Here's what Fred told me: I want to transfer heat energy from a combustion chamber to air enclosed in a dwelling. I want to use my existing baseboard hot water pipes as the heat exchanger, exactly as I did when I was heating the house with oil. So now the question is how to get the heat into the water in the baseboard pipes and circulate it. Okay, those are the givens. Now, here's what I'm thinking; dig a hole in the cellar, 6' x 6' x 6'. Line the hole with concrete, sides and bottom. Mortar in a flue collar for a stove pipe exiting this box. This flue collar will hold a stove pipe that will enter my chimney. In the middle of this concrete in-ground box, build a smaller concrete box, say 3' x 3' x 4'. This smaller box will be my combustion chamber. Fill the space between the two boxes with rocks. Install air intake pipes which will pass through the outer box and into the inner box for make up air and combustion oxygen. These air inlets can be closed to extinguish the fire or opened fully to feed the fire. Design the combustion chamber to create an aggressive, hot, cleanburning fire to charge the rocks. The charge should hold for at leat twenty four hours. Exaughst from the combustion chamber will pass through the rocks before exiting the flue collar and going up the chimney. This should cool the gasses considerably and transfer almost all the heat from the exaughst into the rocks. On top of the rocks lay a network of copper tubes looped into the existing baseboard system. Heat absorbed by the rocks will transfer to the water. By thermal siphon, the heated water in the pipes will flow up into the house and be transferred into the air of the rooms above. This water now having given up its heat to the air in the rooms will return to the rock bed loop and be reheated. A sealed lid will cover the combustion chamber during firing. The rocks will absorb and store the heat energy generated in the combustion chamber during aggressive, hot, quick, clean firings. Barney said, Fred you are assuming that rocks absorb, hold and release thermal energy more effeciently than water. You are assuming the heated water will siphon up to the radiators and back down again. You're assuming the hot water system will work much like a tankless boiler and that having air between the rocks is a better way of holding in the heat than filling the space with water. You are assuming that a horizontal, in-ground fire box insulated by rocks is going to act like a huge heat battery. What makes you thin you can design a gasifying combustion chamber that can burn unsplit wood? Fred, seek the advice and counsel of people better experienced and more informed about these things than you are.