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Why would my normally inoffensive chimney
suddenly start stinking up the house?

Q: We have a fireplace we've used for years. The chimney has always had a good draft. I had the chimney swept in the summer of 2010 to prepare it for use last Winter, but soon after we started using the fireplace, a harsh smoke odor became noticeable at the fireplace and in other areas of our house.

Again this past summer (2011) we had the chimney swept. This time, the Sweep suggested that I use Anti Creo Soot (ACS) when I started using the fireplace again to get rid of burned on cresote. I followed the directions on the ACS and sprayed the logs with the product prior to lighting the fire. So far this Fall we've had two fires and both times the smoke odor returned.

Any idea as to what might be causing this, and what the solution might be? What puzzles me is that there were no odor problems for years. By the way, the Sweep did have a camera probe that he put up the chimney so you can actually see the lining - it was sound with no cracks or breaks.

Thank you for your assistance.

David Wolfersberger
St. Louis, MO

Sweepy  There are lots of factors that might be involved in situations such as yours, but you've given me enough info to spawn a reasonably educated guess.

One reason chimneys are tall is so the top of the chimney will end up in an atmosphere of lower-density air than exists at the bottom. The goal is to create what's called ambient updraft; a slight tendency for air to rise up the flue even when there's no fire going. Ambient updraft is what keeps chimney odors from flowing into the house when the fireplace is not in use.

Ambient updraft isn't very strong. Anything that takes air out of the house might create a powerful enough vacuum to overcome the ambient updraft in the flue, pulling air from the chimney into the breathing space. The usual suspects include any appliance with its own chimney, such as your gas furnace and water heater. Also on the list are kitchen and bathroom exhaust fans, clothes dryers, and the so-called "stack effect", which is prevalent in taller, leakier houses.

So, assuming something (or some combination of things) is evacuating enough air from your house to cause replacement air to flow in from the chimney, why didn't you smell it before? Your question provides an important clue: your Sweep has recommended that you add ACS to your fires. ACS is used to chemically break down Stage 3 (glaze) creosote. Glaze is the densest, hardest type of creosote, and often can't be removed until it has been catalyzed by chemical means.  Reading between the lines a bit here, your Sweep's recommendation that you use the ACS indicates that even though he brushed out what he could, some glaze creosote remains in your chimney.

And guess what? Glaze is ten times smellier than normal creosote.

Here's a synopsis of our story thus far: David's house always causes some degree of backdrafting in his fireplace chimney, but the slight odor from the Stage 1 or Stage 2 creosote normally found in fireplace chimneys isn't particularly noticeable to him. During the winter of 2010, he burns some wet or improperly seasoned wood, causing a layer of glaze to coat the chimney flue. Suddenly, the air flowing down his fireplace chimney into the room is ten times smellier than it ever was before! David has the chimney swept, but the Sweep either doesn't notice the glaze, or doesn't report it to him. In any case, the glaze (and the odor) lingers on. The following year David has the chimney swept again, but this time the Sweep notices the glaze, reports it to him, and suggests he chemically treat each subsequent fire to break down the glaze so it can be swept out. Meanwhile, the odor persists.

If this sounds like it might be your story, there's a happy ending available. Get a moisture meter and determine that the wood you burn from now on is seasoned to less than 25% moisture content. Store it where air can circulate, but rain can't reach it. Continue to add the ACS to each fire (it takes heat to make the catalyst in the ACS "kick"). After about 30 fires, call your Sweep back and have him brush out the catalyzed glaze. Repeat if necessary until every last bit of the glaze is gone. Vow to never again burn wet or improperly seasoned wood, and live happily ever after.


David writes back: Thank you for your very thorough response. I think you really got to what is happening! During the most recent sweep, when the Sweep had the camera, it was easy to see the glaze - it looked hard and shiny. The prior year he didn't have the camera, so he probably didn't notice or report the glaze. This time he did mention that it was a Stage 3 creosote build-up. I understand now why the Stage 3 would cause our problem while the odor of Stage 1 or 2 creosote would be less noticeable.

Regarding the wood: I think you are onto something here too. I usually buy one or two "cords" a year - actually what they call around here a "face cord" that is 4'x8'x18". I normally have some left at the end of the season, which I wrap up in plastic to keep it dry (normally I staple the plastic tight to all sides of the wood pile). I see now that in so doing I've been eliminating any air circulation. I will correct that and only drape the plastic over the top and maybe down a foot or two on the sides but keep in loose.

I really appreciate your comments. I was at a loss as to who to turn to for advice until I found your website.

David Wolfersberger 

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