Well Murray MacLeod you ought visit a still in your neighborhood for a sampling :)

We are talking of a natural convection siphon here. Heat rises.

An existing or newly purchased stove would need have two holes drilled through into the firebox, for the copper tube, at each end of the coil, to have an entrance and an exit. The copper tube of cold water must enter the firebox lower than the hotter exit copper tube. (one in the same tube of copper possibly ten feet long with about ½ inch internal dia.)

This copper tube entering and exiting the firebox has a coil fashioned by you within the firebox. Attempt to get as much copper, coil/coils (heat transfer surface) as possible within the firebox.

The copper coil should be placed latterly as best to accept the infra red spectrum of the fire within the firebox. Take care when installing the copper tube to somehow protect the copper from being deformed while charging the fire box.

A tank/tanks placed (horizontal or vertical) above the stove become another part of this "closed system" and is connected at its bottom (lowest tank tap) to the cold input (lowest) of the fire box. Conversely, the firebox top exit (highest) of hot water is connected to the top (highest tank tap) hot input of the "expansion" tank.

Mount the tank/tanks soundly! (water, hot or cold is heavy! (about seven pounds per gallon.)

A cold water connection from your cold water system (to the lowest possible input tap) will keep the "heat exchanger" "expansion" "tank" filled.

The location of the tap/taps for your usage of the (free) hot water is your choice.

A pressure sensitive safety blow off valve (with manual option) at the top of the "expansion" tank is a good idea as it is possible the system will make steam before you can quickly enough slow the heat source. At this temperature point you would have a nasty bomb in you house! Begin with small fires and work up toward your limitations.

A water temperature gauge on the "expansion" tank gives peace of mind.
As long as the copper is always kept filled with water, there is no danger to the "soft" copper tubing melting from the heat of the fire.

One more thing to consider is the flow velocity of the water thru your hand fashioned "closed loop" hot water system. When the water flows to quickly (as when using a "circulator pump" to move the water), fewer BTU's are transferred to the water. Conversely, to slow a flow (restricted or natural) with to much heat, will make steam. Depending on what flow you find best useful in the ratio of heat vs flow, installing an adjustable flow valve somewhere along the coolest part of the loop will give you aid in obtaining the best economy (if needed) and the most safety. (needed)

Cheers!