Into the Storm
Wood stove water heaters
An alternative energy option, is to adapt an existing wood stove to heat household water. A stainless steel coil is installed inside the stove. Because the coil is stainless steel, it exceeds EPA requirements for drinkable water. To install inside the firebox, a hole saw is used to drill two holes for the ends of the coil (inlet and outlet). The coil can be plumbed directly to a preexisting hot water tank. No special holding tanks are required. When the wood stove is not being used, flip a switch, and the water heater takes over.
If the existing hot water tank is located higher than the stove, hot water can circulate by natural convection. For this setup, the only additional fittings may be a one-way check valve in the line, and one temperature/pressure relief valve at the hot water tank.
If instead the hot water tank is located distant from, or below the level of, the stove, an additional relief valve should be used near the point where the hot water line attaches to the coil, and a circulating pump should be installed along the cold water line leading to the coil.
In both cases, cold water is taken from the drain valve near the bottom of the tank, and the hot water returns through a tee at the top of the tank.
Since the water continuously circulates through the stove, it will get just as hot as electrically heated water. If the stove burns extremely hot, like a coal stove, there are quite a few different ways to "cool down" the hot water. One of the simplest is to install an in-line, variable speed circulating pump. The faster the water circulates, the "cooler" it will be.