Water hammer is more than annoying, it can actually damage your pipes. An occasional strange noise from your plumbing usually isn't anything to worry about. Often it's as simple as air in the lines. But track down and silence a rattle, thump or bang that occurs on a constant basis before it creates a serious problem.
Water hammer is more technically known as hydrostatic shock. It occurs when a valve or other control device closes abruptly, causing the moving column of water inside a pipe to come to a sudden stop. The force, or shock, of the water makes the pipe move, which in turn can cause a noise somewhere along the plumbing run. That thump you hear could be a pipe hitting a stud or other part of a wall. Sometimes the pipes will hit each other, causing a clang or bang. The pipe might also rattle as it vibrates against a pipe hanger or floor joist. It's more than noise; all that movement can weaken pipe joints and fixture connections, causing a leak.
Appliances are the most common sources of water hammer. If the noises started when you installed a new washer or dishwasher, that's the likely culprit. They both have electric solenoid valves that can shut off a large flow of water fast. Toilet valves can also shut off quickly. Unfortunately, unless there's a problem with the appliance, there isn't much you can do about a fast-acting valve. Besides, the real source of all that noise isn't water flow. It's the motion of the pipes.
Check your water pressure. Pressure that is too high can cause water hammer, as well as a host of other problems. Air risers are often installed at troublesome locations like laundry rooms. Over time, those risers lose their air charge. Try draining the pipes to see if that solves your problem. You can also install water hammer suppressors that have a permanent air charge.
Pipes do have to move. Changes in temperature, building stresses, and even the normal operation of appliances will cause movement in pipes. But none of those slight movements should make much noise or cause the pipes to hit anything else. Confirm that you have enough hangers to support the weight of the pipes properly. Check the points where a pipe penetrates a wall, stud or beam for enough clearance. Pay particular attention wherever a pipe changes direction, because that's where movement often translates into a thump. Anything more than slight movement is unacceptable. Not even plastic pipes should sway or bounce.
Water hammer is actually a symptom. The cause is a combination of an abrupt stoppage of water flow that creates pipe movement. Try to figure out when the trouble started and why. To combat water hammer, first find the source of the problem to see if anything can be done to make it less severe. Then investigate the source of the noise, to be sure that no damage has already been done, and to fix any clearance problems so it doesn't happen again.
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