They have been done. Dangerously! There are YouTubes.

But all that I have seen are terrible kludges. There's more to making a safe water heater or boiler than lay people realize. (I'm speaking as a licensed master plumber, and as an engineering degree holder.)

Here are some Youtube's that I like:
https://www.youtube.com/watch?v=9bU-I2ZiML0 (Mythbuster's water heater)

People have tried to discuss them there, and always ended up proposing unsafe things. So . . . I'm not allowing those discussions.

Sorry, but thread closed!