What causes water hammer in pipes?

Water hammer is caused by the sudden stoppage of water flow in a pressurized pipe, creating a shock wave. Contributing factors include high water pressure (above 550 kPa / 80 PSI), fast-acting solenoid valves in dishwashers and washing machines, worn or missing air chambers in older Ontario homes, and loose or poorly secured pipes that amplify the shock impact.

How do you fix water hammer?

Water hammer can be fixed by installing water hammer arrestors at the supply connection of appliances with solenoid valves, recharging or replacing air chambers in older plumbing systems, reducing home water pressure to 345–550 kPa (50–80 PSI) with a pressure reducing valve, and securing loose pipe runs with additional pipe straps or cushioned hangers.

Is water hammer harmful to my plumbing?

Yes. Repeated water hammer events create mechanical stress on pipes, fittings, joints, and appliance connections. Over time, this can loosen pipe joints, damage valves and fittings, cause pinhole leaks in copper supply lines, and stress appliance connections. In high-pressure systems, water hammer can rupture older galvanized or cast iron pipes. Addressing it promptly protects your plumbing system.

What is a water hammer arrestor?

A water hammer arrestor is a small device installed in the supply line at or near a fast-closing valve such as a washing machine hose connection or dishwasher supply. It contains a sealed air chamber with a moving piston that absorbs the hydraulic shock wave when the valve closes, preventing the pressure spike from traveling through the pipe system and causing noise or damage.

How much does it cost to fix water hammer in Ontario?

Water hammer repair costs in Ontario typically range from $150 to $600, depending on the solution needed. Installing a water hammer arrestor at a washing machine or dishwasher connection typically costs $150–$300. Installing a pressure reducing valve (PRV) to lower system pressure costs $350–$700. Repairing pipe damage caused by prolonged water hammer varies based on extent.

What Is Water Hammer? Causes and Fixes

Q: What is water hammer?

Water hammer is a hydraulic shock that occurs when moving water is abruptly stopped or redirected, creating a pressure wave that travels back through the pipes and produces a characteristic loud banging or knocking sound. It commonly occurs when a dishwasher, washing machine, or fast-closing solenoid valve shuts off, or when a tap is closed quickly.

The Physics of Water Hammer

Water hammer — also called hydraulic shock — is a pressure surge or wave resulting from an abrupt change in water velocity within a closed pipe. Unlike gases, water is essentially incompressible. When water moving at speed inside a pipe is stopped suddenly — by a fast-closing valve, an automatic shutoff, or a solenoid — its kinetic energy has nowhere to go. Instead of compressing, the water creates a pressure wave that propagates back through the pipe system at high speed. This wave strikes the next valve, fitting, or change in pipe direction and bounces back, producing the characteristic banging sound and vibration.

The magnitude of the pressure spike depends on the water velocity (related to flow rate and pipe diameter) and how quickly flow is stopped. Slow-closing valves, like manually operated ball valves and gate valves, rarely cause water hammer because the gradual closure allows the pressure to dissipate progressively. Fast-acting solenoid valves in appliances like dishwashers, washing machines, and toilet fill valves can shut off in milliseconds — fast enough to generate significant hydraulic shock.

What Causes Water Hammer in Ontario Homes?

Several factors make water hammer common in older Ontario residential housing stock:

Fast-closing appliance valves: Modern dishwashers and washing machines use solenoid valves that close almost instantaneously when a cycle ends. Older appliances with worn solenoids can also develop faster-than-intended closure behaviour.
High water pressure: Ontario homes serviced by municipal water systems in dense urban areas sometimes experience supply pressure above 550 kPa (80 PSI). Higher pressure means higher water velocity for a given flow rate, and therefore more kinetic energy to be dissipated as a shock wave when flow stops. The Ontario Building Code recommends household supply pressure not exceed 550 kPa.
Waterlogged or absent air chambers: Traditional plumbing systems installed before water hammer arrestors became standard used air chambers — short capped pipe stubs installed in the supply line near each fixture. These chambers trap a pocket of air that acts as a cushion, absorbing the shock wave. Over time, the air in these chambers dissolves into the water and the chamber fills with water, eliminating its cushioning effect. Many older Ontario homes have waterlogged air chambers that no longer function.
Loose pipe runs: Supply pipes that are not properly secured — running loosely through wall cavities or joist spaces without adequate pipe straps — can move when shocked, amplifying the banging sound and, over time, causing wear at connection points.
Long, straight pipe runs: Longer pipe runs give pressure waves more distance to travel and amplify before they dissipate, making water hammer in larger homes or commercial buildings more pronounced.

Water hammer is more than a nuisance noise. Repeated hydraulic shocks gradually stress pipe joints, fittings, and appliance connections. In severe cases — particularly with older galvanized or corroded pipes operating at high pressure — water hammer can cause joint failures, pinhole leaks in copper supply lines, or damaged valves and solenoids in appliances.

