OCT 12, 2018

When water moves through the pipes of your home, it should produce a steady, even sound, no matter where you’re located in the house. Sometimes, it doesn’t.

Water hammer is an apt name for a plumbing phenomenon that usually scares the daylights out of people. If you’ve ever been startled by sudden loud banging or knocking in the walls of your home when water is turned on/off or a washer or dishwasher changes cycles or when the irrigation system kicks on/off, it’s likely due to water hammer.


So how and why does water hammer happen? It’s all a matter of water pressure, flow rates and shock absorption. While water may appear to flow smoothly, inside the pipes it’s churning and tumbling as it moves towards its destination. When a valve snaps opens or closes quickly, the pressure in the pipeline is suddenly changed. Without something to absorb the shock, like an air chamber, the impact can cause a big thump or banging noise that can shake the house, or a big bang followed by a series of echoes. Think of it like a speeding car hitting a wall. Boom!

Many older homes were built with air chambers to help prevent water hammer. While it sounds elaborate, an air chamber is just another pipe connected to the water pipes using a “T” fitting, then capped off at the top and filled with air. It’s like a built-in braking system for water – but since most eventually fail and adding new pipes can be quite expensive, this type of water hammer fix is rarely recommended.


Sometimes, water hammer or knocking in the walls is easily solved simply because it’s due to air trapped in the pipes. In fact, the first step you should take when water hammer happens is to shut your water off and drain the pipes. Here’s how:

  1. Turn off the main water supply to your home.

  2. Open all the faucets inside and outside. Flush all the toilets. Run the washing machine and dishwasher until all water is gone.

  3. Close all faucets and valves. Turn the water back on at the main.

  4. Test the results. If the water hammer noise is gone, that’s awesome!


Water still hammering away? Really strong water pressure could be the culprit. Here’s why: the faster water travels through the pipe, the greater the water hammer when valves are opening and closing. Water hammer will be the least of your worries.

High water pressure also means that water-supplied appliances like dishwashers, washing machines, toilets, and ice-makers are at risk for damage. Water pressure that exceeds 100 psi could potentially void warranties and damage appliances. Install a pressure reducing valve to protect against both high pressure and water hammer.

This cause of water hammer is easiest to correct in a sprinkler system. To fix the water hammer, water pressure must be reduced, because an irrigation valve closes much faster under high water pressure, and the hammering can be quite loud!

Normal water pressure should run between 30 and 55 psi.  A screwdriver can be used to calibrate a water pressure valve to flow below 50 psi. Consider installing a pressure reducer valve at the water main.


If you’re still experiencing water hammer and you can trace it to one specific location, it’s possible that a pipe-mounting strap has become loose or broken. During construction, pipe mounting straps are used to secure pipes to the framing studs to reduce movement and vibration when water is flowing. Clips and plumber’s tape can also minimize the chance of water hammering in the pipeline. Sadly, this issue will require cutting through drywall to repair.

One note: Never use galvanized or steel straps on a copper pipe. The materials will react to each other, potentially creating electrolysis and a damaging plumbing leak.


If your home was built between the 1970s and 1990s, it’s quite likely that there is little built-in protection from water hammer, so you’ll most likely need to call plumbing professionals like the Comfort Heroes to fix things and update your pipes.

Most modern homes (post 1990s) are built with water hammer arrestors connected to the water pipes. Water hammer arrestors contain a spring-loaded shock absorber that cushions the force of the water when a faucet is shut off, greatly reducing the water hammer effect.

If your home is newer, and you’re experiencing water hammer, one or more water hammer arrestors may have malfunctioned (this is rare), or no water hammer arrestors were ever installed.