So imagine that this steel container is the water line that enters your home. (If rainwater is not collected or water is produced from hydrogen and oxygen, you probably have it.) If it’s too cold, the water can freeze and literally blow the pipe. This is bad. Now for some questions and answers.
Why doesn’t this happen more often in the south?
Residential water lines are almost always underground, and that’s good. Although the air temperature can vary drastically from summer to winter, the ground temperature is much more constant. In the southern states, this soil temperature is unlikely to drop below freezing, so pipe water will also be above freezing temperature (and will remain liquid).
But there are some exceptions. In some places with warm climates, not all parts of a water pipe system will be underground and pass through regions of air. (Heck, I have water pipes in the attic and I live in a warmer place). Although there is a small temperature difference between cold water (say 1 degree Celsius) and hot ice (0 C), there is a huge energy difference. It takes a lot of energy to change water from its solid phase to liquid. We call this the latent heat of fusion. For water, it has a value of 344 joules per gram. It may be difficult to understand, then, and an example?
Suppose you have a liter of ice (with a mass of about 1,000 grams). If you want to take this gel at 0 C and turn it into water at 1 C, you will need 344,000 joules of energy (plus a little more energy to raise the water temperature). How much energy is that? Well, suppose you have a smartphone with a 3,000 mAh (milliamp-hour) battery. That equates to 41,000 joules. So you may have enough energy to run your phone for a full day, but you’ll need eight or nine of those phones to melt all that ice.
It’s actually a good thing. It means you can use melted ice to cool your drinks, and you don’t really need that much ice. This also means that you have to remove some thermal energy from the pipes to get them to freeze. A cold night will probably not be enough to blow up the pipes.
Does it help to let a faucet run?
Yes. Okay, imagine you’re inside a water pipe. (Yes, you’re super small now.) If the water is stationary, you may be trapped in a part of the pipe that is exposed to cold air. You could actually freeze it and then you would have to break the pipe. But now suppose it is running water, caused by a faucet dripping slightly. You’re still a small person inside a pipe, but now you’re moving too. It is passed through the cold pipe section and cooled, but not frozen. Instead, just move on to other parts of the house.
Oh, but more water enters from the main underground line into the cold part of the pipe. Would it freeze? Not so likely. Remember that the water pipe is at ground temperature, which is almost certainly not below the freezing temperature. Therefore, the inlet water is not very cold and hopefully it will not freeze.
What about isolation?
Insulation helps. If you wrap some foam insulation around any exposed pipe, it will do the same as the cooler or insulated glass. Insulation decreases the rate of energy transfer from the hot thing to the cold thing through a thermal interaction. If you place a cold drink on a table, the energy is transferred to the drink to cause the temperature to rise. Putting the drink in a refrigerator, on the other hand, increases insulation and decreases the rate of energy transfer so that the drink takes longer to heat up.