I had an interesting time with last week's cold snap.

House-sitting for friends. Xmas day, I'm debating whether to go over or not, but I finally do. Got inside and water was coming from the family room ceiling. Not a lot; I thought it was from an ice dam on the 2nd floor gutter, and water was coming down a wall. I do some more checks, and there's no hot water. I checked the water heater, and it was cold. Something else was going on.

I go outside to the entrance to the crawl space and look around underneath the house. Water is pouring out of the ductwork. Ran back inside and shut off the water.

Took a few days to get a plumber to the house. He's in the crawl space and I turn the water on. It comes pouring out the ductwork in the crawl space like before, but also starts coming down from the family room ceiling again. That should not have happened.

I took a broom and started poking holes in the ceiling drywall. Pulled out some large sections, and found flexible ductwork for air return running over the top of bare copper water lines to the 2nd floor. My friends had set their thermostat to 60F, and that flex ductwork was drawing air from a register directly below a window. With outside temperature around -10F, the result was a pipe that burst.

Here's the absolutely miraculous thing: the spray from the pipe punched through the plastic of the flex ductwork, sending almost all of the water into the ductwork and down into the crawl space. I'm talking several hundred gallons. I should have been looking at a fully flooded first floor - ruined flooring, damaged furniture and appliances, etc - but ended up only having to replace some flex  ductwork and a 1' x 12' section of drywall.

Still need to have some HVAC folks check out the rest of the ductwork under the house, but the furnace seems to be working normally.