I'm building a well-insulated microhouse, and want an indoor wood heater. Calculations show that I need around 1600 btu/hr at -40. An online calculator gives me a flue size of .26 sq". A 3/8 diameter tube would be suitable. That's steady-state, requiring constant fuel management, so I'd plan for higher peak btu's. Flues shouldn't be more than 3x the peak requirement, so just sticking on a 3" flue may not be a good choice. It's not -40 all the time here, so I should figure out my typical btu/hr, and trade-offs between fire tending and fire re-lighting. Has anyone here had experience with small-diameter flues? I just had an idea: multiple flues. If 1/4" tubing is optimum for mild days, and 2" for getting the house warm quickly at -40, I could run the smaller tubing down the center of the larger flue, and damp the larger one when appropriate. Just toying with ideas at the moment. I'm also toying with the idea of active control of the heater: sensing firebox and secondary burn temperatures, controlling air flow to both, and possibly spark ignition to assist secondary burn when necessary. That would give a more consistent burn rate, and thus flue size.
