What is actually happening
Water flowing through a pipe has momentum. When a valve closes suddenly, as solenoid valves in washers, dishwashers, refrigerators and irrigation controllers do, that moving column of water stops all at once. The energy has to go somewhere, so a pressure wave travels back through the pipe and shakes it against framing, straps and walls.
The higher the house pressure, the faster the water moves and the harder the hit. That is why water hammer often shows up in homes low on a slope with high incoming pressure, or in homes where a regulator has started to fail.
Sounds and what they usually mean
| What you hear | Likely source |
|---|---|
| Single loud bang when an appliance stops | Water hammer from a fast-closing valve |
| Rattle or knock as water runs | Loose pipe moving in its straps |
| Ticking as hot water flows | Pipes expanding against framing as they warm |
| Hum or chatter at the main line | A regulator vibrating, sometimes a sign of wear |
| Banging from irrigation lines in the yard | Sprinkler valves closing on high pressure |
How a plumber usually tackles it
- Check the pressure first.A gauge reading tells whether the root cause is high incoming pressure. If so, the regulator comes first.
- Find the trigger.Running appliances one at a time shows which valve sets it off.
- Secure loose runs.Where pipes are reachable in a garage, crawl area or attic, adding or tightening straps reduces movement.
- Add arrestors.Small water hammer arrestors installed near the offending valve absorb the shock with a sealed air chamber.
Arrestors versus fixing pressure
Hammer arrestors
- Targets one fixture or appliance
- Often fitted at washer or dishwasher connections
- Quick fix when pressure is already normal
- Does not protect the rest of the house from high pressure
Correcting pressure
- Reduces hammer everywhere at once
- Also helps toilets, hoses and the water heater
- May involve regulator service and an expansion tank
- Bigger first step, but addresses the cause
Questions to ask
- What is my static pressure, and is it part of the problem?
- Which appliance or valve is triggering the bang?
- Will arrestors be installed at the fixture, and which type?
- Are any pipes loose enough to wear through over time?
- Is a permit needed for the work you are proposing?
Common questions
Can water hammer damage pipes?
Repeated shocks stress joints, fittings and appliance valves. It is worth fixing rather than living with.
Why did banging start suddenly?
A new appliance with a fast valve, a failing regulator or a change in supply pressure are common reasons. A gauge reading helps sort it out.
Is it ever a sign of something dangerous?
Banging itself is not dangerous, but if it comes with leaks or a dripping water heater relief valve, have the pressure checked soon.
General information for Coyote Hills, not a quote or professional advice. In an emergency, call 911.