transfer by either convection, conduction, or radiation. The vacuum makes conduction impossible. The tight stopper prevents air from leaving or entering the flask, making it impossible for convection to happen. The silver reflective lining of the inner chamber pushes back the infrared radiation which is trying to leave the hot liquid.

Heat can’t escape from the vacuum flask making the hot coffee stored inside stay and remain steaming hot for a long time.

The flask will also keep cold beverages cold. Since heat cannot escape from a vacuum flask, nor can it heat penetrate in the flask. This ensures the beverages remain chilled!

How Does a Thermos Flask Work

How Thermos Flasks Prevent Your Coffee from Getting Cold?

Thermos flask doesn’t allow heat to escape from it via all modes of heat transfer, i.e., conduction, convection, radiation. It does so because of the silver coating on its thin walls and the inner vacuum. As the infrared radiation tries to escape the contents, the reflective lining prevents any heat from escaping.

coffee in a thermos

Below is the Physics explaining how a thermos Vacuum flask work to keep your coffee hot.


The air around the flask that contains your coffee is a poor conductor. The vacuum prevents conduction from the taken place because of the absence of air. When there is no air, there will be no atoms available to assist the heat transfer.

The thinness of the silver lining of the walls also blocks heat from leaving or entering the flask via the conduction process.


Convection cannot happen because there is no air circulation in the thermos flask, i.e., the air is trapped inside the flask container. Both the lid, cork stoppers, the vacuum between the flask and the container also prevents heat movement by convection.


The interior of the Thermos vacuum flask prevents heat transfer via radiation from occurring. The bright silver surface both outside and inside the Thermos reflects heat and prevents loss.

Radiation can only happen if the cork stopper is taken off from the Thermos flask.


The cork stopper reduces evaporation from happening because the coffee is not exposed to the air. The vacuum created by the silver walls also prevents evaporation.

How Long Can Coffee be Kept in a Thermos?

Thermos flasks will retain your coffee temperature for 6 hours. The bacteria growth starts if the temperature falls within 40 °F and 140 °F (i.e., danger zone where bacteria start growing). Most dairy products are experience faster bacterial growth. It is recommended you consume your coffee within 5 hours if it contains milk.

Pouring Hot Tea from a Thermos

How long your coffee stays hot depends on the thermostat and the brand of your Thermos. It is good to check the instruction that comes with your Thermos to know how many hours it can keep your coffee warm. Your consumables will only stay safe if they are steaming hot at above 165 °F while you pack them.

A great Thermos brand can offer a flask that is well insulated. This flask will maintain the required temperature you need for your coffee. Preheating your Thermos before filling it with coffee will extend the time it remains hot.

You can preheat your Thermos by filling it with 2 cups of hot boiling water and letting the lid stay tightly covered for 5 minutes. As soon as you pour out the hot water, you can proceed to pour hot brewed coffee in the flask.

How to Take Care of Your Thermos Vacuum Flask?

The best way to keep your Thermo flask working is to maintain, clean, and take care of it regularly. It is a handy container that can give you access to a drink in the desired temperature in and out of your home.

There are concerns about cleaning the Thermos flask due to the structure and construction of the bottles. Below are the best approaches to get rid of the odor and stains to keep the Thermos flask germ-free.

Use a Tea Bag

You can employ tea bags to clean your Thermos flask to keep it in a running condition. Drop a tea bag in your Thermos flask filled with boiled water and leave the mixture overnight. Empty your Thermos flask the next day, and you will love your spotless and clean flask.

Use Baking Soda and Vinegar

You can use either a mixture of baking soda and vinegar or just baking soda alone.

Baking soda can be wonderful to clean your Thermos flask. It will let you clean the flask comprehensively.

If you want to use a mixture of vinegar and baking soda, do the following: Mix a teaspoon of cider vinegar with a teaspoon of soda and warm water. Leave the mixture for a while and later scrub and rinse the flask. It is an excellent way of removing odors and stains from the flask.