How to Diagnose Water Hammer

Identifying water hammer is straightforward: if you hear a single loud bang or a series of knocks in your walls or floors when a washing machine cycle ends, a dishwasher valve closes, a toilet finishes filling, or a tap is shut off quickly — that is water hammer. The sound comes from inside the wall or floor where the supply pipes run.

Distinguishing water hammer from other plumbing noises:

Water hammer: Occurs immediately when flow stops; single sharp bang or brief series of knocks
Pipe expansion/contraction: Occurs when hot water flows through cold pipes; creaking or ticking sound during flow, not at shutoff
Loose pipes vibrating: Rattling or buzzing during flow, not specifically at shutoff
Singing pipes or whistling: Continuous high-pitched noise during flow, caused by worn washers or partially open valves

A licensed plumber can confirm the diagnosis and assess supply pressure using a gauge at the hose bib, which guides the appropriate repair strategy.

Solutions for Water Hammer

Water Hammer Arrestors

A water hammer arrestor is the modern replacement for traditional air chambers. It contains a sealed nitrogen-charged chamber separated from the water by a moving piston. When the pressure wave from a closing valve reaches the arrestor, the piston compresses the gas charge, absorbing the energy of the shock wave. Unlike air chambers, the gas in an arrestor does not dissolve into the water supply — the piston seal keeps them permanently separated.

Arrestors are available in standard sizes (A through F, based on fixture unit rating) and are installed at or near the supply connections of appliances with solenoid valves — particularly washing machines and dishwashers. They can be added to existing supply lines with minimal disruption. For a washing machine, both hot and cold supply lines should receive an arrestor.

Recharging or Replacing Air Chambers

If your home has traditional air chambers that have become waterlogged, they can often be recharged by shutting off the water supply to the home, opening all faucets to drain the system, allowing air to re-enter the air chamber standpipes, then closing the faucets and restoring water supply. This process recharges the air chambers and often temporarily resolves water hammer. However, the chambers will waterlog again within months to years. Installing water hammer arrestors is a more permanent solution.

Pressure Reduction

If supply pressure is above 550 kPa (80 PSI), installing or adjusting a pressure reducing valve (PRV) at the main water entry point can significantly reduce water hammer severity. Lower pressure means lower water velocity and less kinetic energy to produce a shock wave. Reducing pressure to 345–415 kPa (50–60 PSI) is recommended for most Ontario residential applications. A PRV also protects appliances and fixtures from premature wear caused by excessive operating pressure. Most Ontario homes built after 1990 already have a PRV installed; older homes may not.

Securing Loose Pipes

Pipes that are not properly secured to framing members can amplify water hammer sounds significantly. A plumber can access the pipe runs through wall cavities, crawl spaces, or basements and add cushioned pipe straps or foam-lined clips at appropriate intervals. This does not eliminate the hydraulic shock but reduces the noise generated when the pipe moves in response to it.

Water Hammer in Older Ontario Homes

Homes built before approximately 1970 in Ontario are particularly susceptible to water hammer for several reasons. Many have galvanized steel supply pipes (often corroded, reducing effective diameter and increasing velocity), air chambers that have long since waterlogged, and older fixture valves without modern slow-close mechanisms. These homes may also have higher supply pressures if PRVs were not installed when the water main was updated.

For these properties, a plumber's assessment should include pressure measurement at the hose bib, evaluation of the air chamber condition (if present), and a check for visible pipe securement throughout accessible areas. A comprehensive solution — arrestors at appliances plus PRV installation — typically resolves chronic water hammer in older homes.

Cost of Water Hammer Repair in Ontario

Repair costs vary based on the solution required:

Water hammer arrestors (per appliance): $150–$300 installed, including labour and parts. Most appliances need arrestors on both hot and cold lines.
Pressure reducing valve installation or adjustment: $350–$700 installed for a new PRV; $75–$150 to adjust an existing PRV
Air chamber recharging: Typically included in a service call — minimal additional cost if a plumber is already on site
Pipe securement: $150–$400 depending on access and number of straps needed

A diagnostic visit to measure pressure and assess the system — including the recommended repair plan — typically costs $100–$200 and is applied toward the repair cost at most plumbing contractors.

DIY Water Hammer Fixes

Recharging existing air chambers (drain and refill the system) is a simple DIY task requiring no tools. Some water hammer arrestors are also available in configurations that attach to the washing machine hose connections without cutting into the supply line — these can be installed by a knowledgeable homeowner.

However, pressure reducing valve installation or adjustment, supply line modifications, and any work requiring pipe cutting should be handled by a licensed plumber. Incorrect PRV adjustment can result in either insufficient pressure for fixtures to function properly or continued high pressure. Ontario plumbing regulations also require a licensed plumber for any alterations to the water service or distribution system inside a home.