Something is in the furnace flue. Bird, squirrel, chipmunk? Been there three days. We're goin' in. Wish us luck.
Update: It's a squirrel. And the little tree-rat crawls back into the chimney. He's left souvenirs inside about 6 feet of angled-upward exhaust duct: grit and droppings.
We can't shoot anything energetic up the duct, so drop that idea right now. Air rifle? .177, hand-pumped, is all we have -- and that needs to be a clean hit.
UPDATE: We had some good looks at him, the rotten little rodent, but not hits. (Tam has more on Facebook). So...after talking to the pros of Butler M-K, I have made and installed a hardware-cloth (1/4" square galvanized steel mesh) "squirrel screen" as close to the chimney end as I could, held in with sheet-metal screws. They'll have a look at the situation in the morning.
And just think, back when the weather was nice and the days were long, I could've installed a nice squirrel excluder over the top of the chimney!
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