Short definition
A pressure arrestor is a small device installed near a quick-closing valve — a washing-machine fill solenoid, dishwasher inlet, icemaker line, or single-handle faucet — that absorbs the pressure surge (“water hammer”) created when the valve slams shut. It uses a sealed gas cushion behind a piston or diaphragm to convert the surge wave into a damped oscillation.
What it is
When a fast-acting valve closes, the moving column of water has to stop suddenly. Its kinetic energy converts into a pressure spike that travels back up the supply line as a shock wave — the bang you hear in the wall. Pressure spikes from washing-machine fills can reach 4 to 5 times the static line pressure, hammering joints, fittings, and adjacent valves.
A pressure arrestor sits on the supply line near the offender and contains a small chamber pre-charged with air or inert gas, separated from the water by a piston or diaphragm. When the surge hits, the gas compresses to absorb the energy, then re-emits it as a small oscillation that dissipates harmlessly.
Two main types you’ll see:
- Piston-style (Sioux Chief Mini-Rester, Watts) — modern, sealed-gas, doesn’t lose its charge over time. The standard.
- Air chamber — older method, just a vertical capped pipe stub holding trapped air. Water-logs within months as air dissolves into the water, then stops working.
Why it matters to a homeowner
If you hear banging in the walls when the washing machine fills, the dishwasher cycles, or the icemaker valve opens, water hammer is the cause and a pressure arrestor is the fix. The bang is more than annoying — repeated hammer events fatigue solder joints, loosen fittings, and shorten the life of the fixture’s own internal valves.
Modern washing-machine supply boxes (the recessed wall outlet behind the washer) often have arrestors built in. If yours doesn’t, screw-on arrestors that thread into the existing fill-hose connections are a $20–$40 part and a 10-minute DIY install. Under-sink arrestors for a banging kitchen faucet are similar.
A pressure arrestor is not a substitute for a PRV. The PRV regulates static pressure (the resting pressure when nothing’s flowing); the arrestor handles the transient surge from a quick-closing valve. High static pressure makes hammer worse, so if your static reading is over 80 psi you need both.
Common variants and what arrestors are not
- Pressure arrestor vs. air chamber. Arrestor has a sealed gas cushion that doesn’t water-log; air chamber is just trapped air that absorbs over time.
- Pressure arrestor vs. expansion tank. Expansion tank lives at the water heater and absorbs thermal expansion in a closed system. Arrestor handles transient surges from quick-closing valves.
- Pressure arrestor vs. PRV. PRV regulates steady supply pressure. Arrestor handles dynamic surges.