• Plumbpedia™ - How Does A Water Hammer Arrester Work?

Water hammering is caused by the quick shutoff of water supply lines. The energy in flowing water has to go somewhere, and when a valve is shut off, the pipes can flex and “hammer” against anything close--like nearby studs, joists or other water pipes. Solenoid-triggered valves, like the ones in dishwashers, washing machines and water softeners, shut off almost instantly, not only causing the most ferocious hammering but also putting strain on rubber hoses and copper sweat fittings. New style single handle faucets allow the water supply to be turned off very fast which can cause severe water hammering. The solution is to close the handle slowly or install hammer arresters. The old style 2 handle multiple turn faucets usually don’t cause as much hammering because the shutoff is more gradual. Water hammer arresters cure this condition with a cushion of air that absorbs the momentum.

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Water Hammer FAQ

What is water hammer?
Water hammer is usually recognized by the banging or thumping noise that is heard when valves are shut off. Although this is an easy way to recognize the problem, water hammer doesn’t always make these telltale noises. Water hammer occurs when the flow of moving water is suddenly stopped by a closing valve. This sudden stop causes the whole column of water behind the valve to slam into the valve, and itself, like a freight train crashing into a wall. The tremendous spike of pressure that is caused, is called water hammer, and it not only acts like a tiny explosion inside pipes, it can be just as destructive.


How much damage can water hammer do?
Uncontrolled water hammer on a water line of just 50 PSIG, will commonly result in pressure spikes of 250 to 400 PSIG. This pressure spike is not just at the valve, it reverberates backwards from the valve throughout the plumbing system, rattling and shaking pipes, until it is absorbed. If there are no measures taken to control it, water hammer will accelerate the failure of water heaters, valves, backflow preventers, washing machines hoses, pipe, fittings, etc.

What types of valves can cause water hammer?
Any quick-closing valve can cause water hammer. In residential systems, these valves are typically found at the washing machine, dishwasher, the tub/shower, and the ice maker at the refrigerator. Kitchen or lavatory faucets, toilet ballcocks and lawn sprinklers systems can also create the problem. Water hammer can occur on both the hot and cold water supply lines.

Will air chambers control water hammer?
A capped stand pipe or air chamber is not an effective solution to controlling water hammer. Since nothing separates the air from the water within an air chamber, it only takes a few short weeks before the air is absorbed into the water, leaving the air chamber waterlogged and completely ineffective. Laboratory tests confirm that the air is depleted by simple air permeation and by interaction between static pressure and flow pressure.

How can water hammer be controlled?
The most effective means of controlling water hammer is a measured, compressible cushion of air that is permanently separated from the water system, that is, an engineered water hammer arrester. Typical arresters employ a pressurized cushion of air and a dual o-ring piston, in a sealed seamless chamber, which permanently separates this air cushion from the water in the system. When a valve closes the water column is diverted into the arrester thus pushing the piston up the arrester chamber against the pressurized cushion of air. The air cushion in the arrester reacts instantly, preventing the pressure spike that causes water hammer. The piston then returns to its original position after the shock is absorbed, ready for the next occurrence.

Will a centrally located expansion tank or large arrester control water hammer?
A common myth is that one large arrester centrally located will control hammer throughout the piping system. In reality, this does little or nothing to control hammer. The pressure spike of water hammer starts right at the valve or faucet immediately upon closure, and works its way back upstream, damaging everything in its wake. By the time the pressure spike reached the expansion tank or arrester; it would already have done its damage. Further, thermal expansion tanks are not the type of products that are defined by the ASSE 1010 Standard for continuous, maintenance free protection from water hammer. Of all the expansion tank manufacturers in the country, none of them claim that their products control water hammer.

Do you need water hammer arresters on plastic pipe?
When water hammer is present, it must be controlled, regardless of the piping material. It may be true that plastic tube does not seem to transmit as much noise as copper, but the energy from the flowing water is still there. It has to go somewhere. So where does all that momentum energy go when you shut off a valve or faucet? It goes right into the plastic pipe and connections; they take the full brunt of the pressure spike. Every time a valve closes, the plastic pipe system itself is forced to absorb the energy by expanding and contracting over and over again—which is not good. In high flow-rate situations, like lawn sprinkler systems, pipe failures have been seen within a few days of installation due to stress fractures caused by water hammer. In lower flow situations, the same thing is happening, it just takes longer to develop.