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About Water Hammer

Learn what water hammer is and what you can do to fix it

You're getting ready for bed, having just watched a scary movie. Your house creaks, and only after a moment of panic are you able to laugh it off. Then it happens: loud, terrible noises reverberating through the entire house. The dread starts to wash over you. Poltergeists aren't real, right? As suddenly as it began, it stops. You gather yourself and check the kids; it's nothing. The next morning, the same sound. At this point, you're a bit freaked out. Until you see the kids turning the bathroom faucet on and off: when they stop, so does the sound. It takes a minute, but you realize your home might not be infested by the otherwordly. Instead, there might be a problem with your plumbing. And there is - it's called water hammer (or hydraulic shock).


What is water hammer and how does it happen?

It's true that your home's plumbing can make all kinds of noises, but water hammer is unique. Characterized by a series of quick thumps, or a single loud one followed by silence, it occurs only when a valve is suddenly closed. A noise following a valve opening is not water hammer, but likely air trapped in the pipe (which would require the plumbing system to be drained). If the noise seems to be associated with a pump starting, the issue could be either, or even both. Air in the lines usually causes sputtering in faucets and other outlets, and can lead to water hammer as well. If you're hearing noises but don't have any sputtering, there may be a rock caught in the valve, or it could have a loose washer.

Why should something like quickly turning off a faucet cause such clamor? Physics, of course! Water is dynamic and powerful, and in your home travels within only a few hundred feet of narrow piping. With proper installation, good conditions, and with all fixtures operating smoothly, this is a perfect setup. However, should any of the following be the case, water hammer can occur:

Water hammer shakes your pipes
  • High water pressure (usually over 80 psi)
  • Small diameter pipes (under 3/4" diameter)
  • Valves that are sized smaller than the pipe they're connected to

With these conditions present, any valve that closes too quickly - be it a faucet, toilet fill valve or a solenoid valve (like those found on washing machines and dishwashers) - can cause water to create a shock wave through your home's plumbing at pressures up to 10 times greater than normal. With nowhere else to go, the water's kinetic energy is transferred along the length of the pipe, hitting joints and fittings with exactly the kind of force they shouldn't be subjected to. If a supply line carrying just 1 gallon of water at 4 feet per second is suddenly closed off, it's like rolling an 8 lb. bowling ball into a wall at the same speed!

Clearly, this phenomenon is more than just an annoying sound. That sound, in fact, is basically your plumbing screaming. It's worse the less secure your pipes are against joists and studs, and water hammer will work towards just that: the vibrations and movement created by it can loosen mounting brackets and put your plumbing at even greater risk of damage.

It's not necessarily the pipes themselves that incur damage from water hammer (though it can cause them to expand), but joints, fittings and fixtures themselves. While the wave can travel down a solid length of pipe relatively smoothly, once it hits something different, that wave will break and transfer its energy straight to the weakest point of solder or that invisible fracture on a fitting. This is why even PEX plumbing can be susceptible to the ravages of water hammer.

How can I fix water hammer?

So what can be done? It's important to first of all make sure your pipes are secure. If you can, take a look at them while someone turns off a troublesome valve to make sure the "hammer" isn't just noise from loose pipes. If any valves are sized smaller than the lines, replace them to match.

Do you have high water pressure? If you aren't sure, a test gauge is a cheap and easy way to find out. Pressures over 80 psi can lead to water hammer, as well as damage to fixtures. Pressure Regulating Valves (PRV) are required by many plumbing codes, and can guarantee your home's water is at a safe pressure. A PRV must usually be accompanied by an expansion tank to absorb any excess pressure due to thermal expansion. If you're lucky, simply reducing your home's water pressure might keep things quiet.

Because air is compressible (water hardly is) and easily introduced into plumbing, the best way to deal with water hammer is to use air to absorb the shock. Traditionally, this was accomplished with an "air chamber": a vertical length of pipe with a cap at the end, installed on the supply lines. These chambers were initially filled with air, which in the event of water hammer, would absorb the shock and leave your plumbing relatively unaffected. There's one big problem with this design, though: water absorbs air. After a while, the air chamber is no more, and water hammer will take its toll. To remedy the situation, you'd need to drain the entire system and re-introduce the air. Luckily, this design is now considered primitive, and is increasingly rare in plumbing codes. If you're having problems with water hammer and know you have air chambers, you may need to drain the system or clean out the chamber, which could be clogged with scale and residue.

The solution to the air chamber problem turned out to be rather simple: keep the air separated from the water. Enter the water hammer arrester. These devices feature rubber diaphragms or pistons with o-rings that maintain that separation, keeping the air inside available for shock absorption. When a troublesome valve is closed, the water finds its way to the arrester, forcing the diaphragm or piston up, compressing the air inside it until the force dissipates. Arresters should be installed within 6 feet of valves, as they will be less effective the further away they are.


No plumbing system should make an excessive amount of noise. If yours does, it's not something to be tolerated or ignored. Though potentially dangerous and most certainly annoying, water hammer can often be easily remedied and prevented. Not only will you protect your plumbing, you'll (probably) never have to worry about a spectre in your walls again. It's a win-win!


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