Smoke Free Masonry Chimneys
DEAR TIM: My husband and I just moved into a new home with a masonry fireplace. We started a fire and smoke rolled into the room. We checked the chimney for blockages and found none. The damper was wide open as well. What could be the problem? Our fireplace is three feet wide. Is that too small? Can you see anything else in my photos? E.G.
DEAR E. G.: Tell me, did your home construction contract include a strong warranty? You're going to need it. Your photos clearly show several major problems with your fireplace design and construction that are contributing to your smoking problem.
The combustion of wood in a fireplace is very complex. The exhausting of smoke and toxic gases from the fireplace is controlled by three major factors: air pressure, temperature difference between inside and outside air, and the fuel combustion process. Improper design and/or sizing of the firebox, smoke chamber, and your chimney can cause an imbalance which will lead to a smoking fireplace.
Believe it or not, the actual fireplace opening (width and height) controls the size and shape of the firebox, the flue, and the height of the chimney. The width of the fireplace opening controls all other dimensions. This dimension is a function of the size of the room in which the fireplace sits. It is all very complicated. Fortunately, years of trial and error have produced very successful designs and size proportions for smoke free masonry fireplaces.
Several problems are obvious in your photographs. The height of your fireplace opening is too tall in relationship to the 36 inch width of the opening. Furthermore, your firebox appears to be too shallow. This puts the fire too close to the fireplace opening and your room.
Your three foot wide fireplace opening should only be 29 inches tall. The firebox depth should be 16 inches. This depth does not include the thickness of the decorative brick which faces your fireplace. The distance from the floor of the fireplace to the bottom of the damper should be 37 inches. The smoke chamber, the area between the damper and the first flue liner, needs to be a minimum of 27 inches high.
Speaking of flue liners, I think the mason installed one that is too large. In your case, it should have inside dimensions of very nearly 10 inches by 14 inches. Smoke has to push the heavy, cold air up and out of the chimney. An oversize flue liner can make this a difficult process. What's more, my tables indicate that the minimum height for your chimney should be 21.5 feet. That measurement is taken from the top of the last flue liner to the floor of the fireplace.
As much as I hate to say it, I'm afraid that your entire fireplace and chimney need to be reconstructed. The two week process will be long forgotten after the smoke clears.