How do pipes not burst when you turn off a faucet. Shouldnt all that stopped up water build up pressure and explode?

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How do pipes not burst when you turn off a faucet. Shouldnt all that stopped up water build up pressure and explode?

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Since nothing’s pumping water inside your house to the faucets, the pressure at the faucet isn’t going get any higher than the pressure at the inlet to your house (ignoring the effects of gravity). Once the two are at equilibrium, there is no net flow of water (assuming no leaks).

* Imagine a large tank of water with a spigot at the bottom.
* When the spigot is closed, nothing happens.
* The tank doesn’t explode from pressure.
* When you open the spigot, the water starts to flow out of it.
* When you close it again, the water stops and nothing explodes.
* The water system in your home, in most cases, works exactly the same way, just on a very very large scale.

The water is under pressure but that pressure is limited it doesn’t increase due to you not opening the tap.

There is actually a phenomenon known as a [water hammer](https://en.wikipedia.org/wiki/Water_hammer) that can cause pipes to rupture.

I’m not an expert on the subject and can’t give you a proper ELI5, but a section of the linked Wikipedia article titled “mitigating measures” should help explain things.

Inlet water pressure is normally limited by pressure regulator valve. Typical homes will have it set at 60 psi or so which means the pressure in the pipes will never exceed that value. Most pipe is rated for much higher pressures so there’s no reason to expect pipes to rupture unless they’re damaged or corroded.

With most modern community water systems; water is collected, processed, and delivered to your house often with a combination of gravity, pumps, and pressure regulators.
Once at your house, it *should* pass through another pressure regulator. Your household plumbing is designed to take a certain amount of pressure before soldiered joints or plastic couplings would break free or pipes would burst. Your inlet regulator is designed to keep your pressure below that threshold, which also helps keep your taps, toilets appliances etc from being over powered and ruined.
When you shut off the tap, the water is stopped, this sends a small shock wave to the inlet regulator which then restricts the water entering the building before it can add extra pressure to the pipes.
Since water is non-compressible you may notice a momentary jolting sound, called water hammering. That is the result of the minor increase of pressure “hammering” your pipes, but not hard enough to immediately blow your pipes.
As pipes age, particularly copper pipes, this becomes more of a problem because they have less strength to hold the extra pressure.
In modern times, devices have been included to lessen the pressure on plumbing systems, such as an air chamber at the termination of lines to high flow devices such as kitchen sinks. This air chamber allows the extra force of the water to be absorbed by a cushion of air, which can compress in the end of the line. Mechanical devices have also been introduced to plumbing systems to to replace air chambers due to unreliability. Another addition, hose bibs with pressure release valves, these vent extra water to the out side of your house, for example you have your hose connected and a spray gun or valve at the end, with the water flowing full speed. When you suddenly let go of the handle or close the valve, the extra jolt of water is released at your hose bib and sprays to the ground or surrounding area.

With the combination of some or all of these devices, maybe others as well, assuming they are properly functioning, there is little chance your pipes will burst when shutting a tap off, again unless your pipes are old and fragile.

That phenomenon is called a water hammer and despite what some people are posting is an actual problem.

[https://en.wikipedia.org/wiki/Water_hammer](https://en.wikipedia.org/wiki/Water_hammer)

You are absolutely correct is noting that the momentum of the fluid will cause a pressure spike.

One way to mitigate this is with a water hammer arrestor: [https://www.familyhandyman.com/plumbing/plumbing-repair/how-to-use-water-hammer-arresters-to-stop-banging-water-lines/](https://www.familyhandyman.com/plumbing/plumbing-repair/how-to-use-water-hammer-arresters-to-stop-banging-water-lines/)

1. Water is incompressible under normal circumstances.

2. Your water system is pressurized to a certain pressure, and the pumps/gravity in the system won’t be able to supply pressure in excess of that pressure.

3. The pipes in your water system are designed to be able to withstand the pressure your water system can deliver.

Now, the water hammer phenomenon people are discussing is something that happens with flowing water is very suddenly shut off. The momentum of the water, suddenly hitting a closed valve, can cause the pressure in the pipes to experience a spike. This could cause a broken pipe if it is high enough.

Water hammers can be prevented by several methods. The first is to slowly, rather than suddenly, turn off the valve. This gives the water time to decelerate out of the valve and minimizes the pressure spike. The second is to install smaller pipes that minimize the volume of water in the pipe, to limit the inertia of the flowing water. You can also install pressure regulators to your home water supply to prevent it from receiving excess pressure from the water mains. Water hammer arrestor devices can also be installed, which stop a water hammer if it does happen.

Pressure reducing valve in the main line into the house drops pressure to 35ish psi. To pass the building inspection we put at least 100psi on the whole water system for at least 15 minutes.

Most of the time, even without a pressure reducing valve, you won’t see over 40psi. The only issue comes if there’s no check valve and you happen to be right next to a fire hydrant that gets opened frequently. The line is so big the water kind of “pulses” out and that will create back pressure and damage pex fittings or crimp rings.

All this is eastern NC

That is a severe problem with fire hydrants. If you close them too fast, you will brake a pipe.

Generally speaking, there are three kinds of physical energy stored in a fluid: Pressure, Flow and Elevation. I’ll go through them in context of our faucet.

The elevation energy of our faucet and source does not change regardless of the faucet being open or closed. This is typically the energy used to push water through your faucet out of a water tower or reservoir.

The pressure energy on our faucet is mostly negligible as well, as no pressure can be built in a system exposed to atmosphere. While some systems include some form of pressure element, the output of the faucet is not pressurized per say. This is still important for later, so don’t count it out yet.

The flow energy is the energy you are familiar with, and is directly converted from the elevation energy aforementioned. It’s a combination of the velocity the water is being pushed and the mass of water being moved.

Now earlier, I said there’s no pressure in the pipes. That’s only technically true: The elevation of the reservoir is converted into pressure in the pipes when the faucets are closed: Around 40 PSI in a properly regulated system.

When you open the faucet, that pressure energy is pushed into flow energy, as you would expect since water comes out with mass and speed. Now we’re done, and we close the faucet.

That flow energy doesn’t disappear, and is pushed back into the pressure energy in the piping system. This is in addition to the elevation energy from the water tower that normally goes into the system. This is the effect referred to as “Water Hammer”, as everything in your water system gets slammed by that pressure spike. In an unprepared system conveying significant mass and velocity of water, this can spike to 100+ PSI, doing exactly what you fear: Damaging fittings, loosening connectors, even eventually bursting a pipe.

It is however, entirely manageable: The simplest trick is to simply out-engineer it: If you build your pipes to handily ignore 100 PSI (Or include a regulator to put the general pressure well inside safe margins), you don’t even need to be concerned. In many cases however, fragile components are unavoidable, so instead, you add something to “catch” the shock. While water cannot be compressed, Air chambers or a membrane capped spring can be, and are typically installed near faucets to catch the pressure spike.

You can even exploit this effect in a device called a hydraulic ram: You let flow energy build until it slams shut a valve, and use the resulting pressure spike to push water up through a pipe. [https://en.wikipedia.org/wiki/Hydraulic_ram](https://en.wikipedia.org/wiki/Hydraulic_ram)