Q&A Firebox too large

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

    I was told that the firebox in my wood burning fireplace is too large in relation to the size flue I have. In addition, the top of the firebox (leading up towards the flue) is flat, instead of sloped towards the flue. This causes the smoke to billow around the top of the box until it seeps out into the house. I burn very small fires using only the paper-like firelogs (no real wood), which has eliminated the smoke in the house problem. Anyone else experience this problem? Would a mason be able to build up and correct the firebox problems? What about wood burning inserts? Would that be an option? Thanks!!!


    Generally a masonry chimney flue's cross-sectional area should be 1/10 the size of the fireplace opening. Height x width of the fireplace opening gives the area of your fireplace; length x width of the chimney flue tile gives you the cross-sectional area of the flue. If the flue is too small then it cannot remove the volume of air that enters the fireplace. One way to test the theory of improper sizing is to take aluminum foil and tape it over the outside of the fireplace opening at the top and let it hang down 6". This is a fireproof & cheap way to test for this ratio before spending money to fix it. If the fireplace drafts with the 6" strip of foil, try reducing it to 4".

    If the fireplace works well by reducing its size in this experiment then you have a choice of ways to implement a permanent solution. Glass doors are an option as their frame will overlap the opening on all 4 sides to reduce opening size. A less expensive option is a smoke guard, which is a 4" or 6" strip of metal (black or brass) which can be installed at the top of your fireplace opening. An insert (wood, gas or pellet) installed with a chimney lining system would also solve the smoking problem if the ratio of chimney to fireplace is the cause. Be aware other issues can cause fireplaces to draft improperly, such as extremely airtight houses and negative pressure. If the aluminum foil trick doesn't solve the problem, try cracking the window closest to your fireplace to provide extra air to feed the fire. -Karen Duke Duke Chimney Services www.DukeFire.com
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