Water Hammer Control and Diagnosis Reference

Why this matters

Water hammer - that loud BANG when the washing machine cycles or the dishwasher shuts off - is the pressure spike from a fast-closing valve creating a shockwave through the pipes. Left alone, it loosens joints, damages valves, cracks fixtures, and drives customers crazy. The fix is usually an inexpensive arrestor installed at the offending appliance, but knowing WHICH appliance is the cause (and which arrestor type goes where) takes one diagnostic walk-through and one purposeful install.

What water hammer actually is

Water is nearly incompressible. When a valve in a pipe closes quickly (solenoid valves on washing machines, dishwashers, ice makers; single-handle ball faucets slammed shut; toilet fill valves with worn seals), the moving column of water decelerates instantly. The kinetic energy converts to a pressure spike that propagates back up the supply line as a shockwave.

A 60 psi residential supply with water moving at 6 ft/sec can spike to 200-300 psi for a few milliseconds when the valve slams shut. That spike rebounds back and forth, dissipating gradually as it leaks energy into pipe vibration and minor turbulence - but the first big peak is what destroys equipment and produces the audible bang.

The bang you hear isn't just sound - it's the pipe walls deflecting outward microscopically, and the pipe itself moving in its straps until it bumps into framing.

What causes water hammer

Fast-closing valves:

• Washing machine fill solenoids (0.05 sec close time)
• Dishwasher inlet solenoids (similar)
• Ice maker inlet solenoids
• Some single-handle ball faucets (when slammed)
• Pressure-reducing valve overshoot
• Toilet fill valves with degraded diaphragms (close fast as they finish)

Conditions that make it worse:

• Long, straight runs of pipe (less opportunity to dissipate energy)
• Rigid copper or galvanized (less flex than PEX)
• High supply pressure (>70 psi)
• Loose pipe strapping (pipe moves more freely on impact)
• No air chambers / arrestors anywhere in the system

What used to be installed: air chambers

Pre-1980s plumbing often included "air chambers" - short capped vertical sections of pipe above each fixture intended to trap air. The trapped air cushioned the pressure spike.

Problem: air dissolves into water over time. After 1-3 years, the chamber fills with water and provides zero cushioning. The customer reports "the banging just started" - the air chambers have failed.

Modern code (UPC, IPC) requires manufactured arrestors at fast-closing valves, not just air chambers. Most jurisdictions don't require retroactive upgrade, but it's a frequent service call.

Water hammer arrestor types

Diaphragm/piston arrestor (manufactured):

• Sealed pressure vessel with a captive air or gas pocket separated from the water by a diaphragm or piston
• Air pocket can't dissolve into the water - permanent cushion
• Brand examples: Sioux Chief MiniRester (HydraRester), Watts MM-1, Sioux Chief HydraRester
• Sizing per number of fixtures (or fixture units) - PDI WH-201 specs (A through F sizes)
• Lifetime: 20+ years
• Inexpensive per unit

Air chamber (legacy):

• Just a capped vertical pipe (typically 12-18" tall, 1/2" or 3/4")
• Loses cushion over time as air dissolves
• Acceptable only as a stopgap

Whole-house arrestor:

• Large piston-type unit installed at the main supply
• Protects against pressure spikes from any source
• Modest one-time cost
• Common where multiple causes exist OR where access to each appliance is poor

Sizing

Per PDI WH-201, arrestors are sized by fixture units of supply:

Residential: Size A at each appliance (washing machine, dishwasher, ice maker) is the standard.

Installation procedure

For a washing machine:

1. Shut off the hot and cold supplies at the laundry box (or further upstream if no in-box shutoffs).
2. Disconnect the washing machine hoses from the supply stops.
3. Install the arrestor between the supply stop and the hose. Arrestor has 3/4" GHT (garden hose thread) inlet that connects to the supply stop's outlet, AND a 3/4" GHT outlet that the washing machine hose connects to. Common: Sioux Chief MiniRester 660 series.
4. Hand-tighten plus 1/4 turn with channel locks - over-tightening cracks the brass.
5. Restore water. Confirm no leaks at either connection.
6. Run a wash cycle - listen for hammer at the fill/drain transitions. Should be quiet now.

For a dishwasher or ice maker (inside-wall connection):

1. Shut off the supply.
2. Cut the supply line near the appliance valve.
3. Install a tee + arrestor OR a manufactured fitting with an arrestor built into the elbow (Sioux Chief MiniRester EZ).
4. Reconnect, restore water, test.

For a fixture-level install (under-sink, hidden in wall):

• T into the supply, install a low-profile arrestor in the wall cavity. Or use a screw-on arrestor at the fixture stop if accessible.

Whole-house arrestor:

• T into the main supply just downstream of the pressure-reducing valve (if one exists)
• Or T in at the inlet to the water heater (catches spikes from the hot side back through the cross-connections)
• Mount vertically (some types are orientation-sensitive)

Diagnosis - finding the offending appliance

1. Listen to the bang. Loud and from one direction? Localized. Faint from everywhere? Whole-system.
2. Run each appliance one at a time and listen:

• Washing machine fill cycle - listen at end of cold fill, then end of hot fill
• Dishwasher inlet - listen at start of cycle (10-15 seconds after start) when inlet valve closes
• Ice maker - listen 4-8 minutes after a tray release (when fill solenoid closes)
• Single-handle faucet - quick close, listen

1. Note WHICH cycle produces the bang. That identifies the offender.
2. Inspect the appliance's supply lines for loose strapping, copper banging on framing.

When the bang is intermittent or from "everywhere"

• Pressure too high (over 80 psi) → install or service the pressure-reducing valve (PRV). Many homes' PRVs are 15+ years old and have failed open.
• Failed PRV - set pressure to 50-60 psi, customer reports much quieter system.
• Air chamber depleted in an older home - install modern arrestors at each fast-closing valve.
• Pipe strapping loose - secure pipes with cushioned straps (Sioux Chief CTS, MIFAB) at proper spacing.

Common diagnostic mistakes

References

• PDI WH-201 (Water Hammer Arrestor Standard)
• ASSE 1010 (Hydraulic Performance Requirements for Water Hammer Arrestors)
• IPC 604.9 / UPC 610 (water hammer protection requirements)
• IPC 604.8 / UPC 608.2 (maximum residential pressure, 80 psi)
• Manufacturer installation guides (Sioux Chief, Watts, Mifab)