Q&A Calculating Fireplace Dimensions

QandA Posted By QandA, May 23, 2002 at 2:20 PM

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  1. QandA

    New Member 2.
    Staff Member

    Nov 27, 2012

    The main fireplace opening in my 1920 home is 32x49. The depth was 31, however someone stacked fire brick along the sides and back (angled ala rumford) to make it 22 deep.

    The flue is 17x11.5 i.d. and over 3 stories tall. There are two bends to go around the 2nd story bedroom fireplace.

    Recently, I had a contractor rebuild the crown and he installed a 10.5x10 i.d. flue tile at the top.

    By my calculations, this reduces the effective flue size below the acceptable limits.

    Could someone with more expertise than me comment? Any thoughts on the false interior?


    The 10.5 x 10" id flue tile is too small. Here's the generally accepted formula: Square or rectangular flues should be 1/10 the size of the fireplace OPENING. (Overly tall flues or those with a rounded breast might work on 1/12 but the offsets in this chimney will likely work against you).

    The flue size has to be commensurate to the amount of air entering the fireplace. If the flue is too small it cannot draft the fireplace properly; too much air entering the fireplace in proportion to the volume of air that the chimney can remove.

    Height x width of FIREPLACE OPENING x .1 = area needed for flue. 32" x 49" fireplace opening x .1 = 156.8" area needed for flue. The original flue tile measures 17 x 11.5 = 195.5", exceeding the 10% needed, so theoretically that should be fine and probably is, even with the offsets. HOWEVER a 10.5" x 10" flue is only 105 sq. inches, about 1/3 less than needed. I think the chimney will not draft sufficiently with this reduction and wonder why the mason did this; he must have had a reason but I can't guess what it would be. -Karen Duke
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