Thermos on a tree branch
The Reason You Should Never Clean A Thermos With Boiling Water
Boiling water or water heated to around 212 degrees Fahrenheit can melt or damage parts of your thermos, including the seal around the lid.
To ensure the vacuum seal of your thermos is not compromised and its contents are safely insulated, use baking soda and white vinegar to clean it instead.
To clean your thermos, pour in a half cup of vinegar, replace the cap, shake it, and let it sit for a few minutes.
Mix a few tablespoons of baking soda with water and gently scrub your thermos’ interior with a bottle brush. Afterward, rinse it thoroughly with warm water.