Water hammer
When a tap or valve closes suddenly, the moving water has nowhere to go and slams into the pipework, causing a bang. It’s more likely with high water pressure or where a tap or appliance valve shuts quickly.
Loose or unsupported pipes
Pipes that aren’t clipped properly can knock against joists or each other, especially when hot water expands them. This often shows up as ticking or knocking after the heating or hot water comes on.
A noisy fill valve
If the banging starts when a toilet cistern refills, the fill valve (float valve) is often worn. Our guide to why a toilet keeps running covers related faults.
High pressure or a part-closed stopcock
Very high water pressure makes hammer more likely, and a stopcock that isn’t fully open can cause noise too. If you’re not sure where it is, see how to find and turn off your stopcock. Low pressure has its own causes — see why water pressure is low throughout the house.
What you can check
Note when the noise happens, as that points to the cause. Make sure the stopcock is fully open, and look for pipes that visibly move or touch something. Don’t force anything, and don’t overtighten fittings.
When to call a plumber
Call a plumber if the banging is loud or constant, if you see damp or drips near the pipes (check for signs of a hidden water leak), or if you can’t find the cause. Fixes can include re-clipping pipes, replacing a fill valve, or fitting a water hammer arrestor or pressure reducing valve.
Banging pipes you can’t trace? Our Gas Safe plumbers cover South Manchester and Cheshire.
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