Is that gasket material around where the old doors were, sealing that gap well enough? You could be losing draft because it is pulling air from that gasket. Maybe try sealing that with refractory cement instead.
Thank you for the helpful comments, folks. I sealed the hole at the bottom of the chimney, and that seems to have caused a noticeable increase in the draft. We've tried burning it hot today, and haven't had any trouble with smokeback yet. Maybe that will be all we needed to do!
I don't know how to tell whether the superwool is forming a good enough seal in the gap where the doors used to be. I'll try holding a lit match near it to see if there's any air movement toward the seal.
I haven't tried raising the plunger tube yet; as I said above, maybe I won't have to.
I believe in KISS. Keep it simple stupid I tell myself. Wood stoves do not make efficient heaters because of the fire cycle. I had an outside woodboiler that worked efficiently because I used the woodstove fire like a battery charger and had a big water tank as the battery. Then I got good even heat in the house controlled by a normal thermostat and got all the btus out of firewood by burning at full bore and little to no smoke. The draft, water temp, circulator pump etc was all controlled ...all I had to do was throw in big chunks of wood. I run a woodstove now because of a change in living circumstances....the woodboiler was way better. I have to open windows from overheating or I am smoldering the fire. Luckily I have triple wall metalbestos straight ss chimney pipe going straight from top of stove to over 2' above roof and it has a cap on it so I get no downdrafts, great draft no matter the weather (like when it is raining and in most stoves next to impossible to have a fire) and I do not have to deal with creasote. I like things that are as reliable and simple as possible even if not as efficient as a complicated unit. Your woodstove set up as original should be able to roast your house I would think...maybe add mass to store heat? Hook up a hot water exchanger and a water storage tank and make it into a wood stove boiler combo?
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