For about 5 years we have had a propane on-demand water heat for all our hot water needs. Last year we had the chance to get a second hand wood fired water heater. It ain’t pretty, but it sure does the job.
We tapped into the hot and cold lines of our on-demand water heater and plumbed in the wood fired one and connected valves so that we can isolate one or the other.
Since there is no way to regulate the temperature of the water, we piped the pressure relief valve into an old copper water heater (which had belonged to my great-grand parents) that we use as a holding tank. This way if the water gets too hot and blows off it goes into the extra tank (instead of all over the wall….this actually happened and was quite a wake up call). We then can drain the copper tank into a bucket, there is a valve at the bottom that we attached a short hose to, and once it has cooled it can be used to water animals or plants.
One thing that I really love about the wood fired water heater is that we can burn sticks and all sorts of small scrap pieces of wood. The tank holds about 5 to 6 gallons of water, plenty for a nice hot shower. In fact, it works so well that we shut off the propane water heater last winter and it hasn’t been on since!
The copper holding tank is situated behind the chimney so I can't show a picture of it. The plumbing goes in behind the chimney.
Picture of water heater with door removed to show the fire box. We have the heater up on a metal stand to make using it easier.