Our fireplace smells after it rains
9 July 2005
A. Your house is breathing air down the fireplace flue and you probably notice it more after a rain-- especially a warm summer rain-- because the air migrating into the house via the chimney is humid. The moist air picks up the odor of creosote that has been deposited on the sides of your terra-cotta chimney flue liners as a product of burning firewood and wafts it into the house where you smell it.
Air is almost always moving one way or the other up or down your chimney flue in response to pressure changes from inside of the house to the outside. We describe your house as being aerodynamically connected to the exterior via the chimney. The problem can attributed to modern, almost air-tight construction methods that energy conscious building techniques have brought, along with things in our homes that tend to lower or de-pressurize the inside of our homes by their use such as clothes dyers, bath f ans, kitchen fans-- even attic fans-- that pull air from the inside of the building and propel it to the exterior. If you take air from an enclosed space, it lowers the air pressure of that space relative to all the air around it. Air wants to equalize itself so it draws make-up air, as itís called, from wherever it can. In your case itís coming down the chimney. Mere millebars of pressure differential will get air moving in one direction or another.
Understanding the cause of the problem can put you on the path to solution but itís not so easy in this case. Youíre pitting chimney caps and old-fashioned flue dampers against a modern building structure. They canít solve the problem because they were really never meant as a true hermetic seal. Some spring-loaded chimney top dampers that are well gasketed can do the trick. They cost about $300. or more depending upon the height of the chimney.
Sealing the chimney with a masonry sealer to keep water out of the brick isnít really getting at the root cause and, by the way, if you get sloppy applying some of these sealers and splash some on your roofing near the chimney youíll be dismayed to watch patches of your asphalt shingles dissolve.
Trying to be careful while operating exhaust fans by balancing the air through cracking a window open a bit during operation might help so give it a try and see if that makes any difference. Iíve seen folks place candelabra in the fireplace and light ca ndles to try to reverse the air flow through convective air movement from the candle flame. That looks nice. I once received a tip from a lady who said she places a dish of apple cider vinegar in the fireplace and that does the trick. I donít know that f or sure but I have heard from others who say this has merit.
Fireplaces inside of a modern home are a total anachronism and just donít make sense. However, youíre not going to easily get rid of something that appeals to some deep, almost primordial comfort and safety gene lodged deep in our brains that loves the l ook and warmth of a dancing flame on a cold winterís eve. Someday-- in the far distant future-- houses will be routinely equipped with barometrically controlled air-to air heat exchangers to keep the relative air pressures balanced all the time. Then fi replace flues as an air intake vent will no longer be an issue. The technology is here-- itís currently about a $1000.-- and Iíve seen it done.