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Water hammer is a destructive force that can damage
residential or commercial plumbing systems. Not only is "noisy plumbing" an
annoyance, but shock forces due to water hammer can rupture copper supply
lines or cause leaking at joints. This comprehensive Builders Websource® Tech
Note explores the causes and cures of water hammer and other plumbing
noises, helping homeowners, designers, contractors, and plumbers to avoid the
problem during the design phase or resolve an existing serious condition.
Table of Contents
Introduction - What is Water Hammer?
Water hammer is a pressure shock wave induced in plumbing
supply systems whenever there is a sudden change in the steady state
condition of a non-compressible liquid such as water. Pumps, valves,
faucets, toilets, and fast solenoid-activated valves (such as commonly found
in washing machines and dishwashers) are all examples of devices that can
induce water hammer within a typical plumbing system. Water hammer can
result in noisy, banging sounds as pipes rattle and expand to absorb the
pressure wave. Shock waves in typical water pipes travel at up to 4500
feet/second and can exert tremendous instantaneous pressures, sometimes
reaching 150 to over 1,000 PSI. If left unchecked, water hammer can damage
pipes, valves and eventually weaken joints. However, the lack of noise does
not mean that water hammer is not present.
Several factors exacerbate
water hammer and are generally traceable to inadequate system design and
installation. These include:
- Improperly sized supply lines for given peak water flow velocity;
- Excessive system water pressure and lack of pressure-reducing
- 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
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.
Water Hammer 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
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.
To get a sense of how to use Tables 1 and 2, the following examples
Q. What is the capacity of 3/4" type L copper tubing in gallons per
A. Looking at Chart 1, there is .0251 gallons of water in one lineal foot
of 3/4" type L copper pipe.
Q. Suppose a branch must supply water to a new master bathroom,
consisting of a shower, roman tub, toilet, and two sinks. The pipe is Type M
copper and the rated maximum gallons per minute of each valve is as follows:
Shower: 3.5 GPM
Tub: 5 GPM
Sink: 1.5 GPM x 2
Toilet: 1.5 GPM
Total Peak GPM flow: 11.5 GPM.
What is the minimum supply size serving the main branch trunk to avoid water
A. Realistically, the peak flow could be derated since it is unlikely that
all faucets would be turned on at peak volumes simultaneously. Furthermore,
since the shower, tub, and sink are served by both hot and cold water supply
lines, this could also be factored into the derating. However, for the sake
of example, let's assume a worst-case scenario of 11.5 GPM.
Using Chart 2, a 1" type M copper pipe can withstand up to 13.62 GPM flow
rate without violating the 5 foot/second velocity. This would certainly
work. However, by factoring in a reasonable derating and accounting for the
split between the hot and cold water supply lines, 3/4" pipe would likely be
satisfactory, allowing up to 8.07 GPM for both the hot and cold water supply
Q. A 1/2" Type L copper tube is used to supply a washing machine with a 6
GPM fill rate. Will this create water hammer when the machine valve turns on
A. Looking at Chart 2, the maximum flow rate for 1/2" Type L copper pipe is
3.63 GPM. The proposed 6 GPM would increase the velocity to over 8 feet per
second, a condition likely to cause water hammer.
Q. What size should the main trunk line be from the water meter to the
house to avoid water hammer under the following conditions assuming Type K
underground copper supply line:
Sprinklers running @ 6 GPM
Dishwasher filling @ 3 GPM
Washing machine filling @ 6 GPM
Two showers running at 2.5 GPM each
Total peak demand is 18.5 GPM.
A. Looking at Table 2, a 1-1/4" supply line would just meet this
requirement, providing up to 19 GPM at a 5 ft/s peak velocity.
Mitigation in Existing Homes
The above examples provide insight into new construction
where there is total flexibility in sizing supply lines properly at the
outset. But suppose you have an existing home and you are experiencing noisy
plumbing that rattles, bangs, shutters or squeals? These symptoms may not
all be related to water hammer and often there can be multiple problems that
fool novice troubleshooters. Before you replace your plumbing lines, there
are several troubleshooting alternatives starting with a few that are simple
and relatively inexpensive.
Drain Water Supply to Recharge Air Chambers
If water hammer persists and you believe your home has built-in air
chambers (true of most older homes), turn the water heater off or to
"standby setting" and shut off the main household water supply. Drain the
entire household plumbing system (and water heater) at the lowest point in
the system. Turn on all faucets to make sure there is no vacuum that
prevents the water from draining. This will ensure that the supply lines
fill with air.
