Dave, (DLJ) and I did a test when this topic of adding overfire air to keep a fire going, was last brought up. With all due respect to Lee, what works for his large firebed furnace can be dangerous with other types of stoves. This is not a one-size solution fits all type of situation.
Dave tested this with his #6 base heater and I with my range. We both found that while our stoves initially did increase surface and pipe temps, after a few minutes the exhaust temps dropped and our stoves started to slow down. And, without changing the damper settings, they continued to slow down. Especially with stoves not designed to be as airtight as some more modern.
With stoves that have a smaller firebox volume for their stove pipe size than big furnaces, any extra secondary over-fire air can cool the smaller volume of firebed exhaust too easily and too much.
As the exhaust cools, the draft strength drops (as proved by mano readings during the test). As the draft strength drops primary feed air is less likely to be pushed through the resistance of the firebed when there's less resistance for air to go through the secondary's to reach the lower pressure in the stove and chimney system. As a result, the draft that is feeding air to the fire slowly diminishes and thus the fire slowly diminishes causing both to spiral down. Eventually the fire will stall. Or under the right circumstances, possibly even reverse.
Paul