posted 12 years ago
Never done this myself, but a similar idea I had was to build a J-rocket burn core, the insulated chimney, and then pipe the hot exhaust through a 90 degree turn and into the top of a standard, gas water heater chimney and force those hot gases down and then out the bottom (gas burner removed) and horizontally out of the house. The water heater should have a similar effect upon the exhaust gases as the standard 55 gallon drum, which is to rapidly draw off that heat, shrink the gas volume, increase density and thus permit the exhaust to be pushed horizontally. For a bonus, build a greenhouse right next to your outdoor water heater, so that the leftover heat not captured by the water heater can be absorbed by the soil in a greenhouse. It's impossible for your water heater to capture all of the heat, since the temp of the exhaust leaving the water heater cannot be lower than the temp of the water itself. There will always be useful heat leftover, particularly in winter.
Make sure there is a condensate drain hole at the lowest point in your flue; done correctly and completely, the steam in your wood fuel will condense and give up most of it's "heat of evaporation", and this is much of why the RMH can heat with a fraction of the fuel that a "75% efficient" EPA woodstove can accomplish.