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Trapped air could cause noisy plumbing

QuestionWe've had a problem with "water hammer" for a few years whenever a faucet was quickly turned off, especially a hot water faucet.

Years ago I read something about how the plumbing in some houses included vertical dead end pipes with air in the top that acted as a shock absorber to prevent water hammer. I don't know if my house has anything like this, but I remember that such shock absorber pipes could have their air columns restored by draining if they became waterlogged.

I figured I'd give it a try. I turned off the main shutoff and then opened every faucet in the house. While letting the plumbing system drain, I took off the various aerators and let them soak in vinegar to remove some mineral deposits.

I occasionally moved the handle on combined hot/cold faucets from one side to the other. After a couple of hours I put the aerators back on, closed all the faucets, and then slowly turned on the main shutoff.

After the pipes filled, I bled the air out of each faucet one at a time, starting upstairs and working down. Since then we've experienced no sign of water hammer, even when opening just a hot water faucet and closing it quickly. I don't know if draining the pipes will help all cases of water hammer, but it's worth trying first. It's easy and only costs a little time.

AnswerThanks for your information. I'm glad your solution worked, whether it was true water hammering or just air trapped in the pipes, which creates a similar sound.

Your method is normally used to clear waterlogged air chambers found in older homes (they're now prohibited in new construction).

True water hammering occurs when the water flow is suddenly stopped. Something as simple as turning a shower faucet off suddenly can send pressure or shock waves down the water line, creating the "hammer" noise. Experts say that the shock wave travels faster then the speed of sound and creates enormous pressure within the pipe. Over time, it can damage pipes and valves.

Think of it as a train traveling through a tunnel at high speed that suddenly slams into a ton of rocks at the end of the tunnel. The back of the train, unable to stop, continues forward and collides with each rail car in front of it to the point of impact, creating one horrendous crash. Closing a valve suddenly is like blocking the end of the tunnel. The impact causes a big thumping noise followed by echoes in the pipe.

The real culprits are dishwashers and washing machines. Their valves can cause hammer noise, and worn solenoid valves can cause a repeated hammering noise, similar to a machine gun. It is also possible that such appliances demand more water than one or more of the pipes supplying it can safely handle. A simple solution might be to install water hammer arresters at the shutoff valves; they can be purchased at any home center or plumbing store. Another possibility is that the appliances may need larger pipes—if this is the case, I hope this area is not finished. During our remodeling I installed 3/4-inch copper pipes along with arresters on both hot and cold water supplies at the shutoff valves and we haven't experienced any hammer noise.

Noisy pipes aren't always a sign of water hammer. It is also possible that air is trapped in the pipes. Air in pipes can cause a vibrating or ticking sound that can be more annoying than true water hammer. I've been told that air trapped at high points of the pipe system can be very difficult to push out. Your method is one way to flush air out of the system, but it may only be a temporary solution.

Because air rises above water, it may be necessary to find the high spot (often a point right after the water heater), cut the pipe (make sure the water is off and the system is drained), and install a tee with a 1/2-inch ball valve on the tee outlet so it points up. Use a 2-inch nipple between the tee and valve. Make sure the valve is closed when you turn the water back on; the air will rise to the highest point, i.e., to the upright 2-inch nipple. You can then open the valve a little bit and let the air escape; however, be prepared because water will also come out—don't be surprised when it squirts! Finally, be sure to place a cap in the end of the outlet when you're done so if the valve is accidentally opened, it will not leak.

This is not the final word on the subject, but at least you have a starting point if you start hearing strange noises like thumping, machine gun echoes, bangs, clangs, or knocks.

Copyright © 2004, 2006, & 2008 LAF/C.R.S., Inc. All rights reserved.
Question answered by Leon A. Frechette.

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C.R.S., Inc. · Spokane, Washington · USA

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