Obtaining Domestic Hot Water from a Wood or Coal StoveMany folks have expressed a desire to obtain hot water from their stoves.
There are two different types of heat exchangers which can be fitted to stoves and used to heat your domestic water.
1. External Heat exchangers - if the stove has a large flat surface on the rear, then a serpentine can be fabricated that goes against the rear. If it is enclosed with a layer of sheet metal behind this coil, it will provide better heat. I've had them custom made..but the same shops that make DHW coils (tankless heater) for hot water boilers. These coils were made from a finned copper (usually 3/4"), so much the better for heat exchange. You could make your own by using 180 degree copper bends, but use high-temp (silver) solder so the coils don't come apart if they ever hit a very high temperature.This would only happen if they ran out of water and the stove was VERY hot. Input would be into the bottom of the coil, and output from the top. A pressure relief valve should be installed next to the coil...WITH NO VALVES BETWEEN THIS PRESSURE RELIEF VALVE AND THE COIL.
2. Internal Heat Exchangers - A few companies make such an item, although it may not be easy to find. The best ones are small tanks or coils made of stainless steel. The kits come with instructions and a pressure relief valve. In order to install an internal heat exchanger, a hole must be drilled into the stove body. This may be a job for a professional, as you don't want to compromise the safety or integrity of your stove.
In many installations a pump and control will be needed. The only exception
is when the unit is close to a tank and can be set up for a thermosyphon (the
rising and falling of heated and cooled water) loop. I have used 1/100 HP brass
circulators (available from Graingers) along with an aquastat to make the pump
turn off and on when the water at the top of the coil heats and cools. I usually
set this control to go "on" at 140 degrees and "off" at approx. 120. This then
circulates heated water back to the hot water storage tank.