kkct wrote:I see your point and will have to do more research on calculating the size (by any chance do you have an links on calculating the correct size?) the keeping hearth is fairly straight as I can see the sky when I'm standing inside it, the others I'm sure have curves since they all come together by the time they are at the roof line.
There are so many calculators on the web that I would simply Google "fireplace flue size" and find one you like. I expect it is possible that the original mason may have erred on the side of caution when it came to selecting flue size. It is also fair to assume that "trial and error" "rules of thumb" existed in 1720 and were widely known.
Too large a flue can create excessive draft, this results in a bigger hotter fire. This may have been advantageous when actually used for heating. My house has 6 fireplaces and a chimney which tapers to 5' x 5'. If that huge pile of bricks could be heated to 90 degrees, that would radiate an awful lot of heat into the house.