If you already have a wood-burning stove at home that heats your water, this is not going to be of much interest to you…
If however, you rely on gas/electricity to heat your water, this is a great little emergency preparedness project for backup hot water in emergencies. It would also be useful for remote/off-the-grid log cabins etc. With this system, you effectively have a free, constant supply of hot water stored in an insulated water tank (old water heater), much like folks with built in wood burning stove boiler systems.
You can pick up those old water heaters for nothing, or next to nothing from FreeCycle and Craig’s list etc. They often get wasted or recycled when people get their heating systems replaced for new modern ones.
Endless hot water without electricity!