As I said in an earlier post, I wanted to burn the fireplace more this season, as I had moved the computer into the fireplace room and the TV is in the same room. Also, the t-stat for the coal stove is in the F/P room.
The problem appeared, with my first fire, after the weather got cold. While the F/P was burning, the coal stove t-stat temp. rose to about 80°, which kept the coal stove dampened off. But, at the same time the oil furnace t-stat, located in the adjacent dining room, dropped. Thus, the oil furnace was running, on and off, to keep the remainder of the house up to temp. (no coal stove influence) .
My first thought was, that the heat was trapped in the F/P room, due to an arched doorway between the two rooms (80° in F/P room and 71° in adjacent room). I tried a fan pointing in all directions to get the heat out of the F/P room, to no avail! I couldn't get the coal stove thermometer to drop, no matter what I did. However, blowing air into the F/P room, would raise the temperature in the adjacent dinning room a degree or two. Next, I tried a 2 stage t-stat in the dinning room, to control both the coal stove and oil furnace. The short coming of this, was that the coal stove ran nearly all the time, as it was set to be the "primary". Due to the slower recovery of the coal, it took a long time in the morning to get the whole house up to temp..
My second thought was, that the F/P was pulling the heat out of the rest of the house for combustion air and/or the natural flow of air of the F/P, even though, I have an outside combustion air source for the F/P. So, I tired a piece of plastic over the archway ... it didn't even wiggle, so it didn't appear, that house heat was being sucked into the room to satisfy the F/P.
I was stumped ... there didn't seem to be a solution!
Then it hit me last night ... the t-stat and the wall it was mounted on (directly across from the F/P) were being heated by "radiation" from the F/P and the t-stat wasn't really being influenced by the room's true air temp.! I saw another example of this yesterday: The outside temp. was 20° or so, the sun was bright, and the ice on my sidewalk was melting.
Hopefully, the final solution will be to move the coal stove t-stat to the dinning room, with the addition of a small fan to move the air around within the F/P room.
In hind sight, I don't think, this is so baffling. But, I talked to a couple of guys, who do HVAC and they failed to bring this to my attention.