wet fireplace smell

Since we repaired the floor drain in our basement, the smell of sewer gas has thankfully gone away. But now we have a new, mysterious odor that pops up seemingly without any pattern: what we can only describe as "wet fireplace," like cement and wood ashes. Our house has no fireplace. The chimney is in use only by the water heater vent, and it has a cap to keep rain out. We'll go for days without smelling this smell, and then it will suddenly permeate the living area (on the first floor) without any rain in the forecast, without the HVAC being on. It's as if a fireplace has been secretly hiding behind our sofa, and it was unable to suppress a fart. We are at a loss to explain it. Any ideas?

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  • The intermittant nature of this smell's appearance implies that there must be some pressure that induces the smell into the house. I would certaining want to do a worst-case depressurization test.  I would also makr a careful log of atmospheric conditions when the smell appears. You were on the right track when you noted that" there was no rain in the forcast", but there could certainly be other drops in atmospheric pressure that could exacerbate the situation.

    Here is another possible explanation: back in the 90s when I was doing Radon Training it was reported that there is strong correltion between full moon cycles and interios radon levels.  An explanation can be found at https://www.theguardian.com/science/2006/mar/16/thisweeksscienceque...:  "the lunar cycle changes the amount of stress in the Earth's crust and drives the water table up and down. Combined with ocean tides, which squeeze and relax the ground, this can change radon emissions during a full moon and a new moon by up to 46%."

    The same force would pull damp moldy smells into the house as well.

    Moon affects deadly radon levels in homes
    Thousands of homeowners could be exposed to higher levels of the deadly gas radon than they realise because of a bizarre interaction with the moon.
  • You may have guessed correct Ben with the hidden fireplace chimney. Is your existing chimney protruding from your roof a brick & morter style?

    Perhaps the dwelling is an older one?

    If so, the mortar joints may be so deteriorated that water/moisture is indeed penetrating the chimney outside, gravitating downward through your wall over a long period of time and collecting within the wood frame boards inside your wall.

    You might want to inspect the condition of this chimney at the roof and expose it open in a few easily accessible areas down inside your walls to check its condition. 

    By doing this, I think you will unhappily find the source of your problem. (Just a hunch though)

    • It is an older house (1925), and the chimney is brick and mortar. However, we were able to inspect it thoroughly both above the roof when we put the cap on and at the attic level before we walled it up, and there was no evidence of moisture on the outside. Also the chimney guy was able to vacuum about 10 gallons of dry ashes out of the bottom of the chimney, which implies it isn't wet on the inside either.

      • Ben,

        I would hire a certified home inspector to isolate your "wet fireplace smell".

        They have the proper experience and equipment to identify many types of residential issues.

        A 1925 house with a brick chimney had more of an original purpose than what it is currently being used for (water heater exhaust) .

        Good Luck

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