Rod,
Yep, enough alarms for a bank! :-)
At times when the damper and draft settings are very low, or closed down, the draft in the exchange zone can indeed draw fumes from a leak in the firebox/combustion chamber. I learned this the hard way. My first attempt to seal this point failed and stunk up the joint. That's when I found a better steel pipe and fixed that issue once and for all period.
Eventually I hope to build a bigger and better version of this taking advantage of the slope I own to increase airflow rates. I can then bury the ductwork and have a central system like city folks, except that mine uses no electricity. With an ample firebox, and heating water too, one might get several days between wood refills. A speed controlled auger and wood chips, pellets, coal, corn or whatever solid fuel employed, one could elminate the firedoor, except for a tiny lighting port and sealing ash removal point. Making the design much more effiecent. You oil burners might give it a thought.
In my thinking before building this system I considered a water heating jacket, actually no more than a 30 gal steel drum, to sit in one corner and just soak up heat. If the fire died down I could still get some residual heat from the water and the blocks. Maybe the ideal way to build this is to make the walls and roof from tanks. Along with the convection the mass of water would be a backup. If one could raise 750 gallons of water to 175° F it would surely hold that heat for a long time with any kind of insulation at all.
Closing thoughts;
With the convection system a series of solar collector convection boxes can be added to the show. A little aluminum dryer venting, a little glass, and there goes another rick of wood needed. Build enough panels and the wood would only be needed occasionally. Like I've said, I got the slope facing that southern sun. Using the heated water thermo syphon recirculation, the releasing radiators (placed in the hot air feed ducting) could have the air forced through them by the convection, thereby adding to the hot air stream sent to the living space, which feeds the whole cycle. Simple bypass valving could provide a heat exchanger for domestic hot water.
I've many thoughts and much wood to cut....
