This is a common question for people who are looking to make an energy drink. Some sources say that replacing the liquid with boiling water is most effective, while others insist it is not a safe procedure. It also depends on which type of thermos we are using. Whichever you choose, be sure to practice caution and read the instructions on the thermos. 

Construction of a Thermos 

1. Glass Walls With Silver Surfaces 

The walls of a vacuum bottle are made of glass and the surface is silvered. The silver layer prevents the water from coming in contact with any other wall to conduct heat. The walls and silver layer place a barrier between the boiling water and the inside of the thermos.

2. Vacuum Insulation

A vacuum bottle is specially designed for insulation. This quality enables it to transfer heat toward its inner silver surface for cooling while preventing heat from traveling through its outer walls.

3. Bottom Venting System

To prevent the water from boiling over, a thermos has a bottom venting system. The vents allow air to flow into the bottle to replace the pressure that is lost as water boils. However, this option is available in some types of thermoses.

4. Inner Sealing Cap

The inner sealing cap closes the top of a thermos to seal the liquid inside. The cap is equipped with a narrow opening, which restricts where steam can go as needed for temperature control.

5. Double Walled Metal or Plastic 

Some types of thermoses are constructed of a double wall between the inner and outer container. The insulating material is usually stainless steel, but it can also be made from plastic for heat retention or insulation.

6. Sealing Rings

O-ring seals ensure a good fit between the cap and the outer body of the thermos bottle. A tight seal prevents any leakage of heat from escaping through a loose fit. 

Will Boiling Water Damage the Thermos? 

Adding boiling water to the thermos is a common practice for energy drink makers. However, manufacturers of thermos bottles advise against boiling water in the bottle. The following reasons may help explain why:

1. Stainless Steel or Plastic? 

Some types of thermos bottles are made of stainless steel, which is prone to absorbing the smell and taste of various liquids. If you add boiling water to a thermos made from stainless steel, the water may impart a metallic taste to the drink.

2. Paint Discoloration 

Some thermos bottles, with plastic as their raw material, are discolored by the heat. For example, one study found that plastic bottles painted with a white enamel were discolored by temperatures that were not hot enough to melt the enamel. Discoloration of the metal or plastic can lead to rust over time. 

3. Leakages 

Water leakage is highly likely if you add boiling water in a thermos that has a loose seal or does not have an O-ring. 

Which Thermos Is Most Susceptible to Damage From Boiling Water?

Glass-lined thermoses are the most susceptible to damage. They are best used with cold water or hot water that does not exceed 160 degrees Fahrenheit (71 degrees Celsius).

Aluminum thermoses are less susceptible to damage than glass-lined thermoses. Thermal shock is the main concern with aluminum thermoses, so it’s important not to let the heat transfer occur too quickly. They can be used for hot liquids that do not exceed 185 F (85 C).

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Boiling water may not damage your thermos permanently. However, it can lead to discoloration of the metal and plastic and develop a metallic taste if the bottle is made from stainless steel or plastic. You may be better off using cold water with a thermos than replacing the liquid with boiling water. Some thermoses, however, are quite good at retaining scald water.