So, imagine this steel container is instead the water line going into your house. (Unless you collect rainwater or make water from hydrogen and oxygen, you probably have one.) If it gets too cold, the water can freeze and literally burst your pipe. That’s 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 a good thing. Although air temperatures can vary drastically from summer to winter, the ground temperature is much more constant. In the southern states, this ground temperature isn’t very likely to get below freezing—so water in the pipes will also be above freezing (and stay liquid).
But there are some exceptions. In some places with warm climates, not all parts of a water pipe system will be underground, and will pass through regions of air. (Heck, I have water pipes in my attic, and I live in a warmer location). Although there is a small temperature difference between cold water (let’s say 1 degree Celsius) and warm ice (0 C), there is a huge energy difference. It takes quite a bit of energy to change water from its solid phase to a liquid. We call this the latent heat of fusion. For water, this has a value of 344 joules per gram. That might be difficult to comprehend, so how about an example?
Suppose you have a liter of ice (with a mass of about 1,000 grams). If you want to take this ice at 0 C and turn it into water at 1 C, it would take 344,000 joules of energy (plus a tiny bit more energy to raise the temperature of water). How much energy is that? Well, let’s say you have a smartphone with a 3,000-mAh battery (milliamp-hours). This is equivalent to 41,000 joules. So, it might have enough energy to run your phone for a full day, but you would need eight or nine of these phones to melt all that ice.
It’s actually a good thing. It means that you can use melting ice to cool off your drinks—and you don’t actually need that much ice. That also means that you need to remove quite a bit of thermal energy from your pipes to get them to freeze. One cold night probably isn’t going to be enough to make your pipes burst.
Does It Help to Leave a Faucet Running?
Yes. OK, imagine you’re inside of a water pipe. (Yes, you are super tiny now.) If the water is stationary, you might be stuck in a part of the pipe that is exposed to cold air. You could actually freeze, and then you would have to break the pipe. But now suppose it’s running water, caused by a faucet that is slightly dripping. You are still a tiny person inside of a pipe, but now you are also moving. You pass through the section of cold pipe and you get cold—but you don’t freeze. Instead, you just move on to other parts of the house.
Oh, but more water from the main underground line is coming into that cold part of the pipe. Would it freeze? It’s not as likely. Remember, the water pipe is at ground temperature, which is almost certainly not below freezing. So, the incoming water isn’t super cold, and hopefully it won’t freeze.
What About Insulation?
Insulation helps. If you wrap some foam insulation around any exposed pipes, it does the same thing as your cooler or insulated drink cup. The insulation decreases the rate that energy is transferred from the hot thing to the cold thing through a thermal interaction. If you put a cold drink out on a table, energy is transferred into the drink to cause it to increase in temperature. Putting the drink in a cooler, on the other hand, increases the insulation and decreases the rate of energy transfer so that it takes longer for the drink to warm up.