Water Hammer: Why Your Pipes Bang and What Fixes It

What causes water hammer in house pipes?

Water moving through a half inch line has weight and speed behind it. Shut a valve on it in a fraction of a second and that momentum has nowhere to go, so it turns into pressure and throws a shock wave back up the pipe. Watts puts it the same way in its own spec sheet: the banging is the shock of high speed water when a fixture is suddenly closed.

The size of the spike surprises people. Sioux Chief's engineering guidance puts the pressure rise in metal pipe at roughly 60 times the flow velocity in feet per second, which makes a common half inch supply line at 6 fps good for about a 360 psi jump on top of whatever the house already sits at. That lands on the pipe, the solder joints, the fittings, and every appliance hose downstream.

How hard it hits you depends on how fast the valve closes, how fast the water was already moving, and whether the pipe is free to move when the wave arrives.

Why does the washing machine set it off and not the faucets?

A faucet closes as fast as your wrist turns, which is slow. A solenoid valve snaps shut in milliseconds. Watts names dishwashers, clothes washers, and fast closing positive shutoff valves as the sources of water shock, and that matches what we find in the field. Irrigation zone valves and ice makers belong on the list too.

Which is why the bang usually keeps a schedule. It lands mid cycle on the washer, or at the same point in the dishwasher run, or every time the sprinklers change zones at five in the morning. If the noise is tied to one appliance, you have already found the valve.

Do water hammer arrestors actually work?

A certified one does. An arrester is a small piston or bellows device with a sealed, pre-charged air cushion behind it. The wave hits the piston, the air absorbs it, and the shock dies at the valve instead of traveling the length of the house. Watts charges its Series 15 arrestor to 60 psi at the factory and seals it permanently, and the sheet says the sealed chamber prevents the loss of air to the water.

That sealing is the whole reason the old trick quit on you. Houses used to be plumbed with a capped vertical stub of pipe above the fixture, called an air chamber, and the trapped air was supposed to do the cushioning. Nothing separates air from water inside a bare stub, so the water absorbs the air and the stub fills up. Draining the house down recharges it for a few weeks, then the banging is back.

Placement decides whether a new one helps. Watts tells installers to put the unit as close to the source of shock as possible, one on the hot line and one on the cold. An arrester tucked in at the water heater does nothing for a washer bang at the far end of the building.

Will strapping the pipes stop the banging?

Sometimes on its own, and almost always as part of the fix. A pressure wave makes pipe jump. If a run is loose in the crawlspace, hanging between joists with a strap missing, or passing through a drilled hole with nothing between copper and framing, that jump becomes a hammer blow against wood. What you hear is the house, not the water.

Get under the floor or into the attic while somebody runs the washer through a fill. You can usually watch the pipe move. Cushioned clamps at the loose spots, a strap where one fell off, and a foam sleeve where a line rubs a joist kill a lot of noise for very little money. Do not crank a clamp down tight on bare copper against wood or steel, because then thermal movement grinds instead of banging.

Older houses around here hand you more of this to find. Plenty of the places we open up still run their original 1960s galvanized supply lines on 1960s hangers, and galvanized that has rusted down to a narrow bore pushes water through the opening it has left that much faster. At that stage strapping is a patch on a pipe that is finished. A whole house repipe in copper or PEX runs two to four days on most single family homes, and we do offer financing on work that size.

Are water hammer arrestors required by code?

The model Uniform Plumbing Code language calls for an approved arrester where quick acting valves are installed, and the devices that count have to be certified to ASSE 1010 or PDI WH-201. The Watts unit above carries both listings plus IAPMO. California adopts the plumbing code with amendments, and the water hammer section is one HCD did not take up, so whether an inspector writes it on a retrofit comes down to your local building department. On new work and remodel permits, plan on it.

Read the listing before you buy. A capped stub sold as an air chamber carries no certification and no sealed charge, and a screw-on laundry gadget with no ASSE or PDI mark on the box is not an arrester.

Could the real problem be pressure instead?

Often it is. Hammer gets worse as static pressure climbs, because the same pipe is moving water faster to begin with. Houses sitting below a storage tank in Marin, or up on the hill neighborhoods around Santa Rosa, start high. Put a gauge on a hose bib before you spend a dollar on arresters: the walkthrough is in high water pressure in the house, and anything over 80 psi gets fixed first.

One bang from one appliance is a cheap afternoon with a strap and an arrester. Banging at every fixture, a shudder that runs the whole length of the house, or pipe you can hear hitting framing from another room says something is wrong back at the main. Call CNTRline at (707) 308-5599 and we will put a gauge on it and tell you which one you have. Same day in most cases, and we answer 7 days a week.