Dear Straight Dope:
Does allowing a faucet to drip really keep the pipes from freezing during cold weather? Water freezes at 32 degrees Fahrenheit so why would moving water to freeze at a lower temperature than stationary water?
Craig Watts, Murfreesboro, Tennessee
Letting the pipes drip (actually, slightly faster than a drip is best, but it’s not my water bill) will help keep your pipes from freezing. You are correct: the freezing point of water is 32 degrees F at sea level under standard temperature and pressure (0 degrees C for our metric friends) and the fact that the water is moving barely changes that (the friction between the water and the pipes creates a negligible amount of heat). It’s not that the motion changes the freezing point–it changes the amount of water to be frozen.
First, the obvious: water does not freeze in an instant (except in cartoons). It takes some time for the water to lose its ambient heat to the surrounding pipes. Metal pipes are particularly sensitive to exterior temperature changes–think back to the summertime, when it’s darn near impossible to get a cool drink of water from the tap. The longer a certain amount of water sits in the pipes, the more the pipe acts on it, drawing out the heat until the water approaches the temperature of the copper pipes, which can be well below water’s freezing point. The water that is in direct contact with the pipes will give up its heat first, then the water next to that, and then next to that, etc, until the very center of the pipe freezes.
So, two solutions to frozen/burst pipes present themselves. Either you can remove the cold surface of the metal pipe, or keep changing the water that is next to the metal part. If you want to keep the pipes warm, the Big Orange Home Store has stuff you can wrap your pipes with (mostly insulator wrap, but they do make electrically-heated pipe wrap), but the easier way is to keep the water moving, away from the walls of the pipe, and not letting it sit long enough to give up the residual heat. The trouble with this method is that in very severe cold (Arctic cold or Chicago in February) is that the pipes can be cold enough to freeze the water on contact, making the whole dripping faucet thing a moot point.
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