Parents tend to gravitate more towards that designs that use boiled water because they are simple and straightforward in how they work. Users simply fill a stainless steel thermos with boiling water to keep it heated for an impressive 6-8 hours at a time. When it is time to eat, parents pour the water around their formula container the heat it to the desired temperature. The downside to these models is if parents don’t have access to boiled water, they have no way to heat their formula.