I was inspired by Hobartian’s construction of a Dick Hill Stick Furnace, discussed in this forum. www.hearth.com/talk/threads/my-progress-in-building-a-wood-fired-boiler-based-on-the-design-by-richard-c-hill.68593/ I have been trying to come up with a design for something similar but easier to build, as my skills are nowhere near to his. I think I have come up with something possible that just might work. However, I have not seen anything like it anywhere else; it’s a bit of a mongrel, so I’d really appreciate some expert input. The construction of the Rocket Mass Heaters, as described at www.rocketstoves.com using cob, seems a bit simpler but a lot of work and, I suspect there is a knack to it. The internal chimney effect seems a great idea and I am hoping the draft from that mechanism can allow a simpler forced draft mechanism for my design. As I am not too sure about cob and pumice is readily available here, I was thinking of building mostly in pumice concrete, with high alumina cement. Has anyone experience of this material? It is a bit soft but I am trying for a design where everything comes apart rapidly to allow cleaning and repair, so I’d simply replace any worn parts. If I do a careful job on the moulds, spares would be cheap. Also, as I am not in a very cold climate, the furnace won’t get huge demands placed upon it. Has anyone got any figures for R-Values for different types of refractory and high temperature insulation? What total R-Value should I be aiming for? I am going to build outside, next to the wood shelter, so I can make it much larger than the Dick Hill design, in physical size that is, not in power. The stick feed chamber and the primary combustion chamber will be similar in size to Hobartian’s. The horizontal tube will also be similar. However, the tube will then pass into a large hot updraft chamber. The idea is that this will act as both a long-duration hot combustion zone and a hot insulated chimney. The flow is then passed down over a flue gas to water heat exchanger. Passing the cooling gas downward adds to the chimney effect and also means a thermosyphon can be in counter-current flow. Because I have the space, I was considering making a real meal of the time delay and the chimney effect, by making the riser chamber about 150cm high and expanding the flow to 40 to 50cm in diameter. Given that the whole chamber is well-insulated, how much do I need to worry about stagnation in corners? Is there an issue with getting interrupted combustion in these corners? One reason for making it rise so far is to give me room for a simpler heat exchanger. If corners etc are not too much of an issue, I was considering mounting the heat exchanger inside the basic hot chamber, at one end. It would be constructed of very thick spigotted rings of insulating refractory, stacked to form a riser tube nearly the whole height of the chamber. Flue gas will pass down the tube and exit though the bottom of the chamber. Concentric to and inside the refractory tube, will be a large diameter steel pipe, which passes through the roof of the hot chamber and is suspended from it. This pipe is blanked-off at the bottom. The cold water will enter through a smaller concentric pipe, which leads down most of the length of the outer pipe. The flow then passes up the inside of the hot outer pipe. (The inner pipe should be insulated. Can anyone suggest a suitable insulating material? It needs to be quite thick so the annulus is small enough for good turbulent flow. It has to tolerate flowinf near-boiling water as well.) The heat exchanger should be reasonably simple to build and also easy to dismantle and clean. The contact area could be comparable to Hobartian’s, if I can find a piece of pipe 35cm in diameter that is. I am hoping the Rocket stove -type internal chimney will allow me to do away with the need for a draft inducer at the end of the flue system. I am thinking of introducing forced, pre-heated secondary air into the primary combustion chamber end of the horizontal tube, in such a way that it induces the primary air flow into the primary chamber, as well as helping to force the combined wood gas and secondary air into the horizontal tube.