The thermosiphon (thermosyphon) loop is a passive way to heat water for domestic use and other purposes. It's a slow process, it will not produce hot water on demand like a domestic hot water coil found in boilers nor will it have the recovery rates of a standard water heater. While the basic example here uses a coal fired application the same method can be used for any solid fuel heating application like wood or wood pellets, it's also the same method used in a thermal solar setup. The only thing you really need for this to work effectively is a constant source of heat. The more heat and heating surface area available the more effective it will be. It will not be effective in a gas or oil fired application because there is not enough constant heat.
A thermosiphon loop will have one small section of pipe or tubing used to heat the water, for solid fuel applications they can be purchased as kits. In the example depicted for a coal fire application there is a U shaped stainless steel pipe that is about 3 feet long inside the burn chamber. As the water temperature rises in that small section it will naturally rise into the top of the storage tank and is replaced by colder water from the bottom of the tank. In households where hot water usage is managed or there is a small demand the thermosiphon loop may meet all of that demand.
If you use a lot of hot water or want to insure you have hot water on demand it can also be used to supplement your existing hot water heater. At the very least it can maintain and increase the temperature of the water. Utilizing a timer that will only turn on a hot water heater at times when recovery rates are important can increase it's effectiveness.
The tank should be located as close as possible to the stove and elevated for optimal performance.
WARNING: Improper installation of a thermosiphon loop can cause severe injury or even death. It can also damage hot water heaters not designed for high temperatures or systems.