Neither taste nor consume the contents of your broken Thermos flask. The fibers from the internal part of the flask might have contaminated the food.

General Thermos Flask Maintenance Tips and Tricks

  • Never use chlorine bleach to clean your Thermos to avoid damages to the steel weld.
  • Do not dip your Thermos flask in water to prevent water penetration into the walls of the vacuum.
  • Always leave some space for the cork and stopper when filling your flask.
  • Do not use a microwave to preheat your flask. Instead, use either cold water for cold beverages or boiled water for hot drinks.
  • Ensure you always rinse your Thermos flask with warm water if it has been a while since it is in use.

How to Reduce Thermos Flask Hazards?

Yes, Thermos flask explosion can happen. But why do Thermos flasks explode? Can you avoid and prevent vacuum flasks explosions?

Woman Holding Thermos Cup outside

Thermos flask explosion happened when you leave something within the flask for a long time. The temperature drops, and bacteria begins to grow. The gas that is produced by the bacteria will build up pressure within the flask during this process. The more the pressure, the higher the chances of an explosion occurring.

The probability of a thermos flask explosion can be minimized if you follow these tips:

  • Do not expose your flask to unnecessary vibrations from loud music or any equipment.
  • Never drop or smack your Thermos flask that is already loaded with food or content.
  • Ensure you anchor your flask with a suitable clamp.
  • Check your Thermos flask regularly for flaws like scratches and cracks before usage.
  • Ensure you stick to using a smaller Thermos flask. They are unlikely to explode.
  • Make sure you read your Thermos flask instruction before usage.



Never use a microwave to preheat your Thermos flask. It can cause an explosion

Best 5 Thermos Flasks to Keep Your Coffee Hot

Below is our list of the best thermos flask of all time that will keep your coffee hot.

1. Stanley Classic Vacuum Bottle

[amazon box=”B000FZX93K”]

  • Keep your content hot or cold for 24 hours.
  • Its 18/8 stainless steel construction is tough and durable.
  • The leak-resistant lid is multipurpose, and it doubles as a cup and Thermos flask.
  • The product comes in multiple sizes: 1.1qt, 1.5qt, and 2qt.
  • Built for long-term use and comes with a lifetime warranty.

Contigo Autoseal Vacuum Travel Mug

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  • The drink will stay chilled for 18 hours and cold for 7 hours with the thermal-lock insulator.
  • You can hold it with a hand and push a button to release the drink.
  • Its auto-seal technology made it to be leak and spill-proof.
  • It is 100% BPA-free.

Cresimo 68 Oz Stainless Steel Thermos Coffee Carafe

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  • It is made with excellent stainless steel and is designed superbly.
  • Easy to clean and refill.
  • Simple push-button that allows a single hand coffee pouring.
  • It has an Insulation vacuum technology that will lock in the vacuum’s heat.
  • Beverages are guaranteed to stay hot for 12 hours and cold for 24 hours.

THERMOS Stainless King Travel Mug

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  • It is dishwasher safe.
  • Insulation technology that keeps liquids hot for 7 hours and a beverages cold for 18 hours
  • It has interior and exterior stainless steel that is rugged and durable.

Stainless Steel Coffee Mugs

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  • Easy to clean with premium quality, it will last for years.
  • The air-seal design will keep your drink hot and beverage cold.
  • Made of 18/8 304 stainless steel.
  • Zero toxins and BPA-free.
  • Coated stainless-steel design looks great and can never rust.


Now that you know how the Thermos vacuum flask works to keep your coffee hot, you can take care of it and use it effectively to your advantage.

The double-walled silver-coated inner chamber of the flask is a poor conductor of heat. The double wall of the flask forms a vacuum that does not allow heat flow via convection, while the silvery walls prevent loss of heat via radiation. These processes help the Thermos flask to maintain and retain the heat of its content.

A thermos vacuum flask is protected with a silver-coated container. The content’s temperature is kept constant by the vacuum which prevents conduction, convection, and radiation from occurring. The silvery layer of the inner wall deflects the heat via radiation to keep the content hot.

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