I was asked (in TBN Carry-all for firewood ) to put up some information about my combination wood/gas heating system. I will attach a pic of the wood 'boiler' that I installed and plumbed in with my gas boiler such that either can heat the house by themselves, or the two can be in series, with the gas as a back-up if the water gets below temp, now set at 100 F.

I installed this system in 1980 (thereabouts) when I had an oil-fired boiler, but since have changed to gas-fired unit (MUCH more dependable). I can maintain heat in the house with wood at temperatures in the -20 F, if around to feed it with wood, which in a normal cold winter takes about 6 cords (4x4x8 full cord) of wood.

There are three pumps operated by thermostats in the living room, bedroom hall, and rec room downstairs. The wood boiler just maintains hot water up to about 150 - 170 F, after which a temp sensor cuts off power to the intake-air damper, and it closes until the water temp cools enough for power to the damper opens it. When the thermostats call for heat, a pump kicks in and pumps water at whatever temperature it is at, hopefully it is warm. If power goes off, the damper will default to closed, unless manually opened and manually controlled. So there can be heat in the house if there is a power failure.

The boiler is just two steel cylinders, one inside the other, with a plate welded on the back, and the door bolted to the front. There is about a 9 gallon water jacket around the inside cylinder that gets heated by the wood fire. The system has been outstanding, and unfortunately the company is no longer in business. In fact, because the govt. has become involved in requiring all such hot water units to be certified as steam units if used in a house, one cannot afford to buy one as the expense to certify is sky high. That is one reason the outside free-standing wood fired boilers are so popular.

Because of the water jacket, there is not an excess of heat around the unit. There is the added advantage that wood heat is stored in the water, and only used as needed in the house when the thermostat calls for heat and a pump kicks in. Unlike wood fired heaters that put out the heat whether one needs it or not.