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The Humble U-Bend: How It Keeps Your Drains and Sewers Working

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Look under your sink or at the profile of your toilet and you’ll see a twist in the pipe. This isn’t just for the style and look of your drain. It actually has a very important reason why it takes up room under your sink.

A Rose By Any Other Name…

…Would still prevent smells, at least when it comes to U-bends. It’s also called an S-bend or P-bend, but the technical term is the trap. The turn traps water inside the drain, and there is a reason other than to splash you when you remove take the trap off. That water forms a pressure seal on the drain, which has enough force behind it to keep gas in the pipe and out of your drains. Without that bend, the sewer gases would rise back up the pipes and cause all sorts of horrid the smells.

The trap isn’t alone on this. Every drain system has vents, usually sticking out the top of your roof. This provides air to equalize the pressure. Otherwise the water going down the drain any time you flush the toilet or run a shower would pull water through the trap, breaking the seal of liquid.

Helping You Avoid Clogs

Solids, such as sand, can cause a lot of trouble and clogs in your drains. If these happen when the drains are in your floors, walls, or halfway to the sewer, that clog can cause a lot of trouble. Your friendly neighborhood U-bend helps you avoid these clogs. Solids sink to the bottom of the trap, while liquids flow right through it without any trouble.

This does mean that you need to clean your trap out every now and then. Fortunately, plumbers anticipated this by building traps so they’re easy to take apart and reassemble. Just remember that there’s water inside the trap, so put a bucket underneath it to catch the water.

Giving Gravity a Hand

If drains were only a straight pipe, then the most force that could be generated to pull the water and waste through the system would be from the weight of the liquid and items going down the drain. This is where the U-bend steps in and adds a little more force by creating a siphon.

You’ll see this use of the U-bend on toilets. The level of drain pipe on the other side of the bend is higher than the water level in the bowl. As water is released into the bowl from the reservoir, the water level rises, creating weight. This weight pushes the water level over the top of the drain. The water rushed through it, creating a siphon that sucks the water and everything else in the toilet down the drain.

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