BridgeMan

For anyone who's interested--a 10" round has a net surface area of 78.54 sq. inches, while a 12" square has a net surface area of 144 sq. inches (not counting a slight rounding deduction at the corners).

Not quite the same. While the circular shape may be more efficient than a square in venting flue gases, I doubt the 10" can make up for the 54% net area deficiency it suffers compared to a 12" square.

paul52446m

Some people have a hard time with math! You are right ,if you have a regular size fire place you should have a 12 x 12 clay liner. The man should check and see if the clay liners are cemented in. If they were cemented, then that why they are cracked. Clay liners are suppose to just set on top of each
other so they can expand and contract with the temp change. The joints between the clay liners do not need to be sealed. Heat will not go out through the seems, the draft in the flue will draw air in through the cracks. Paul

Michaelolding1

Most clay liners average maybe 10 1/2" x 10 1/2" effective inside area for a nominal 12 x 12 flue.

A round flue is more efficient due to the nature of fluid dynamics and most drop in flue systems measure the flue inside diameter not outside diameter like is done on the clay tile so the actual effective area of a clay tile is about the same as a 10" steel sleeve.

It is likely that by the time you deduct the joints and inconsistencies of a clay lining system that any seamless round flue will be more efficient than a square tile flue.

BridgeMan

Sorry, but the numbers don't quite add up--a 10.5" square tile flue has a surface area of 110 sq. in., compared to the 78+ sq. in. of a 10" round, meaning the square flue is 40% larger. And you're not being very realistic if you think the round steel flue will have no joints, offsets or other interior irregularities. I've looked at dozens of round pipe flues over the years, and have yet to see one that's one clear, straight shot from bottom to top.

Michaelolding1

Smoke rising in a flue travels in a circular pattern so the corners of your clay tile are effectively useless which must be deducted from the over all total. Really it isn't so much the area as it is the location of the chimney in terms of surounding topography, prevailing winds, etc and most importantly the chimney height. The difference in air pressure between the ground and the top of the chimney is a lot more important to adding draft.