How to clean a Thermos flask

From biodegradable soap to white vinegar and baking soda – here are the best ways to clean your flask to make it last as long as possible.

How to wash a thermos flask

Is there anything better than a hot drink when camping, or a perfectly chilled iced coffee on a hot morning hike? More than just comfort, a hot drink can do wonders for warming you up if you’re camping on a cold day. A well-insulated flask can keep your drink warm for between 12 and 24 hours, so you don’t need to be in the proximity of a Starbucks to get your caffeine fix.

Like most things, the shelf life of your flask depends on how well you take care of it. A good flask can last years, but to prevent yours from staining, rusting, or getting that horrible musty smell reminiscent of wet dog, you need to clean it properly, and regularly, and rinse it after each use. 

It’s also important to note that flasks aren’t just for drinks! They can double up as a lunchbox and are a great way of ensuring foods with lots of sauce don’t leak all over your backpack. Premake your noodles or porridge and enjoy a hot lunch on a day hike, without needing to pack your stove.

Editor’s notes

A high-quality Thermos flask should last years – but that’s assuming that you look after it properly. Learn how to properly clean your hiking flask and get as much life as possible out of your trusty thermal hiking companion. 

How to clean your flask while camping

On short trips, simply rinsing your flask during your travels should be good enough. But if your trip is a little longer, or you’re mixing funky flavours (e.g. you’re drinking coffee in the mornings and soup in the evenings), you’ll want to use a washing-up liquid that’s biodegradable, since you’ll likely be washing your flask in rivers or streams. 

Man Holding a Thermos Flask When Camping

We particularly like Dr Bronner’s Castile Soap, which triples up as a soap that you can use to wash your hair, body, and pots and pans. Sea to Summit sells good, lightweight pot scrubbers that last ages.

How to clean your flask at home

You can clean your flask with ordinary washing-up liquid when at home. Make sure you rinse it thoroughly, especially the stopper, to stop your hot drinks from tasting soapy. A good way to get into the small grooves in your Thermos stopper is to use a toothbrush. 

Some flasks are fine to go in the dishwasher, but bear in mind that washing your flask in this way will decrease its performance over time. To dry your flask, it’s best to upend it and let it air dry, before giving it a final dry using a tea towel.

Is your Thermos stained or smelly after a few days in your hiking backpack? Pour half a cup of distilled white vinegar and a tablespoon of baking soda into your flask, add warm water, and leave it to froth for about 10 minutes. Make sure you rinse it really well afterwards, otherwise it won’t taste great! Never use chlorine or bleach to clean a stainless steel flask, as this will damage it. 

If you’re using a glass flask rather than a stainless steel flask, washing it with just vinegar and water is generally sufficient. 

Struggling to scrub the bottom of the flask with a regular sponge? Use a long-handled bottle brush.

How to clean flasks with straws

Straws look as though they’d be really difficult to clean, but this is deceptive – it’s very simple. If your flask has either a stainless steel, silicone, or glass straw, you can clean it by placing it in boiling water for 10 minutes. 

Alternatively, a steel straw brush is a great tool to remove any stubborn residue. These are also great when cleaning reusable drink straws at home.

5 flasks that deserve a spot in your hiking pack

Looking for a new flask to add to your outdoor gear? Here are our top picks to keep your food and drink tasting their best.

1. Stanley Go Everyday Tumbler

For everyday use and solo trips, the Stanley Go Everyday Tumbler is compact and nifty. The double-wall insulation keeps iced drinks cold for up to 20 hours. 

2. Hydrapeak Sportboot Straw 

The Hydrapeak Sportboot Straw is so sturdy that it feels invincible, and it looks stylish too. The only downside is that it’s rather heavy. 

3. Thermos Ultimate Flask 500ml

The Thermos Ultimate Flask keeps hot drinks hot for up to 24 hours, double that of most flasks on the market. At under 300g, it’s also lighter than you’d expect. 

4. ESPRO Travel Coffee Press

If you can’t go without proper coffee on an adventure, the ESPRO Travel Coffee Press is your new best friend. The double wall insulation keeps drinks hot for up to six hours, but the selling point here is the coffee filter, for a perfect brew on the go.

5. YETI Rambler One Gallon Water Jug

Feeding an entire expedition party on soup? The YETI Rambler One Gallon Water Jug is enormous and has a convenient carry handle. Not one for ultralight backpackers, but perfect for those wanting to go all-out with their hiking and camping gear.

FAQ: How to clean a Thermos flask

Still have a question about how to thoroughly clean your Thermos hiking flask? Check out our FAQ below where we aim to answer some of the most popular cleaning questions. 

We recommend washing your flask after each use with a good washing-up liquid, then performing a deep clean as and when necessary. You can use a mixture of distilled white baking soda, and warm water to do this.

Some flasks can be cleaned in the dishwasher, but this is likely to decrease their performance over time. We recommend checking the instructions on your flask, and hand washing whenever possible.

Steel straw brushes work really well to remove stubborn residue in flasks. You can loosen this dirt by soaking the straw in boiling water first.

Still have a question about the best way to clean your hiking flask? Reach out to the Adventure Pending team and we’ll get back to you ASAP with tailored advice!

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