Fast acting valves will cause the sudden ’shut-off’ of the water flow. The two most common valves are in the toilet and the clothes washer. A garden hose ( with the spray nossle) could be another source.

A vacume in a water line implies empty space. Once an opening to this space would be present air would be drawn into the vacume space. However, in a water piping system in a home there is no empty space. All lines have water, even if the source is not drawing water at that very moment, the pipe to that source still has water in it. Also, if a vacume did exist in your water piping system then the gravity from the world around it would result in the pipe being totally crushed for the section having the vacume. A vacume is kind of like “reverse pressure” or suction – it will suck everything into the vacume in an attempt to fill the vacume, and gravity is only helping to accomplish this.

The only way to have a pipe with no standing water in it ( on a supply line) is if there is a shut-off valve which is shut off. Beyond the shut-off is a source which has been opened to drain the standing water in that specific section of pipe leading to the shut-off. As this space becomes empty it is filled with air – there is no vacume in the water system.

You may need more than one water hammer arrestor, each should be installed close to the suspected problem source. I’d get two and install to the toilet and clothes washer sources.

It has nothing to do with the pipe leading outside.

If you are getting this noise when the water is running, it is not water hammer. It’s probably plastic water pipes vibrating in the walls. Not much can be done for that, not cheaply and easily anyway.

Improperly sized supply lines for given peak water flow velocity;
Excessive system water pressure and lack of pressure-reducing apparatus;
Inadequate strapping or securing of plumbing to structure;
Excessively long straight runs with no bends;
Lack of expansion tank or other dampening system, such as water hammer arresters;
Newton’s law states that for “every action there is an equal and opposite reaction.” If water is flowing into a washing machine then is suddenly shut off, the kinetic energy of the flowing water reverses direction and must be dissipated during the transition to a steady state. This energy is initially reflected back through the plumbing system in a direction opposite to the original flow, creating an oscillating shock wave. Depending on the extent of the shock wave, a loud banging or rattling sound can be heard as pipes expand and move as the shock wave dissipates.

If there were no friction losses and if the pipes had no expansiveness, the shock wave would continue indefinitely. However, as water flows through the pipes, friction due to internal pipe surface irregularities helps to slow the water, resulting in energy that is converted to heat. In addition, virtually all pipes — including drawn copper tubing — have some measure of elasticity. As the plumbing system encounters a sudden pressure shock wave, the pipes expand slightly to absorb the shock. Pressures in excess of several thousand PSI are possible during this brief instant, which is why water hammer can burst pipes and joints unexpectedly.

Failure to properly address water hammer can yield the following dangers, according to the Plumbing and Drainage Institute:

Ruptured piping
Leaking connections
Weakened connections
Pipe vibration and noise
Damaged valves
Damaged check valves
Damaged water meters
Damaged pressure regulators and gauges
Damaged recording apparatus
Loosened pipe hangers and supports
Ruptured tanks and water heaters
Premature failure of other devices
Clearly, repairing any of these conditions “after the fact” is more expensive and inconvenient than designing a system right from the start.

Traditional Solution to Water Hammer Traditionally, the solution to water hammer has been to install pipe risers inside the wall at each faucet or valve junction. Sometimes these risers would be as high as 24″ or more, depending on the pipe diameter. In theory, the risers would trap air as the plumbing system is first activated. The column of air acts as a natural damper, compressing as it absorbs residual shock waves from a sudden change in the supply flow.
Many, if not most older homes today have such systems and some plumbers continue to install air risers because “that’s the way they’ve always done it.” However, what is not readily understood is that these risers eventually fail due to water logging. Over time, the trapped air in the risers dissolves into the water supply itself and the water level gradually rises until the air chamber is completely void of air at all. For this reason, some people who have never experienced the sound of water hammer may suddenly start complaining of “strange noises inside the walls.”

The only real solution is to completely drain the home’s water supply system at the lowest point and gradually re-pressurize it. However, this solution is temporary at best, since the air chambers will eventually become waterlogged once again, thereby eliminating their effectiveness.

What’s more, further studies have also found possible health problems associated with air risers, such as an accumulation of rancid water, bacteria, minerals, and other muck that festers in the dark, dead-end chambers. If left unchecked and untreated, this could eventually contaminate the entire household water supply causing unexplained illness.

Therefore, modern practice and, indeed, many plumbing codes, now prohibit air chambers in new construction. In any case, if you are remodeling or building a new home, DO NOT use air chambers to mitigate water hammer problems. Rather, design the system right from the start and you’ll never have to worry about it again. A combination of proper pipe sizing and water hammer arresters are all that’s necessary in most situations.

Prevention by Design If air chambers, then, are not an approved method of dampening the impact of water hammer, what is the correct method? First, let’s consider new construction where there is no existing legacy plumbing.
In the case of a new home, there is total flexibility in sizing the water supply lines properly. Empirical studies conclude that water hammer is greatly mitigated when water velocity is kept below 5 feet/second within the pipe. This is a conservative number. And while CPVC plumbing is increasingly common (and is somewhat more forgiving at higher velocities due to its increased expansiveness), copper plumbing is still the mainstay of most commercial and residential plumbing systems today and is the focus of this technical note.

To assist contractors and plumbers in sizing the water supply line diameter at each branch of a plumbing supply system, Builders Websource has constructed several tables comparing Type K, L and M copper tubing. While type K is rarely used in residential construction, type L and M are more typical. Type K copper, denoted by its green labeling, has the thickest wall of all three types, whereas type M (denoted by red labeling) is the thinnest. Since the outer diameter for all three types is fixed, type M actually provides slightly greater internal cross section for a given pipe size due to its thinner wall size. While this can help to reduce water hammer by a small fraction, the thinner wall size is more susceptible to bursting and pin hole leaks over time. Type L copper is the best compromise for price/value and is a proven all-around performer for commercial and residential use above ground.