I have a problem with a down draft from my fireplace. My fireplace was recently redone, at the same time as other renovation work, on the main floor of my house. What may be causing smoke to come back into my house even after the fire has gone out.
There may be several reasons for a downdraft to occur in a wood-burning fireplace. The problem may be as simple as a damaged or broken damper or as complex as a poor fireplace design or location. It is very difficult to determine the problem without a visual inspection or assessment. I will offer some possible causes but will recommend that Mr. Neeler consult a WETT certified specialist to inspect the fireplace and offer suggestions for reducing or eliminating the downdraft.
According to “A Guide to Residential Wood Heating”, published jointly by Natural Resources & CMHC, there are three main reasons that wood-burning systems smoke. The first, already mentioned, is “bad system design”, the second is “extreme negative pressure in the house”, and the third is “improper appliance firing technique”. Smoke spillage from a fireplace may be due to any or all three of these factors.
Older masonry chimneys have specific design features to improve the draft of the smoke being sent up the chimney. If these features are poorly constructed, damaged, or blocked by debris, the fireplace may experience poor draft and spillage. A damper, to control cold air entering the home when the unit is not in use, may be too small or not fully operable. The damper may also be poorly located and many experts suggest that it should be located at least 6” above the bottom of the breast (the top of the fireplace opening) for good draft. The damper controls may also be damaged and not allow it to be opened fully. A damper should always be fully opened when a fire is lit.
Some fireplaces have mechanical dampers located at the top of the chimney flue. If these controls are damaged, or just due to the design, these dampers can restrict airflow and reduce draft. Check the fireplace for metal cables hanging down in the firebox to determine if the damper is this type.
Lower air pressure within a house relative to outdoors can also cause a downdraft. If the pressure is greater outside, air and smoke can be drawn down the chimney, rather that going up. This is most common in basement appliances where the chimney is located outside the foundation wall. Newer homes experience this more often as they are more energy efficient and have fewer gaps around the doors and windows to draw in outside air. This may also be part of the cause in this case, as the recent renovation work may have closed up former sources of outside air.
The final of the three common causes is the most difficult to determine. The homeowner may not be lighting good quality, hot burning fires. The lower the fire temperature and the longer it is allowed to smoulder, the greater the possibility for spillage. As the fire burns down and the chimney cools, the draft is reduced and may even cause the smoke to back up into the house.
The final suggestion for the problem in question is to look at the fireplace and chimney location. If the chimney is not high enough above the roofline, is blocked by trees or other buildings, or is subject to high winds, the draft may be affected. Only evaluation by a fireplace expert can determine this.
A good publication, as well as the one previously mentioned, “All About Wood Fireplaces” is available through Natural Resources Canada. Both of these publications are free and can be obtained by writing to Energy Publications C/O Canada Communications Group, Ottawa Ontario, K1A 0S9 or from the Office of Energy Efficiency Website: www.oee.nrcan.gc.ca.