Fortunately, I have such a bathroom system. After the a pipe burst two years ago on a -30 degree day, we had a plumber install shutoff and drain valves, so we can completely turn off the system when the temperature is slated to drop into the artic regions. Last nights temp was only -4, but I was extra cautious and shut it off anyway.
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The big question is, who enclosed the porch and decided to build a bathroom (a room with a lot of water) with no crawlspace under it! This is the upper midwest, you know, and it does get cold...
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Speaking of the cold, a couple of projects for the near future include caulking the windows with some rope caulk, and adding more weather stripping to the doors. But, I think I'll wait for the temps to reach the double digits before attempting those tasks.