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Water Hammer in Thornton, CO Homes: Causes and Fixes
A sharp knock when you turn off a faucet or when a washer valve closes is water hammer. Moving water has weight and speed, and when it stops suddenly, the energy goes into the pipe.
By the Thornton Plumbing Co. team ·
What water hammer is
When a valve closes quickly, the water in the pipe slams to a stop and sends a pressure wave back along the line. The pipe moves and knocks against framing, and the noise echoes through the house.
Over time, repeated shocks can loosen fittings, wear washers and even cause leaks. It is not just noise.
High pressure
Pressure that is too high makes the shock bigger. Many homes should run between 50 and 70 psi. A gauge on a hose bib shows yours. A pressure reducing valve near the main can lower it.
If the regulator has failed, pressure may be above 80 psi. A plumber can test and replace it. See water line repair for the supply side.
Loose pipes
Pipes need straps or hangers to hold them still. If a strap is missing or has loosened, the pipe will rattle. Open an access panel or a basement ceiling and look for pipes that move when the water shuts off.
Adding padded clamps is an easy fix. In walls, a plumber may need to open the drywall.
Causes and fixes
Most water hammer has one of these causes.
| Cause | Sign | Fix |
|---|---|---|
| High pressure | Gauge reads above 80 psi | Install or replace a regulator |
| Loose pipes | Rattle after shutoff | Add padded straps |
| Waterlogged air chamber | Banging got worse over time | Drain the system to refill with air |
| Fast-closing valves | Bang at the washer | Add an arrestor |
Air chambers and arrestors
Older houses had short vertical pipes called air chambers that cushion the shock. Over time they fill with water and stop working. Draining the system lets air return: shut off the main, open the highest and lowest taps and let all the water drain, then close the taps and turn the main back on.
Water hammer arrestors are small devices with a piston or bladder that absorb the shock. They install near washers, dishwashers and fast-closing valves.
Fast-closing valves
Washers, dishwashers and ice makers use solenoid valves that shut quickly. That is a common source of knocking in the laundry room. A hammer arrestor on the supply to the appliance helps.
Single-lever faucets also close fast. If the noise happens at one fixture, a local fix is likely.
When to call a plumber
If the pressure is high, if you have repeated banging or if you see leaks at fittings, call. A plumber can test the pressure, install a regulator or arrestors and secure loose pipes.
Ask them to check the expansion tank at the water heater too. A waterlogged tank can add to pressure swings. See water heater repair.
What is happening inside the pipe
Moving water carries weight and momentum. When a valve snaps shut, for instance a washing machine solenoid or a dishwasher valve, the moving water slams to a stop and the shock travels back through the pipe as a bang. In a well built system, a cushion of air or a mechanical arrestor absorbs the hit.
Over time the air chambers that older homes rely on fill with water and stop working. The result is the knocking that begins after you turn a tap off.
Quick fixes to try first
Shut off the main, open the highest and the lowest faucet in the house, and let the system drain. Then shut the taps and reopen the main slowly. This refills the air chambers. It sometimes quiets pipes for months.
Check that the straps and hangers hold the pipes firmly. A pipe that is free to move bangs against framing when the shock arrives. A little foam between the pipe and the joist can quiet it.
Arrestors and pressure
Water hammer arrestors are small sealed devices that fit on the supply to a washer, a dishwasher or a fixture group. They have a piston or bladder that moves to absorb shock. A plumber can install them in a short visit, and they are a better long-term answer than an air chamber that will fill again.
High pressure makes everything worse. If hammer is paired with a hissing sound, high gauge readings or a regulator that is older than the house's last remodel, have it checked. See water heater repair as well, since expansion in a closed system can add its own noises.
When knocking is a different problem
A tick or a pop when hot water runs is usually thermal expansion in the pipe, not hammer. A rumble from the water heater points to sediment, which is common with hard water. A rattle that begins when the toilet fills is a fill valve. Sort the sound by what triggers it, and the cause usually follows.
Washing machines and dishwashers
Most of the banging in a house comes from fast solenoid valves. Look at the supply hoses behind the washer, and consider screw-on arrestors that thread between the hose and the valve. They cost little and install in minutes. If the bang happens when the dishwasher cycles, the shutoff valve under the sink may need an arrestor as well.
Check that each appliance's shutoff valve is open all the way. A partly closed valve speeds the water through a narrow gap and makes noise on its own.
Pressure tank and well systems
Most Thornton homes are on city water, so the issues above are the usual ones. If a property has a private irrigation well or a booster pump, a pressure tank with a failed air charge can also cause pulsing and banging, and it needs a different check.
Quick answers
Asked about water hammer
It can damage fittings and cause leaks over time. Fix it soon.
The washer solenoid snaps shut fast. Fitting an arrestor on that supply line helps.
Often. Check pressure, secure pipes and drain air chambers.
Often, but not always. Test it with a gauge.
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