What Is Water Hammer and How Do You Fix It?

That bang after the washer shuts off? That’s water hammer. Water moving through the pipe hits a dead stop and sends a shockwave down the line. It’s not just an annoying noise, either — the plumbing code treats it as a real problem. Some folks think they’ve got a ghost in the wall. I’ve been called out on three of those jobs. Every single time it was a bad air chamber, not a spirit.

What Causes Water Hammer

Water moving through a pipe carries momentum. Close a valve fast, like a washing machine solenoid or a quick-close faucet, and that moving column of water has nowhere to go. It rams into the closed valve. That sends a pressure spike back through the line. Hits elbows. Hits tees. Hits pipe straps. That’s your bang.

The ICC IPC (International Plumbing Code) requires water hammer arrestors on quick-closing valves for exactly this reason. Section 604.9 in most adopted versions of the code calls for protection on fixtures like washers and dishwashers that use solenoid valves. Doesn’t matter if your house passed inspection thirty years ago under an older standard. You’re not up to current code without them.

The Three Things That Make It Worse

• High static water pressure. Anything over 80 psi at the meter and you’re asking for trouble. I check this on every single inspection with a $10 gauge that threads onto the outdoor spigot. Takes two minutes. Skip it and you’re guessing.
• Loose pipe strapping. Copper and PEX both need a strap every 6 feet horizontal, 10 feet vertical, under most local amendments to the IPC. Loose straps let the pipe move, and moving pipe amplifies noise you’d barely notice otherwise.
• Missing or failed air chambers. Older homes built before the 1980s relied on capped vertical pipe stubs — air chambers — to cushion the shock. Over time these fill with water and quit working. That’s the number one cause I find on service calls in houses built before 1985. Every time.

Water Hammer Arrestors vs. Air Chambers

Homeowners ask me all the time why their old air chamber stopped working and whether they need a new one or something different. Here’s the straight comparison. No sales pitch.

IAPMO’s Uniform Plumbing Code has recognized mechanical arrestors as the acceptable solution for decades now. If you’re renovating a kitchen or laundry room and your plumber suggests skipping this, that’s a red flag. Push back. I won’t leave a laundry room without one installed. Never have.

How I Diagnose Water Hammer in the Field

First thing I check: water pressure at an outdoor spigot. Over 80 psi? That’s my first suspect. No arrestor on earth holds up against a well pump or municipal system cranking 100 psi into old galvanized pipe. Second, I run every fixture in the house one at a time and listen for exactly when the bang hits. Washing machine solenoid valves are the usual trigger. They snap shut in under a second.

Third, I check strapping in the crawlspace or basement. Copper pipe loose in its hangers will slap against floor joists on its own. People blame that on “water hammer” plenty of times when it’s really just an unsecured pipe run.

Red Flags

• Banging on every fixture, not just one. That points to high system pressure, not a bad arrestor.
• Banging that started right after you installed a new washer or dishwasher. The new solenoid valve snaps shut faster than your old fixture ever did, and now you need an arrestor you never needed before.
• Banging paired with pipe you can see moving in the crawlspace. That’s strapping, plain and simple. No arrestor fixes loose strapping.

How to Fix Water Hammer

1. Test your static water pressure first. Above 80 psi? Install a pressure-reducing valve (PRV) at the main. This one fix solves a huge chunk of the calls I run.
2. Install mechanical arrestors on washing machine boxes, dishwasher lines, and anything with a quick-close solenoid valve. Kohler, Moen, and Delta all publish spec sheets confirming their quick-close faucet cartridges need an arrestor installed upstream. Not optional on modern fixtures with fast-acting internals.
3. Secure loose strapping wherever you can reach it. $20 in pipe hangers if the run’s accessible.
4. Flush the system if you’ve still got working air chambers. Shut off the main, open the highest fixture in the house, let it drain completely. Resets trapped air in the old-style chambers. Temporary fix. Nothing more.

The Water Quality Association notes that sediment buildup in old galvanized pipe narrows the interior diameter and speeds up water velocity. That makes hammer worse. Still got galvanized supply lines in your house? That’s a conversation with your plumber that goes beyond the noise complaint.

When It’s More Than Annoying

Water hammer isn’t just noise. Repeated pressure spikes stress soldered joints. They loosen fittings. They shorten the life of your fixture valves and supply lines. I’ve seen a hammer spike split a soldered copper joint at 2 AM in a finished basement. That’s not a knock in the wall anymore. That’s a flood. If your house bangs every time the washer shuts off and you’ve let it go two years, you’re gambling on a joint failure eventually. Don’t wait for that.

Get this looked at by someone who’ll actually test your pressure and check your strapping. Not just sell you an arrestor kit off a hardware store shelf. Find a licensed plumber in your area through localto.co, or if you’re checking out this site from somewhere else, localto.co covers other markets too.

— Frank Mercer, Licensed GC (Ret.) | HAAG Certified Roof Inspector