i've considered a motorized flue damper that closes even when the draft control is open. When heating with a woodstove, once the fire is going good i tend to leave the flue damper closed and use the draft control to alter the temperature of the room. I find a regular indoor woodstove without a damper to be noticeably more inefficient than the same stove with a damper installed. For this reason, i would like to install one on my owb as an experiment to see if more heat gets transferred into the water rather than lost up the smokestack.
I think it would take some fiddling, and perhaps some logic controls to decide what the proper combination of draft and damper settings provide the longest burn times. Variable draft/damper control motors would be ideal.
When I want a longer burn time, I just throw a much larger diameter piece of wood in. I don't even own a damper, stopped using them around..hmmm..73 or so.
Never used or fed an OWB so can't comment on if they need one or not. Haven't had a lick of trouble from regular woodstoves though, since I stopped using a damper. Used to putz with the stoves and pipes all the time, trying to get them to burn well and not be smoky and gunk up stuff, got sick of it, yanked the damper out, started paying better attention to my wood supply, make sure I have a real large variety of species and sizes instead of trying to make a piece of wood burn the way I wanted it to. Wood burns best wide open and..that's it. If it is too much, cut the quantity down or switch to a less dense species, etc. Need more heat, smaller splits or rounds of better wood.
and etc
works for me.
Now I could very well change my opinion on that if I was using some epa stove or owb, so I can't really say there. I have only run various incarnations of the zogger smogger....