When a fireplace allows smoke to back up into a room instead of going up the chimney, it is usually caused by insufficient draft in the flue. There are many possible causes for a weak draft in a chimney.
If there haven’t been any changes to the fireplace lately and the smoking problem just began, it is most likely that there is a lack of updraft due to some sort of restriction or blockage in the chimney. The chimney restriction could be something as simple as not having the throat damper open enough or a malfunction of the top damper if the chimney is so equipped. Excessive creosote build up in the flue, or the presence of a bird’s nest or a bee hive in the chimney are also fairly common causes of a poor flue draft. These types of obstructions would ordinarily be resolved during the annual chimney inspection and cleaning, but they can recur long before a year has passed.
Poor draft can be caused by other factors besides blockages in the flue. It can be caused by a chimney being too short. In that case, the chimney may have to be lengthened or a chimney fan may be required. There may be some other design error in the fireplace, such as the flue being too small for the fireplace, the opening of the fireplace being too large or the lintel (the top of the fireplace opening) may be too high. In those cases, adding a smoke guard to the face of the fireplace might help the fireplace to function properly.
Not every smoky fireplace is caused by blockages in the flue or design errors. If a home is too well sealed, a slight vacuum can be created in the room by the smoke going up the chimney, which eventually impedes the draft. It may be necessary to slightly crack a window to relieve the negative pressure and allow the chimney to work properly. When a home has more than one fireplace, small amounts of smoke that has gone up the working chimney properly can be sucked back down an idle chimney and be released into the home even though the throat dampers on the unused chimneys are closed, because even when closed, throat dampers don’t seal completely. In those situations, it may be necessary to add top sealing dampers to the unused chimneys.
Poor burning techniques, like placing the grate too close to the front of the firebox can also create a smoking fireplace. Burning green or unseasoned firewood, burning paper and cardboard or pine can create more smoke than the chimney is designed to handle. Having too much ash in the fireplace can keep air from circulating well and getting to the firewood, resulting in a cool burning fire with not enough heat to keep the updraft going.
If the problem only happens when it is raining or soon after a rain, it could be a problem of too much water getting into the top of the flue. Generally, it is harder for smoke to rise up a flue that is wet on the inside, because the water takes so much heat out of the smoke that it reduces the smokes ability to rise, stifling the draft. The addition of a chimney cap to the top of the flue normally resolves that condition.
Troubleshooting chimney problems or chimney diagnostics can be a bit tricky, but well within the ability of a professional chimney sweep, who has the support of a dependable experienced chimney company.