Size of fireplace to chimney size?

Old 04-15-00, 10:52 AM
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Can anyone provide me with some help. I am about to install a new Cotswold stone fireplace sorround and would like to increase the size of the firplace openning. Is there any limiting ratio of fireplace openning to flue size?
Old 04-23-00, 07:04 PM
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Yes there is a ratio between flue size and fireplace opening size.

When using square or rectangular flue tile the flue must be 1/10th min. of the fireplace opening size.

When using oval or round flue the ratio is 1/12 of the opeing size. Round flue vents more efficiently because gasses will not swirl into the inside corners of the squared flue tiles, therefore reducing the effective size of the tile.

For example: If you have a 2 foot high x 3 foot wide fireplace opening, = 6 sq. ft. or 864 sq. inches your flue size must be 73 sq.inches or 1/12th or the opening when round, and 87 sq. inches when sqare.

Flue sizes are figured kind of strangely. There is one standard for round flue and one for square flue. Round is measured on the outside of the tile and square is measured on the inside. Why, I cannot offer, it's just the way it is done.

Your masonry supply store can tell you what flue sizes are available and the dimesions. But your chimney is already installed so you just need to calculate the opeing size and match it to your vent requirements.

Err on a smaller inside fireplace opening size rather that larger. It is better to have the flue try to pull more gas out of the firebox, (increased draught), than to have the gas backup becauise there is too much flow from an opening that is too large.

Plumb & Level Masonry

Check out some of our custom built Fireplaces at:
Old 05-19-00, 10:20 AM
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Many thanks.
Great to think I can get advice from the US on my Cotswold chimney.

Mark Johnson


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