# Why is there water hammer after repiping, and how is it fixed?

> Water hammer after repiping: why new lines bang when valves close, how pressure and supports drive it, and the fixes short of a redo. Get a pressure check.

URL: https://fullboredallasrepiping.com/guide/water-hammer-after-repiping/

Learn

A bang when the washer valve snaps shut is a pressure surge, and new pipe can make it louder. The cause is usually pressure, support or quick-closing valves, and the fix is targeted, not another repipe.

Last Updated: November 10, 2026

Key takeaways

- Water hammer is a pressure surge when moving water stops suddenly, usually at a quick-closing valve in a washer, dishwasher, ice maker or single-handle faucet.
- After a repipe it often shows up because the restriction is gone and flow is higher, supports are loose, or the old air chambers were removed.
- High static pressure makes every surge worse. The plumbing code calls for a pressure-reducing valve where static pressure exceeds 80 psi.
- Fixes include water hammer arrestors at the offending valve, added supports, and PRV and thermal expansion work. A full redo is rarely the answer.
- Air sputtering right after a refill isn't water hammer and usually clears.

Every time the washing machine finishes filling, something in the wall goes bang. Water hammer after repiping is a pressure surge: when a valve shuts fast, the moving column of water stops and slams back through the pipe. New pipe can make it louder because full-bore lines carry faster flow than scaled old pipe, supports may need adjusting, or old air chambers came out. The fix is usually aimed at pressure, support or the valve itself.

[Fullbore Dallas Repiping](https://fullboredallasrepiping.com/) handles [PEX repiping](https://fullboredallasrepiping.com/pex-repiping/) and [whole-house repiping](https://fullboredallasrepiping.com/whole-house-repiping/) for Dallas homes, and we plan surge control into the job. Here’s how to work out what’s banging and what usually fixes it.

## What’s actually banging

Water moving through a pipe carries momentum. A valve that closes in a fraction of a second, like the solenoid in a washing machine, dishwasher or refrigerator ice maker, or a single-handle faucet snapped shut, stops that water almost at once. The pressure spike runs back through the line and shakes it against the framing. You hear a bang or a run of knocks.

Higher pressure and faster flow both make the spike bigger. Long straight runs and loose supports make it louder.

## Why it started after the new pipes went in

### The restriction is gone

Old galvanized pipe narrowed by tuberculation throttled your flow. When [galvanized pipe replacement](https://fullboredallasrepiping.com/galvanized-pipe-replacement/) brings full flow back, water moves faster and stops harder. Pressure the restriction used to hide now reaches the fixtures.

### Supports and routing

PEX expands and moves more than copper. If it isn’t supported at the spacing the code and the maker set, or it passes through framing without sleeves, it can knock against wood. A long unsupported drop in a wall behaves like a drum.

### The old air chambers are gone

Many older systems had capped vertical pipes behind fixtures meant to cushion surges. Most waterlogged long ago, but taking them out still changes how the system behaves. Today the answer is a mechanical water hammer arrestor at each quick-closing valve.

### Pressure and thermal expansion

Service pressure varies from house to house in Dallas. The plumbing code calls for a pressure-reducing valve (PRV) where static pressure exceeds 80 psi. Once a PRV or check valve closes the system, heated water has nowhere to expand, so the water heater needs a thermal expansion tank. Without one, pressure climbs between uses and every surge gets worse.

Hearing new knocks since your repipe?

We start with a pressure reading and which appliance or fixture sets it off, then fix that valve, that support or the PRV.

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## Is it water hammer or something else?

| Sound | When it happens | Likely cause |
| --- | --- | --- |
| Single bang | Washer or dishwasher fills stop | Quick-closing solenoid valve |
| Knock at a faucet | Single-handle lever shut fast | Fixture valve, local support |
| Sputter and spitting | First use after water was off | Air in lines, clears with use |
| Ticking in walls | As hot water starts or stops | Pipe expansion against framing |
| Hum or chatter | While a fixture runs | Loose washer, PRV or cartridge |

Air after a refill is normal and fades as you run the fixtures. Our [water shutoffs guide](https://fullboredallasrepiping.com/guide/repiping-water-shutoffs/) covers what to expect when the water comes back.

## How we track it down

Note which appliance or fixture sets it off, whether it’s hot, cold or both, and where in the house you hear it. A static pressure reading at a hose bib with everything off shows whether pressure is high, and a reading that climbs overnight points to thermal expansion. Our [low pressure vs low flow guide](https://fullboredallasrepiping.com/guide/low-water-pressure-vs-low-flow/) uses the same gauge from the other direction.

