That would be a Baby Bear, pre 1980, before the UL listed stoves. The redesigned post 1980 stoves are the ones with the baffle inside. yours is designed to heat 1000 sf.and tales logs to 18". It should be a rear vent model, sitting out in the room
, a steel plate covering the fireplace opening, with a 6 inch hole for the horizontal pipe to go through. These stoves were actually designed to go ACROSS the fireplace mantel to cover the sheet metal covering the hole you would be looking at. They had a side outlet, not too high, but higher than the door to prevent smoke coming in the door when opened. To give you an idea how well these drafted; The 4th stove made (a Papa Bear) was installed 8 feet away from the chimney with 2 elbows and still drafted OK with no smoke out the door or intake. This was in a kitchen with a high chimney entrance, (back of fireplace) so it was probably pitched upwards as much as possible. The sale of the stove depended on it working. (the guy wouldn't pay if it didn't do all the things Bob Fisher claimed it would do)
I would bet sticking it back in that hole, the heat around it gets intense. The heat can only move away from the stove in one direction.
Forward. So the air is actually moving away from the stove from ONLY the front. This could be creating enough low pressure to allow the combustion gasses to leak out the front vent. Air pressure outside is taking up the space of the air moving away from the stove, counteracting the draft upwards.......You probably used to have a much larger chimney area, slowing the draft down in the larger flue. This would create a higher velocity out the stack, and into the air intake. Now, by making it smaller, less rising heat and gasses go up. This is less air flow through fire and through intake vent. Lower velocity air through the intake vent. This can now be overcome easier by th elow pressure area in front of the stove created by the heat moving away from stove, and is probably why the problem is occuring with the liner. Also if the air is rising rapidly out of that hole, pulling air up from the floor over the stove front. Upwards moving air, = moving air = low pressure. Again right at the intake area. This is very similar to "stack effect" where the heat moving away from the stove and rising into higher places of the building rapidly is enough to pull air from the room, and down the stack. Smaller opening of the intake vent, (draft cap) the less air space to draw the smoke and gasses through the vent into the room. Also the more it's closed, the less heat the stove puts out. Less heat, less air moving away, low air pressure around stove is gone. All these factors are probably putting that stove door in a lower pressure area than the top of the chimney. Atmospheric pressure wins. I would first try getting it out into the room, so if it's a top vented model, you won't be able to do it. They are designed to be connected to a chimney and go straight up. If you can't use it, better sell it to me
Someone asked about the most efficient Fisher stove. That would be one made after 1980 with baffle. 40 to 60 grams smoke for every kilogram burned pre 80, only 6 grams smoke per kg. burned in the redesigned stoves. (Baffle rolls smoke back into heat and flame to burn cleaner) The mobile home , or manufactured stoves that are HUD approved would be more efficient since they do not use inside air for combustion. All the combustion air is drawn from outside, used in the firebox, and exhausted up the chimney. Just like an unvented heater needs inside air, so a window must be cracked for enough oxygen. This is where the 100% efficiency is lost due to cold air coming in for combustion. I have a 4 inch PVC pipe in basement leading to my Goldilocks for combustion air intake. 20 degree or colder air ices up this pipe on the outside in the basement and requires a drip tray under it to catch condensation ! My basement is not heated, and I consider the air down there during winter cool and dry. That shows how much cold air is required for good combustion. This stove would draw the same amount of air through every crack it could find throughout the house if it used inside air for combustion. So this is a big part of efficiency often overlooked.
Also to make a Fisher more stingy with wood; They were designed with a triple door seal that did not use a gasket. The raised area around the edge of the door makes contact with the bottom of the channel on the stove, and the raised edges of the channel contact the flat door creating 3 contact surfaces. They were considered air tight as built. However there is enough air leakage, that many will not hold a fire overnight. By adding a THIN gasket to the channel bottom, an air tight seal is achieved, and a longer burn time is achieved. You will be able to rake the coals around and add wood in the morning, instead of starting a new fire in a warm stove ! They act like a different stove. The gasket material should be the flat type used to install glass in doors. If the round rope type gasket material is used, it can cause problems closing the door. The only problem this can create is when replacing the gasket material. ALL the old cement should be removed from the channel due to making the replacement gasket too thick, adding more cement to the old can cause door closing problems.