You'd have to ask the experts, we just try to heat our house cheap.
If I close my upper damper fully, as long as the intakes are controlling air up to 1 turn open, it doesn't make much stack temperature difference fully open or fully closed. With air intakes open more, it controls the burn rate by throttling the exhaust up the stack, slowing down the intake air. We use that mode for cooking, but it soon gets too hot in the house heating that way.
There are some antique dampers with very small holes when shut that plug up quickly causing problems. Newer dampers have the correct size hole for closed position. Otherwise the chimney draft should be strong enough to keep the air flow going the right direction through intakes. Mechanical fans in the house (bath and kitchen range fans) can over power the draft causing smoke from the intakes. Clothes dryers vent inside air out depressurizing the house as well and can use the chimney as an air intake into the home. Power vent gas heaters are culprits pulling air down chimneys as well. So make sure everything is off when this happens. An outdoor air source near the stove prevents these problems. If it works fine, and starts doing this, a partially clogged elbow or screen at chimney top are the common causes.
The Grandma and Grandpa called for an inline damper for open door use with a screen in place. The Bear Series with single door and 6 inch chimney did not use a damper. However, the manuals and stoves were designed for connecting to an existing chimney and insulated efficient chimneys didn't exist. So I find with a straight up, no bends, very good drafting insulated chimney, a damper helps in all the models, contrary to the pre UL manual. The UL listed Fireplace Series III did not require an inline damper with the smoke shelf baffle after 1980.
In the Hearth Wiki, under Fisher, you can read both the older manual written for all 5 models and the newer manual for double door stoves, type III. The Goldilocks and Teddy Bear were available in 6 and 8 inch versions, and they were the only 6 inch stoves requiring a damper. These both had smoke shelf baffles, so I don't know why the damper was dropped in the full size Grandma and Grandpa III.
The damper operation for open door burning is given in the Goldilocks and Teddy Bear manuals. Both double door stoves similar to GM and GP but a Pedistal version instead of legs, called radiant heaters with doors closed, Fireplace with doors open.
Quoted from manual;
To Operate as Fierplace Stove;
1. The fireplace screen must be in place to retard sparks.
2. The manual damper in the flue adapter is used to control the stove temperature when the doors are open. To set the damper, rotate the damper handle slowly until smoke starts to come out of the opening. Reverse the rotation until the smoke stops. Adjust for your preferred temperature.
(This adapter is Part #1679, a Dura-Vent 6 inch long double wall pipe with damper that becomes the first piece of pipe coming off the stove. It's still available, but a manual damper in the first 6 inches of pipe is the same)
To Operate as Radiant Heater;
1. Turn manual damper lever to vertical position.
2,3, and 4 gives fire lighting instructions.
(entire manual is available in pdf format with a request including an email address to send to. The file is too large to send through PM)
