I did set the baro with a manometer - set it to maintain just below .05 measured with the manometer. I can get another thermometer and check the pipe temp. I did move the thermometer to the side of the stove and there was about a 50* temperature difference - it read 350* when on top it had read 300* just before I moved it.
When the intake air lever is set to "0" it is not completely closed, there is a sliver of light that I can see when looking through the ash door to the back of the stove (maybe open 1/2"?). Could it be that the cold temps outside are causing an increased draw on the chimney which compensates for the closed down intake? I've also gone through a lot more coal since closing down the air intake - I typically fill the hopper twice a day and I filled it this morning at around 7 and just had to fill it again at around 3 which is different. So, that would suggest that the fire is actually burning hotter and not simply that I was losing most of the heat up the chimney before shutting down the air intake. Before I closed the air intake down yesterday it had been at 1/2 open.
Decided to come out of lurking mode to offer my 2 cents. This is my first post to this forum and I am still new to burning coal myself. If I say something incorrect I am sure some of the experienced users will correct me and I wont be offended in the least.
The way these stoves control the burn rate is by restricting the air getting to the coal. If it can't get air it can't burn. The purpose of the baro is to provide a consistant draft. With a manual damper or no damper the draft will vary depending on outside temp, flue temp, wind, etc. That would cause the air flow through the coal to vary despite having a constant inlet setting. With the baro a given inlet setting should always provide the same air flow.
A manual damper works by restricting air flow from the stove. Burning coal with a limited air supply can produce carbon monoxide due to incomplete burning. With a baro the CO will go up the flue but with the manual damper it could leak out into the room if you close the damper too far.
Your stove is obviously getting enough air to keep a good fire going even with the temperture control set to zero. It is getting hotter in part because less heat was allowed to escape up the flue. It is using more fuel because the higher temperture of the coals promotes faster burning, provided it has enough air. To throttle it down a bit you need to further reduce how much air it takes in.
I was surprised to read that the air inlet was still open when it was adjusted to zero. Is there a stop that prevents closing it any further? Might there be some method of adjusting further? I think you should be able to get it for all practical purposes closed.
The other way to reduce the airflow is by reducing the draft. Try setting the baro back down to .03-.04 and see what you get. Then if you need more air flow open the inlet.
The bi-metal bar on the thermostat adjusts the inlet opening automaticly. If it starts to cool it opens further and closes if it gets warmer. If you have a fan aimed at the stove it can carry the heat away from the bi-metal bar causing it to behave as if the room were cooler, making it open the air inlet to make it hotter.