Key Takeaways
- Water hammer is a pressure shockwave — fast-closing valves slam moving water to a stop, and the shockwave bangs through your pipes.
- Decode the sound: single THUD = classic hammer; rapid BANG-BANG = loose pipe; CLUNK when starting/stopping = thermal expansion or a failing arrestor.
- Check pressure first: anything above 80 PSI invites hammer. Ideal household range is 40–60 PSI.
- The fix ladder: lower pressure → drain air chambers → install arrestors (one per problem fixture) → replace the PRV if pressure won’t hold.
- Arrestors go as close to the problem valve as possible — not one unit at the main for the whole house.
What Is Water Hammer?
Water hammer is a pressure shockwave created when flowing water is stopped suddenly — typically by a fast-closing valve in a washing machine, dishwasher, or single-handle faucet. The moving water’s momentum converts instantly to pressure, and that shockwave slams through the pipe, rattling and banging everything it passes.
It’s not just noise: repeated hammer fatigues joints, loosens hangers, and can eventually crack fittings or burst supply lines. A “little bang” ignored for years becomes a leak inside a wall.
What Does Your Banging Sound Mean? Decode It
Different sounds point to different problems — listen before you fix:
| Sound | Likely cause | First move |
|---|---|---|
| Single loud THUD when a valve closes | Classic water hammer (pressure shockwave) | Check pressure; install arrestor at the fixture |
| Rapid BANG-BANG-BANG | Loose pipe rattling against framing | Secure the pipe with straps/clamps |
| CLUNK when water starts or stops | Thermal expansion or a waterlogged/failed arrestor | Check expansion tank; replace arrestor |
| Hiss + vibration | Excessive pressure (80+ PSI) | Test pressure; adjust or replace PRV |
| Ticking as pipes warm up | Normal expansion against tight holes | Usually harmless — note it, don’t chase it |
What Causes Water Hammer?
- Fast-closing valves. Modern solenoid valves (washing machines, dishwashers, ice makers) snap shut in milliseconds — the #1 trigger.
- High water pressure. Above 80 PSI, even normal valve closures hit hard. Many Denver homes sit high because of elevation-fed municipal zones.
- Waterlogged air chambers. Older homes have vertical pipe stubs (“air chambers”) meant to cushion shock — over years they fill with water and stop working.
- Trapped air pockets. Air compressed elsewhere in the system can amplify the shockwave (and cause sputtering faucets).
- Long straight pipe runs with no cushioning — momentum builds over distance.
Sudden onset? If hammer appeared overnight, suspect a failed PRV (pressure spiked), a newly installed fast-closing appliance, or a waterlogged arrestor — not “the house settling.”
How Do I Fix Water Hammer? The Fix Ladder
Work cheapest-first. Most hammer dies at step 1 or 2.
Step 1: Test and Lower Your Pressure
Screw a $10–$15 pressure gauge onto a hose bib with no water running. Above 80 PSI is a problem; 40–60 is ideal. If high, adjust your pressure reducing valve — or replace it if adjustment doesn’t hold. Learn how to test water pressure properly first.
Step 2: Drain Waterlogged Air Chambers
Shut off the main, open every faucet (highest first), drain the system, then close everything and reopen the main slowly. This recharges old-style air chambers with air. If hammer returns within weeks, the chambers are undersized for modern appliances — move to step 3.
Step 3: Install Water Hammer Arrestors
The placement golden rule: one arrestor per problem fixture, installed as close to the fast-closing valve as possible — at the washing machine valves, the dishwasher supply, the problem faucet. A single arrestor at the main does almost nothing for a hammer born at the laundry room.
Arrestors are sealed piston or diaphragm units ($15–$40 each) that absorb the shockwave. Washing-machine arrestors screw directly onto the supply valves — a 10-minute DIY job.
Step 4: Replace a Failing PRV
If pressure tests high and won’t stay adjusted, the PRV has failed internally. Replacement is a plumber’s job and solves hammer plus a dozen other pressure symptoms at once.
Water Hammer Arrestor vs. Expansion Tank — What’s the Difference?
| Water hammer arrestor | Expansion tank | |
|---|---|---|
| Solves | Sudden shockwaves from fast-closing valves | Slow pressure rise from heated water expanding |
| Goes | At the problem fixture | At the water heater |
| When you need it | Banging when valves close | CLUNK sounds, dripping T&P relief valve |
They’re not interchangeable. An expansion tank won’t stop valve-slam hammer; an arrestor won’t fix thermal expansion. If your T&P relief valve drips, you need the expansion tank checked — not an arrestor.
Can I Prevent Water Hammer?
- Keep pressure at 40–60 PSI — the single best prevention.
- Choose slow-close valves when replacing faucets and supply stops.
- Secure pipes properly during any remodel — loose pipe turns mild hammer into a drum solo.
- Service the PRV — test pressure yearly; PRVs last 7–12 years.
Frequently Asked Questions
Is water hammer dangerous?
Yes, over time. Each shockwave stresses joints and hangers. Ignored hammer causes loosened fittings, cracked supply lines, and eventually leaks inside walls — far more expensive than an arrestor.
Will a water softener fix water hammer?
No. Softeners address mineral content, not pressure shockwaves. Don’t let anyone sell you one for banging pipes.
Why did water hammer start suddenly?
Check three things: a failed PRV (test pressure — sudden spikes mean PRV failure), a newly installed appliance with fast-closing valves, or a waterlogged arrestor that finally gave out.
Can high water pressure cause other problems?
Plenty: dripping faucets, running toilets, premature appliance failure, pinhole leaks, and water heater T&P discharge. Pressure regulation is whole-plumbing preventive medicine.
Hammering pipes? Professional Plumbers Denver diagnoses the cause — pressure, arrestors, or PRV — and fixes it in one visit across the metro. Call (720) 390-2652, 24/7.
