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Thermos flasks are a household essential in many Kenyan homes—keeping mostly tea, uji, or porridge hot for hours.
But despite their convenience, many people unknowingly make simple mistakes that affect the flask's performance and durability.
Whether you use it for daily tea, for storing baby's porridge, or during travel, here are five common thermos flask mistakes you’re probably making—and how to fix them.
1. Not Preheating the Flask Before Use
Many people make the mistake of pouring hot tea or porridge directly into a cold thermos flask. While this may seem harmless, it significantly affects how long the contents remain hot.
When a flask is cold, the inner chamber absorbs a portion of the heat from the hot liquid, causing the temperature of your drink to drop immediately.
This means your tea could start cooling down within an hour instead of staying hot for six or more hours, depending on your flask’s design.
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To avoid this, always preheat the flask. Simply boil a small amount of water and pour it into the flask. Close the lid and let it sit for 2–3 minutes, then pour the water out before filling the flask with your drink.
2. Closing the Lid Tightly Without Letting Steam Escape First
After pouring hot liquids like tea or soup into your thermos flask, it’s common to seal it immediately. However, this traps hot steam inside, building pressure that can cause problems over time.
The increased internal pressure may result in leaks, especially around the rubber seal or lid, and in extreme cases, could damage the flask’s insulation or cause a vacuum seal failure.
To prevent this, allow the steam to escape for a minute or so before sealing. Leave the lid loosely placed on the flask, giving it time to release excess pressure.
This small delay protects the integrity of the rubber seal, helps the lid fit properly, and ensures the flask can maintain temperature without any strain on its structure. It also makes it safer to open later, especially if it has been carrying boiling liquid.
3. Overfilling the Flask
Trying to fill your thermos to the brim may seem like a good way to maximise its use, but it often leads to spillage and inefficient heat retention.
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When a flask is overfilled, there’s no space left for steam expansion. This means when you try to seal the flask, pressure builds up and may push the contents out or cause the lid to malfunction.
Additionally, overfilling can create a mess, especially when carrying the flask in a bag or using it for travel.
The lack of headspace also means the flask's vacuum mechanism has less room to regulate temperature changes, which might reduce how long your drink stays hot.
To fix this, always leave about 1–2 centimetres of space from the top. This space allows for safe expansion, reduces pressure on the lid, and improves thermal efficiency.
4. Using It for the Wrong Types of Liquids
Thermos flasks are designed for specific types of liquids, and using them for the wrong contents can shorten their lifespan or affect hygiene.
For example, many flasks are not suitable for thick porridge (uji) or carbonated drinks. Uji can leave residue that’s hard to clean, especially in flasks with narrow openings.
On the other hand, carbonated drinks can build pressure, increasing the risk of the flask popping open or even exploding if not carefully managed.
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Dairy-based drinks like milk tea or soup with cream can spoil quickly and leave behind persistent smells, especially in plastic-lined flasks.
It’s important to use the right flask for the right job. Wide-mouthed flasks are better for porridge and soups, while stainless-steel options are ideal for tea and coffee.
Always read the manufacturer's instructions before use, and avoid storing liquids in the flask for more than 6–8 hours unless you’re certain it’s safe.
5. Poor Cleaning Habits
Many people rinse their flasks with water after use, assuming that’s enough to keep it clean. However, this habit allows bacteria, mould, and odours to build up—especially around the lid and silicone seal.
Over time, residue from tea, uji, or soup can form a film inside the flask or under the lid, which not only causes unpleasant smells but may also be harmful to health.
A dirty thermos also loses its efficiency. The inner wall may develop a thin coating that interferes with heat retention. To maintain cleanliness and performance, wash your flask daily with warm soapy water.
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Use a bottle brush to clean the inner walls and scrub the lid thoroughly, paying attention to the rubber seal.
For deep cleaning, mix baking soda and vinegar or use a thermos cleaning tablet. Rinse thoroughly and let it dry with the lid off to prevent musty smells.