How the two differ in kind
Two devices, both described as dealing with pressure, addressing opposite phenomena.
A PRESSURE-REDUCING VALVE addresses STATIC pressure that is too high all the time. Mains supply pressure varies enormously with location, elevation and the network, and it can sit well above what taps, mixers, flexible connections and appliances are rated for — which shortens their life, causes leaks at fittings, wastes water at every outlet, and makes showers uncomfortable. A PRV throttles the incoming supply to hold the downstream pressure at a set value continuously, and codes commonly require one above a stated supply pressure.
A WATER HAMMER ARRESTOR addresses a TRANSIENT. When a fast-closing valve stops a moving column of water, that column's momentum has nowhere to go, and the pressure spikes sharply for a fraction of a second — enough to bang pipes against their supports, damage fittings and, repeatedly, fatigue joints. An arrestor is a sealed air or gas chamber that absorbs the spike by compressing.
What makes them non-interchangeable is what drives the spike. Its magnitude depends on the water's VELOCITY in the pipe and on how quickly the valve closes — not on the static pressure. That is why a system at entirely normal pressure hammers when a washing machine's solenoid valve snaps shut, and why a system at high pressure with slow-closing taps does not.
The practical consequence is that fitting a PRV in response to hammering usually helps a little and does not fix it: reducing the pressure reduces the flow velocity somewhat, which reduces the spike somewhat, and it does nothing about the closing time. Conversely an arrestor does nothing for a system that is simply at too high a pressure all day.
And there is a third, cheaper answer that addresses both: pipe sizing. Velocity too high is a design fault, and it causes hammer, noise and erosion together.
The factors that actually differ
Show
| Pressure-reducing valve | Water hammer arrestor | |
|---|---|---|
| What it addresses | Static pressure that is too high continuously. | A transient spike lasting a fraction of a second. |
| What drives the problem | The supply network's pressure at that location and elevation. | Water VELOCITY and how fast a valve closes — not static pressure. |
| Symptom | Fittings leaking, appliances failing early, splashing taps, high water use. | A bang or a series of knocks when an appliance or a tap shuts. |
| Where it goes | On the incoming supply, near the point of entry, with an isolating valve and usually a strainer. | Close to the offending valve — which is where it works and where it is often not fitted. |
| Does it fix the other problem | Partly and incidentally — lower pressure means lower velocity, so the spike is smaller. It does not fix hammer. | Not at all. |
| Maintenance | A strainer to keep clear; the valve itself can drift or fail and wants checking. | A sealed chamber is maintenance-free; an old-style air chamber waterlogs and stops working. |
| Side effect to plan for | Closes the system, so thermal expansion has nowhere to go — an expansion tank becomes necessary. | None. |
| The cheaper alternative | None; high supply pressure has to be reduced. | Pipe sizing. Velocity too high is a design fault, and it causes hammer, noise and erosion together. |
| What else helps | Nothing else reduces static pressure. | Slower-closing valves, securing pipework properly, and removing loose runs that amplify the noise. |
| Both may be needed | Frequently — high pressure and a fast-closing appliance valve are independent problems. | Yes. |
Which one, and when
Choose pressure-reducing valve when…
- The measured static pressure exceeds what the fittings and appliances are rated for.
- Taps splash, fittings weep, and flexible connections or appliance valves fail repeatedly.
- A code requires one above a stated supply pressure, which many do.
- Water consumption is high because every outlet delivers more than it needs to.
Choose water hammer arrestor when…
- A bang when a specific appliance or tap shuts — a washing machine, a dishwasher, a solenoid-valve outlet.
- The static pressure is normal, which rules out the PRV as the answer.
- Where pipework cannot be resized or re-secured and the transient has to be absorbed.
- As close to the offending valve as possible, which is where it actually works.
Now run your own numbers
This page holds no prices on purpose — a national average is wrong for almost every real project. Quantify both options with your dimensions and your local quotes.
Frequently asked questions
- Why doesn't a PRV stop water hammer?
- Because the spike is not caused by static pressure. When a valve closes quickly, the column of water behind it is still moving and has momentum; stopping it converts that momentum into a pressure surge, and the surge's magnitude depends on the water's VELOCITY and on how abruptly the valve closes rather than on the pressure the system was sitting at. So a system at an entirely normal pressure hammers if a solenoid valve snaps shut on a fast-moving column, and a high-pressure system with slow-closing taps does not. A PRV helps a little and incidentally: lower pressure means a slightly lower flow velocity through the same pipe, so the spike is smaller. It does not address the closing time at all, which is usually the dominant term.
