eli5 why do pipes make cracking/knocking/banging sounds in cold months?
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It’s knocking when steam (hot water) reacts with still (cold) water causing the steam to change phase instantly to water which leads to that knocking noise.
Usually, thermal expansion.
Heat something up, and it will expand. For solid objects, usually it’s just a tiny little bit. But it’s not zero. Chilling an object has the opposite effect.
Pipes are made up of many individual segments all coupled together, so they have loads of tiny little joints where two individual components meet. They also are fitted together extremely snugly. I mean, they have to be to be waterproof/airtight. So if you have a length of pipe running through a space, there’s not going to be a lot of give between the components.
So let’s say we have a pipe that is sitting minding its own business at ambient room temperature, then all of a sudden, you start to run hot water through it. The heat from the water will heat up the pipes, causing them to expand. The expansion amount is tiny, but over the length of an entire pipeline, it could add up to millimeters, even centimeters of length. So you now have a pipe that used to be snugly fit into a very specific spot, and now it’s a centimeter longer than it used to be. *Something is going to have to move.* And of the options, many of them probably do not want to move, and will require tremendous forces to move even just a tiny bit. But physics is a harsh mistress who cares not about the plight of pipes and fittings, so something eventually gives and slips, making an audible sound when it does so. This is the source of the majority of random bangs and clunks you hear in piping systems.
It tends to be more prevalent at night because, well, when it’s night time, it tends to be colder than it was during the day. And all of the things in and around your house, having spent the day being all warmish, start to cool down. This makes them contract, causing the same phenomenon but in reverse, leading to more clunking and banging. Colder months have the same effect, but much more pronounced, and causes it around the clock instead of just at night.
This isn’t limited to just pipes either; they’re just the primary culprits, as they can be carrying liquids or gases that are at wildly different temperatures than the ambient air around them. The components that make up a building can also creak, clunk, and bang for all the same reasons. It’s the source of nearly all of the random sounds that go bump in the night for seemingly no reason whatsoever.
Except the noises *you* hear in *your* house. That’s obviously just a ghost. /s
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