Ice stalagmite. My furnace drowned during the recent floods in Boulder, and I replaced it with a high-efficiency furnace. The furnace discharges into a plastic pipe, which the technicians ran up the old flue. We are nearing the end of the first cold snap of the year, with nighttime temperatures running below 0 °F, and the condensation from the pipe evidently caused the ice stalagmite on the roof.
