This one needs a licensed pro
Call a plumber: knocking is likely water hammer
Knocking or banging right when a tap or appliance shuts off is usually water hammer in the plumbing, not a failing heater. It is not urgent, but a plumber can fix it.
Why this happens
A. O. Smith says knocking from a heater is likely water slamming into the heater's internal shutoff valves too quickly, and that it is probably not a disastrous problem. Rheem says banging or knocking usually points to the plumbing system rather than the heater, caused by sudden valve closure, high water pressure or loose pipes. Bob Vila says it is not an immediate problem but can damage pipes over time. A. O. Smith and Rheem both name a water hammer arrestor as a fix.
What to do right now
- If you smell gas, or hear hissing near a gas line or the burner, leave the house and call 911 and your gas utility from outside.
- Note which tap or appliance sets it off, and whether it is hot water, cold water or both.
- The pipes banging page walks you through what you can check on pipes you can see. Do not adjust a pressure regulator or open a wall yourself.
- Ask a licensed plumber about a water hammer arrester, and about checking the water pressure and any loose pipes.
- If you see water or steam from the relief valve pipe, keep away from the end of the pipe and call a licensed plumber right away. For a leak or a damp wall, stop using the heater and call a licensed plumber.
When to stop. Water or steam from the relief valve pipe means keep away from the end of the pipe, never block it, and call a licensed plumber right away. A drip or a damp wall or ceiling means stop using the heater and call a licensed plumber. Smoke, flames, sparks or a burning smell near the heater mean leave the house and call 911 from outside. A gas smell, or hissing near a gas line or the burner, means leave and call 911 and your gas utility from outside.
What to tell the plumber
- When it knocks, which fixtures or appliances set it off, and hot, cold or both.
- Whether it started suddenly or after plumbing or appliance work.
- Whether you know of an arrester, a pressure regulator or an expansion tank.
- The heater's make, model and age from the data plate.
Build the full list: what to tell the plumber.
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