There have been lots of threads on this, though I have never experienced it, until now. Normally I have our wood stacked and drying in the shed by late spring. Our chimney stays clean year after year. Well this last summer it was different. We started on a new garage project. Unfortunately, with tearing down the old garage we needed a place to store the stuff that resided in the garage. Even after a massive purge there was still stuff to store. It got put in the house, on the porch and in the wood shed. The garage was "supposed" to be done by September and that would have been fine. I had a large 2 cord pile of split fir that was drying out nicely. Unfortunately, Murphy's law kicked in. Between the contractor and the county, the project got delays adding up to over a month. And then the rains came. Long story short, for the first time in years we have been burning less than ideal wood. The tarps on the locust and maple leaked and the faces of the cords got soaked in what turned out to be a non-stop wet Nov. record setter. So we burned the best we could. The fir got into the shed in late Oct. and started to dry out. Not perfect, but with its high oil content, totally burnable. But here we are mid-season and the symptoms of a clogging flue are not ignorable. I post this to help others that may be in a similar situation. Over the past two weeks, draft has weakened. At first it was just slower secondary performance and a little longer delay before being able to close off the air. Then a hint of smoke spillage started creeping in. It didn't take long to get worse. I went on the roof yesterday and checked the cap screen. It's not fully clogged, but there is clear evidence of it starting to fill up the gaps. I'll take some pictures tomorrow. Today it is impossible to start a fire without smoke spillage whenever the door is opened. The fire starts as if I have the air closed off. Tonight is our last fire until things are cleaned. This is after only a cord of wood burned. Tomorrow will definitely be cleaning day. I'll take some pictures. This has been quite an education. Air control wide open, stove burning like it's half-closed. Stove top temp struggling to reach 500F. And a nasty smell of smoke lingering in the house. Tomorrow is cleaning day for sure.