Quick Answer:
If your shower makes loud knocking when you first turn it on, the most common causes are trapped air in the pipes after a shutoff or a sudden pressure change (water hammer). Try purging the air, open the valve slowly, and test hot versus cold to narrow the source. If the banging is violent, recurring, or accompanied by leaks, stop and call a plumber.
Why This Happens
There are a few typical reasons a shower bangs when started:
- Air trapped in the lines after a recent shutoff or repair can move and hit pipe fittings when flow starts.
- Water hammer — a fast change in flow or a closing valve — sends a pressure shock through the system that sounds like a knock.
- Loose pipe straps, worn valve cartridges, or a failing pressure regulator can let pipes move and strike framing when pressure changes.
- If the problem began right after the water was turned back on at the main, trapped air or pressure surges are especially likely; see the note on recovering from a shutdown and related issues with **pipes banging after turning the water back on**.
Step-by-Step What to Do
1. Note when it started and check other fixtures
Ask: did this begin after a planned shutoff or any plumbing work? Run another faucet (sink, tub, outside spigot) and see if they make the same noise. If multiple fixtures knock, the issue is likely in the main supply or a common branch.
2. Test hot versus cold
Try the shower with only the cold side open, then only the hot. If knocking happens on one side only, the problem is probably on that branch (a valve, cartridge, or pipe near the water heater for the hot side).
3. Purge air from the lines
Trapped air is common after a shutoff. To purge air safely:
- Open the highest faucet in the house (usually a second-floor sink) and the lowest faucet (basement or outside spigot).
- Run both hot and cold until the flow is steady and free of sputtering. This helps push air out of the system.
- For targeted work on the shower, briefly run the shower after purging to check if the knocking stops. For more detail on full-house procedures, read about how to **clear air from pipes**.
4. Open the shower valve slowly
Start with the handle at a low flow position and increase it gradually. A sudden wide-open turn can trigger a pressure shock. If slow opening prevents the knock, the valve or cartridge may be worn and letting pressure spike.
5. Watch for water hammer symptoms
Water hammer typically shows as a single hard knock when a valve closes, or a repeated banging if a check valve or air chamber is failing. If knocking occurs when you turn other appliances on or off (dishwasher, washing machine), suspect a pressure shock issue.
6. Observe and document
Note timing, which side (hot/cold), which fixtures, and whether it started after a shutoff. This information makes a repair much easier and safer for a plumber if you call one.
What Not to Do
- Do not keep snapping the valve open and closed to “make it stop.” That can worsen wear and hide the real cause.
- Do not ignore violent or recurring hammering — it can stress fittings and lead to leaks or bursts.
- Do not clamp pipes blindly or hammer on pipes to stop the noise without diagnosing the problem first.
When to Call a Professional
- If banging is loud and persistent despite purging and slow opening.
- If you see leaks, wet spots, or loose pipe movement when the noise happens.
- If the problem started immediately after a shutoff and purging didn’t clear the air, or if the issue involves the water heater or main supply.
- When you’re uncomfortable working near shutoffs, the pressure regulator, or the water heater — a licensed plumber can test pressure, secure loose pipes, and install protective arrestors safely.
Safety Notes
- When testing hot water, be cautious of scalding. Run hot slowly and check temperature with your hand at a safe distance before prolonged exposure.
- If you shut off the main to work on the system, relieve pressure by opening a lower faucet first and close the main carefully.
- Do not attempt to remove or disassemble valves or cartridges unless you have the right tools and know-how; parts under pressure can spray hot water.
Common Homeowner Questions
- Q: Is a knocking pipe about to burst?
A: Not immediately, but ongoing hammering increases leak risk and should be addressed. - Q: Will running the shower more make it go away?
A: Not reliably — it may mask the issue and allow damage to continue. - Q: Can I fix this by myself?
A: You can try purging air and opening the valve slowly; call a plumber if the noise persists or if you find leaks.