- Where should an arrestor be fitted?
- As close to the valve causing the problem as possible, which is the detail that decides whether it works. The arrestor absorbs the surge by compressing a sealed gas cushion, and it can only absorb what reaches it — so an arrestor fitted at the other end of a long branch is absorbing a surge that has already travelled the pipe, banged the supports on its way, and dissipated some of its energy into the structure. Fitted at the appliance connection, it takes the surge where it is generated. Practically that means at the washing machine or dishwasher connection, at a bank of solenoid valves, and at the end of long runs serving quick-closing outlets. Several small arrestors close to the sources work better than one large one at the main.
- What does fitting a PRV do to the rest of the system?
- It closes the system, which creates a problem that has to be solved at the same time. A pressure-reducing valve — like a check valve or a backflow preventer — prevents water flowing back toward the main, so when a water heater warms its contents and the water expands, that expansion has nowhere to go. In a sealed volume of nearly incompressible water, a small expansion produces a large pressure rise, and the relief valve on the heater lifts. The symptom is a relief valve weeping every time the heater runs, appearing shortly after the PRV was fitted, and the fix is a thermal expansion tank on the cold feed to the heater. It is a predictable consequence rather than a fault, and it belongs in the same job as the PRV rather than as a later call-out.
- What is the cheaper fix for hammer?
- Pipe sizing, in a new or reworked system, because excessive velocity is a design fault that causes several problems at once. The surge from a closing valve scales with the water's velocity, so a pipe sized for a reasonable velocity produces a much smaller spike from the same valve than an undersized one does. Reasonable velocity also removes the rushing noise that carries through the structure and reduces erosion-corrosion at bends and fittings. Two other measures cost very little: securing the pipework properly, since a loose run amplifies the noise by banging against joists and brackets, and choosing slower-closing valves where there is a choice. None of them removes a solenoid valve's abrupt closure, which is why arrestors still have their place.
- Do old-style air chambers work?
- They did when installed and most of them no longer do, which is why they are worth recognising on an older system. An air chamber is simply a capped vertical stub of pipe above a connection, holding a pocket of air that compresses to absorb a surge. The problem is that the air is in direct contact with the water, so it gradually dissolves into it and is carried away — the chamber fills with water, and a waterlogged chamber has nothing compressible in it and absorbs nothing. That is why hammering frequently returns in a house that had none for years. Draining the system down lets the chambers refill with air and restores them temporarily. Modern sealed arrestors solve it permanently by separating the gas from the water with a bellows or a piston.
- How do I know whether my pressure is too high?
- Measure it, with a pressure gauge on an outside tap or a washing machine connection, and take the reading at a quiet time — pressure in a distribution network is usually highest overnight when demand is low, and a daytime reading can miss the peak. Compare it against what the fittings and appliances in the house are rated for and against whatever your local code sets as the threshold above which a PRV is required. The supporting evidence is symptomatic: taps that splash, fittings that weep, flexible hoses failing, appliance inlet valves failing repeatedly, and a mains-fed shower that is uncomfortably fierce. High pressure also wastes water continuously, since every outlet delivers more than it needs to for the same use.
- Can hammer damage anything?
- Yes, and the damage is cumulative rather than immediate, which is why it is easy to live with until something fails. Each surge stresses the pipework, its joints and its supports, and repeated cycles fatigue soldered and compression joints, loosen fixings, and work pipes against the structure. Appliance inlet valves and flexible connections take the spike directly and are common failure points. On a larger system the surges can damage valves and gauges. The noise is also a nuisance in its own right, particularly at night in a dwelling. None of this is dramatic on any one occasion, and a system that has hammered for years has been accumulating stress for years — which is the argument for dealing with it rather than accepting it as a characteristic of the house.
- How is a PRV sized and set?
- Sized for the flow it has to pass at an acceptable pressure drop, and set to a downstream pressure that suits the fittings and the building's height. Undersizing is the common error: a valve too small for the demand starves the system at peak flow, so the house has adequate pressure with one tap running and very little with three. The setting has to account for the static lift to the highest outlet — pressure falls with height, so a setting comfortable at the valve can be inadequate on an upper floor. Installation details that matter: an isolating valve and a strainer upstream, since debris is what makes them fail; accessible location; and the expansion tank that the now-closed system requires. The set pressure should be recorded, since a drifting valve is otherwise hard to diagnose.