There are different design considerations on the size and location of barometric dampers. Where the damper is located and it's size depends on the fuel type and the number of appliances of the single fuel type drafting to a single chimney flue. With coal not only does the appliance exhaust combustion gases it also contains fly ash. This fly ash does not make it to the top of your chimney. It's heavier than the flue gases and will fall out. Where it falls out is determined by the flue gas velocity. Ideally you would want all of the ash to be transported to the chimney stack. That way the change in gas velocity occurs where the smaller diameter stove pipe meets the chimney thimble and the ash falls down to the chimney base clean out. Think of it this way, it's kind of a shop vacuum cleaner. Only with the AHS coalgun it's a blower. The stove pipe (technically called a convector) blows combustion gases and ash to the inlet to the chimney stack. There the larger chimney size causes the velocity to slow and the ash falls out.
So far so good. But it's not that simple. When the AHS coalgun combustion blower is not running natural draft takes over. It's sucking the now idling coal fire combustion gases and heat out of the boiler. Not good especially on a windy day. You need to regulate the amount of "suck" on the boiler. That's what the barometric damper does. It shunts a portion of the draft to pull air out of the room not the boiler. The amount is regulated by the barometric damper flapper door. Usually by some kind of movable weight. Properly set it still drafts enough to suck the combustion gases out of the boiler but not so much that it causes the coal fire burn more. One way to determine the proper setting is the measure the draft "over the fire" with a tube draft gauge temporarily inserted over the fire. With the AHS coalgun the flapper door on the front of the boiler opens allowing room air to enter. This changes the path of the air flow in the boiler. Air is no longer flowing through the coal bed, it's flowing over the top of the burning coal. Just what you want. A boiler that idles well when heat is NOT needed.
The ability of a barometric damper to regulate flow is dependent on it's physical size. Technically you want the flapper door to be operating in the flat part of it's curve. What size damper needed is based on the chimney stack size. Large diameter chimney, large barometric damper. For AHS coalgun boilers it should be located at the chimney stack thimble. The Fields 5 inch RC barometric damper is specified for 5 or 6 inch "convectors". Use 5 inch stove pipe so that the velocity is kept high and the ash is conveyed to the chimney stack. Make sure there is a slight uphill to the chimney thimble. You don't want a place for combustion gases to collect. This will contribute to the "puff back" condition described in the Bureau of Mines Report. If you have a very, very large diameter chimney (12 x 12, 12 x 16, etc.) you will need a larger sized barometric damper.
For other than AHS coalgun installations understand the principles I've described; size and locate the barometric damper appropriately. For example if it's a natural draft appliance you might want the barometric damper near the stove so that you are aways "sucking" the fly ash to the chimney stack.