How the heat a large amount of water that is safe to use?

I would like to know how heat up a large amount of water ( assumption : 100 liter ) that people could use to take their bath and also it would not be dangerous even if they accidentally ingested it ? . 


My idea is that to design a metal container and have the chemical in it . I'll react the chemical that is inside the container and then I'll put the container into the water , which will eventually heat up the water through thermal equilibrium . 