## What usually fixes it

- **Water hammer arrestors** at the washer box, dishwasher and ice maker connections, sized and listed for the job.
- **Added supports or sleeves** where a line moves against framing, reached from the attic or an access opening.
- **PRV adjustment or replacement** when static pressure is above 80 psi or the existing PRV has failed.
- **Thermal expansion tank** at the water heater in a closed system.
- **Fixture valve changes** where a cartridge closes unusually fast.

None of these means replacing the new piping. A full redo is only the answer if the installation is wrong throughout, and a support and pressure check will show that.

## Old fixtures on new pipe

A [partial repipe](https://fullboredallasrepiping.com/partial-repiping/) leaves some old pipe in service, and full flow from the new section can reach fixtures and valves that were used to a trickle. An old washer box, an aging ice maker valve or a worn toilet fill valve may start to chatter or knock where it never did. Those get fixed at the fixture. Listing which fixtures and valves stay on your estimate makes it clear what was renewed and what wasn’t.

## When it’s more than noise

A bang followed by a drip, a wet ceiling, or water from the water heater’s temperature and pressure relief valve isn’t just noise. Shut off the water at the main and call for service. Repeated surges can also wear on older fixtures and on pipe kept in place after a partial repipe; our [repair or repipe guide](https://fullboredallasrepiping.com/guide/repair-or-repipe/) covers when that pipe is worth replacing too.

## Before and after a repipe

If you’re planning a repipe, make sure the estimate lists the PRV, the expansion tank and arrestors at the quick-closing valves. If the noise started after one, start with a pressure reading. Our [Dallas home repiping guide](https://fullboredallasrepiping.com/guide/dallas-home-repiping-guide/) covers the full job, and you can [get an estimate](https://fullboredallasrepiping.com/contact/) for a follow-up visit.

Questions about your house?

Call (469) 846-9377

Related service

[PEX repiping](https://fullboredallasrepiping.com/pex-repiping/)

We repipe with one listed PEX tubing and fitting system, laid out as trunk-and-branch or a home-run manifold and routed to handle Dallas attic heat and winter freezes.

## Questions about this guide

### Why did my pipes start banging after a galvanized repipe in Dallas?

Old galvanized pipe is usually narrowed by internal scale, which throttled flow and softened surges. New pipe brings full flow back, so when a washer or dishwasher valve snaps shut, the faster water stops harder. Checking static pressure and adding arrestors at the quick-closing valves usually solves it without touching the new piping.

### Does PEX stop water hammer better than copper?

PEX flexes a little and can soften the sound, but it doesn't remove the surge, and loosely supported PEX can move and knock against framing. Copper carries the sound more. In either material, pressure, flow velocity, supports and arrestors at quick-closing valves decide whether you hear hammer.

### How do I check my Dallas home's water pressure for water hammer?

Thread a pressure gauge onto an outside hose bib with every fixture and appliance off, and read the static pressure. The plumbing code calls for a pressure-reducing valve where static pressure exceeds 80 psi. A reading that climbs overnight can point to thermal expansion in a closed system, which needs an expansion tank.

Want this answered for your house?

On the first visit we trace your lines from the main shutoff to every fixture and show you what the work would involve.

Call (469) 846-9377

## More guides

- Learn [Low water pressure vs low flow: which one does your house have?](https://fullboredallasrepiping.com/guide/low-water-pressure-vs-low-flow/)Pressure is the push behind the water; flow is how much arrives. Your weak shower can come from either, and only one of them is fixed by new pipe.
- Learn [The Dallas home repiping guide](https://fullboredallasrepiping.com/guide/dallas-home-repiping-guide/)Leaks that keep coming back, rusty water, a shower that dies when the washer runs: what is going on in your pipes, and how we plan, permit, build and close up a repipe in a Dallas house.
- Decision Guide [PEX vs copper pipes: which suits a Dallas repipe?](https://fullboredallasrepiping.com/guide/pex-vs-copper/)Both are allowed under Dallas's plumbing code, and neither wins in every house. Your route, your attic, your fixtures and what you care about most decide which fits.
- Decision Guide [Should you repair or repipe a house with leaking pipes?](https://fullboredallasrepiping.com/guide/repair-or-repipe/)One leak is a repair. A pattern of leaks on the same pipe is a decision. Here's how to tell which one your Dallas house has, and what sits between the two.
- Learn [What causes recurring copper pinhole leaks?](https://fullboredallasrepiping.com/guide/recurring-pinhole-leaks/)A copper pinhole is the end of a slow process inside the pipe wall. When they keep coming, where and when they show up tells you more than any single leak.