We put in a new furnace a couple of years ago. It’s the fancy new kind that doesn’t have a flue, just a PVC pipe running out the side of the house.
That pipe runs out the house, and under the deck, and finally comes out the side of the deck.
And it drips, as it must. As Peter has explained, burn natural gas, you get carbon dioxide and water.
None of which mattered when we first got the furnace, since it was just dripping on the lawn. Then I put in the yuge patio. I was concerned that the drips would stain the concrete, so I put a trash can under the pipe in the winter.
A lot of cold and not a lot of wind, and the above giant ice dildo is what happens. Supposed to be near zero for another week. I wonder how high that thing will end up.