I tried it. 50' of 3/4 copper coiled in the top of a 55 gallon drum. I
tried
wood and coal

I had also considered it for a Rube Goldberg type of hot water heater.

This thing would boil 30-40 gallons of water pretty quick, certainly fast
enough to supply normal hot water needs but when you have 300 gallons in an
open tub it takes a lot to make a difference.
If you put the tamk above the heater you would not even need a pump. It would
thermal siphon up there. Make damn sure you have a pop off valve so it doesn''t
blow up on you.