A couple years ago we were given a gas fired water heater. Upon inspection it was apparent that the entire burner assembly was useless. So a week or so later I removed all of the burner and the insulation from the bottom foot or so, at first thinking the tank might be useful. About that time I watched a couple videos on using wood fired water heaters and decided to see what I could do with it.
Other than keeping the heat shield used to hold the burner assembly, I found a old unused steel tire rim that the shield fit into snuggly. The lower half I filled with dirt and gravel to help keep it in place. The opening in the shield where the gas burner had entered I opened a bit and used for the wood feed.
Given the fear of over pressure and possible explosion fed by all the warnings we had seen on the net, I only filled the tank with water and left the lines unattached. After filling the “fire box” and lighting it using small branches and kindling, we found that to heat the initial forty gallons took approximately a hour. After, it took less time, refilling the used water, keeping the fire running with only small pieces of fire wood.
At the time we did not have a water heater so it was a godsend for us. Much easer than heating the water on the stove or a fire a gallon at the time. Some time later we were given an electric water heater that with some parts worked and so we stopped using the wood fired one. Given the setting up of our outdoor kitchen I moved it to that location and am setting it up once again for use there. This time I will not have any fear of running it under pressure as it is fairly easy to tell when it is getting hot enough and the pressure relief valve is attached. Given the fire box is less than a cubic foot does not hold enough for a long burn time.
One last thing the warnings online also mentioned the chimney being too small to work accurately. We had no trouble with it on ours nor do we expect to. My guess on others experiences is they were trying to have to big a fire going in it. As we were not in a hurry to have the water heat up we did not over feed it.
I hope this helps any that are interested to try there hand at it though only if you feel comfortable with attempting it.
Silver has said if anyone has questions feel free to post them and He'll do His best to answer them.