Once you are sure that there is no residual water in the supply lines,
shut off all open faucets. Carefully restore the main water supply and
slowly turn on the faucets at the end of each branch run. This process will
restore water into the system, trapping fresh air in the risers. If the
risers are clogged with mineral deposits and other muck, this procedure
won't help much, unfortunately.
When the system is fully recharged, restore your water heater to its
normal operating temperature range. This recharging procedure will have only
a limited impact as air chambers typically become waterlogged over time,
requiring a repeat of the process described above.
CAUTION!: When draining the water heater separately, the water
temperature could be scalding (140-160 degrees) resulting in serious burns.
Always wear protective clothing and protect your face and eyes with goggles.
Be very careful when draining a water heater and carefully follow all
Install Low-Flow Fixtures
Since water hammer is correlated to total flow velocity, one solution in
older homes is to replace existing fixtures with low-flow types or water
restrictors. For example, replacing an old 4.5 GPM shower head with a newer
2.2 GPM version will cut the peak flow rate in half. Similarly, replacing
tub and lavatory faucets or using flow restrictors could be enough to
minimize water hammer.
Check Washers, Valves and Toilets
Sometimes other factors can create resonance in a water supply system
which amplifies through the pipes. These include faulty washers in faucets
or supply shut off valves. If nothing has changed in your home and suddenly
you are experiencing noisy plumbing, try isolating the fixture that has the
problem and inspecting the valve mechanism. Resolution to your problem might
be as cheap as a five-cent rubber washer.
If the problem appears to be related to the toilet, you may need to
change the ballcock, fluidmaster or fill valve. Or the shutoff valve to the
toilet may have a bad washer.
Secure Loose Pipes
While this may not always be convenient due to poor accessibility of
hidden pipes, long straight runs are generally the worst offenders. Where
accessible, tightly secure all pipes at frequent intervals to minimize
rattling against hard surfaces. While strapping will help to alleviate some
of the rattling sounds resulting from water hammer, it does not address the
water hammer condition itself.
Install Pressure Regulator
If your home does not have a pressure regulator and your water pressure
is high (in excess of 60-80 PSI), adding a pressure regulator could help to
reduce water hammer. By reducing household pressure to 40 PSI, you will
retain sufficient pressure while reducing the likelihood of damaging water
hammer. This should be done by a professional, licensed plumber.
Adjust Existing Pressure Reducer
Some people report a "squealing" noise in their plumbing. This should not
be confused with water hammer. If your main water supply line already has a
pressure regulator, you may need to adjust the set screw on the regulator to
eliminate the squealing sound. If it's adjusted all the way down to the
lowest pressure, this can create a condition called cavitation.
According to Utah State University's School of Engineering, "Cavitation
is a liquid phenomena based on the formation and collapse of vapor cavities
in the fluid passing through a valve. The vapor cavities begin to grow in
low pressure regions such as areas of separation and collapse downstream of
the low pressure regions. Cavitation can produce the effects of noise,
vibration, and erosion or damage to the valve and downstream piping."
Alternatively, experts at HandymanWire.com advise, "if you have a shutoff
valve prior to or after a pressure reducing valve, and they are of the
washer variety, check it for a bad washer and make sure it is open all the
way. If you can narrow it down to a watered valve and changing washers does
not help, replace it with a ball valve."
Install In-Wall Water Hammer Arresters
process can be more invasive and requires that you have access (or create
access) to the fixture supply line where water hammer is most prevalent
(such as a dishwasher, washing machine, icemaker, or other fast-acting
valve). Water hammer arresters are similar to air chambers, except that the
air pocket is fully isolated and sealed from the water, typically with a
piston type plunger and lubricated O-ring. Water hammer arresters help to
absorb the pressure shock wave, virtually eliminating water hammer. The
Plumbing and Drainage Institute offers a comprehensive document on Water
Hammer Arresters. Interested parties may download this document as a PDF
file at: Standard PDI-WH201
Water Hammer Arrester Standard (3663 KB).
manufacturers make compact water hammer arresters for residential and
commercial use. For example,
Precision Plumbing Products
makes convenient wall boxes for washing machines and ice makers. In
addition, they offer handy "mini angle stop" versions that are easily
accessible making them ideal for retrofit applications at sinks and toilets.
The version at the right, is known as the "mini-rester" and is made by the
Manufacturing Company. It is a cost-effective solution for a single
valve, such as a washing machine. Other versions are available that either
screw in or can be sweat soldered, such as the Hydra-rester. Typical prices
for water hammer arresters range from $15 to over $125 each depending on
For optimum performance, water hammer arresters should be installed
inline with the branch supply at the fixture tee with no intermediate bends.
Furthermore, sizing must be done by a qualified professional working with
the manufacturer. Failure to properly size the water hammer arrester will
result in sub-optimal performance and wasted effort.