The knocking started six months after the homeowner moved in. Loud banging from inside the walls every time a washing machine cycle ended, every time a dishwasher stopped drawing water, every time anyone closed a faucet quickly. The homeowner had a four-month-old who slept through most noises but woke up startled every time the pipes banged. The homeowner was also a shift manager, so we got the call at 11 PM on a Wednesday.
We pressure-tested the system at the hose bib and the meter at 102 PSI. That was the first number. The second number came when we looked at the water heater - no expansion tank on what was clearly a closed system with a PRV and a meter-side backflow preventer. Both numbers pointed at the same diagnosis.
What Water Hammer Actually Is
Water hammer is the shock wave that travels through a pipe when flowing water is forced to stop or change direction suddenly. When you close a faucet quickly, the column of water moving through the pipe decelerates from full flow to zero in a fraction of a second. The momentum has to go somewhere, and it becomes a pressure spike that travels back through the pipe at the speed of sound in water - about 4,800 feet per second.
That pressure spike shakes the pipe, rattles against the framing, and at higher pressures can actually damage fittings, valves, and water heaters. You hear it as a bang or a thump from inside the wall. In extreme cases it can rupture a fitting or crack a pipe.
Water hammer happens at any pressure. What changes with pressure is the severity. At 50 PSI the shock wave is manageable. At 80 PSI it is noticeable. At 100 PSI it is the kind of bang that wakes a baby.
Why This House Hammered So Loudly
Two reasons, both fixable.
Pressure over 100 PSI. The PRV on the main was either failing or undersized for the household demand. The static pressure at the meter read 102 PSI, which is well into the range where every fixture closure creates a measurable shock wave. Residential plumbing is designed for 50 to 80 PSI. Above 80 PSI, every other component in the system is operating outside its design envelope.
No expansion tank on a closed system. A PRV plus a backflow preventer at the meter creates a closed system - water has no path back to the supply side. As water heats in the water heater, it expands. In an open system, the expanded water pushes back through the PRV into the supply. In a closed system, the expanded water has nowhere to go, so it raises the system pressure. The expansion tank is a small pressurized tank that absorbs that expansion. Without an expansion tank, every heating cycle raises the system pressure above what the PRV is set to deliver, which means the PRV has been working harder than it should.
The combination of unregulated pressure and no expansion tank is the textbook setup for water hammer. Every fixture closure creates a shock wave, and the system has no buffer to absorb it.
What We Did
New pressure regulating valve. Sized to the household’s peak demand, set to 65 PSI static, with a pressure gauge on the downstream side. Pressure test at four fixture locations to confirm the new setting holds under different flow conditions.
New expansion tank. Sized to the water heater capacity, pre-charged to match the static house pressure (which we set first), and installed on the cold line at the water heater. The expansion tank gives the heated water somewhere to go without raising the system pressure.
Water hammer arrestors at the washing machine and dishwasher. These are small in-line shock absorbers that fit on the supply lines at fixtures that close valves quickly. They absorb the shock wave at the fixture, which is the source, rather than letting it travel back through the pipe. The arrestors are inexpensive and they eliminate the residual hammer that the PRV and expansion tank alone do not catch.
Pressure check across the whole system. Static pressure at every fixture read 65 PSI after the install. The system held 80 PSI for 15 minutes on a pressure test with no measurable drop. We left the homeowner with a working pressure gauge and the recommendation to call us if the static pressure ever reads above 75.
The Result
The homeowner called the next morning to say the house was quiet. The baby had slept through the night for the first time since they had moved in. The washing machine cycle ending, which had been the loudest event, was now a soft click.
That is what a properly configured pressure and expansion system sounds like. Not silent - fixtures still make the normal sounds of water flowing - but without the bangs that the homeowner had been hearing for six months.
The Number
| Line item | What it covered |
|---|---|
| Pressure test | Static pressure at meter, hose bib, water heater, fixtures |
| Pressure regulating valve | New PRV sized to demand, set to 65 PSI |
| Expansion tank | New expansion tank pre-charged, installed on cold line |
| Water hammer arrestors | Two arrestors, washing machine and kitchen |
| Pressure verification | Static and dynamic pressure at four fixture locations |
Total: $1,180 installed. Most of the cost was the PRV and the labor to install on the main. The expansion tank and the arrestors are inexpensive parts.
What We Tell Homeowners With Loud Pipes
If you hear banging or knocking from inside the walls when fixtures shut off, the cause is almost always one of three things.
Pressure over 80 PSI. Static pressure test at the hose bib takes about 30 seconds and a $15 gauge. If the reading is above 80 PSI, the PRV is either failing or undersized. Both are fixable.
No expansion tank on a closed system. If you have a PRV and a backflow preventer at the meter and no expansion tank at the water heater, the system is closed and the pressure is rising every time the water heats. An expansion tank is a small part and a 90-minute install.
Quick-closing valves without arrestors. Washing machines and dishwashers close their supply valves electronically in a fraction of a second. That closure creates a shock wave at the fixture. A water hammer arrestor at the fixture absorbs the shock wave at the point of origin.
Any plumbing company that diagnoses loud pipes as “you need a repipe” without checking pressure first is selling the wrong scope. The diagnosis starts with a pressure gauge and ten minutes at the meter.
If your pipes bang when fixtures shut off - request a pressure and hammer assessment or read more about what causes water hammer in residential plumbing.
