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Fireplace opening and chimney sizes
The correct size of your fireplace opening with regards to the size of your chimney. Usually there is an existing chimney and a fireplace is either being built or remodelled. In this case the size of the opening of the fireplace must be tailored to the size of the chimney. A common error is to make the fireplace opening too big. This results in poor draw and a smoking fireplace.

The fireplace opening should be no more than 8 times the area of the flue if your chimney is square or rectangular. If you have a round metal chimney liner then the fireplace opening should be no more than 10 times the area of the liner. (A liner offers less resistance to the flue gases than a traditional chimney, so the fireplace opening can be larger in proportion to the liner's area).

As an example:

Someone wants to install a fireplace with an opening of 0.5 M by 1M. They have a square chimney.

The area of this opening is 0.5 x 1 = 0.5 M squared

We divide by eight to give the minimum chimney area 0.5 / 8 = 0.0625 M squared.

This is the minimum cross sectional area of the chimney. As the chimney is square, we square root this number to find out what the minimum internal dimensions of the chimney would have to be.

√ 0.0625 = 0.25 M

So the minimum internal measurements of the square chimney would have to be 0.25M (25cm).