Make a heating coil: 1/2" copper or stainless steel tubing wound into an open flat spiral of approximately 3 turns with an over all diameter of about 12" with several ft of straight pipe on each end. Leave at least a 1" air space between each turn. Use car 1/2" heater hoses and connect one end to the bottom of a thermally insulated hot water holding tank. Could use an old gas house water heater. The other end of the heating coil is connected using the 1/2" heater hose near the top of the holding tank (to be under water). The bottom of the tank needs to be placed higher than the heating coil so that water is always forced into the coil as it boils.
Place the heating coil under or near any fire or hot item. If you cook with electricity this coil could be permanently attached to the back side of the bottom and side reflecting plates. If you build a fire this could be placed in the bottom of the fire or under it. The point is, to capture as much of the waist energy escaping to the bottom and sides of any cooking operation as you can. With a small portable container this process can be used to provide hot water when camping-traveling between settlements. Note: Bigger and smaller tubing can be used as appropriate. For example the gas tank out of a car and some of the steel and rubber gasoline lines could be used in a pinch to produce a small version. Warning: Make sure all the gasoline has dried out before using these items near open flames.
Another way: If you have a wood or oil burning space heater, wrap your tubing around the inside, or outside of a hot gases vent pipe, to capture the heat that would normally escape the room.
Offered by Mike.