The dryer vent trick my old boss in Scranton showed me, and I use it on chimneys now
Three years back I was on a job in Scranton and the homeowner had a wood stove that smoked into the room every time the wind picked up. I tried reseating the pipe, checked the cap, even ran my brush down twice, and nothing helped. My old boss watched me fuss for about 20 minutes, then grabbed a leaf blower and taped it to the cleanout with a rag stuffed around it. He ran it on low for maybe 30 seconds and a wad of creosote and a bird nest the size of a softball came flying out the top. Turns out the flue was mostly clear but that one soft plug was sitting right at the offset, and brushing just pushed it around. I still do a quick blow test on any stove that smokes weird, before I start tearing things apart. Does anyone here run into those hidden plugs at offsets, and how do you find them without pulling the whole run apart?