Quick Answer:
If pipes bang only while the water heater is recovering (not when you shut off faucets), the disturbance is almost always from thermal expansion shock waves. Confirm the timing, correlate the noise with pressure peaks using a max-needle gauge, and correct the expansion tank sizing/charge first. If noise continues after the pressure is stable, add hammer arrestors at the worst branches.
Why This Happens
When a closed domestic hot-water system heats water, the water expands. That extra volume raises system pressure briefly during a heater recovery cycle (the period after the heater refills and reheats). If the system has no room to absorb that expansion, the pressure spike sends shock waves through the pipes that sound like banging. This is different from the usual “water hammer” you hear when a faucet or valve is shut off quickly; those sounds coincide with fixture use, while expansion shock waves occur during heater recovery.
Step-by-Step What to Do
Step 1 — Confirm the sound happens during heater recovery
- Listen when the heater is running its recovery cycle (often after hot water use stops and the tank refills). Note whether the banging starts while the heater is heating, not when you open or close fixtures.
- Test by using a hot-water demand: run a shower or faucet until the heater fires and then stop the fixture. If banging begins after the fixture is off, during the recovery time, that points to thermal expansion.
- Compare with fixture shutoffs: purposely close faucets and watch for immediate banging. If you don’t hear banging with fixture shutoffs, the problem is likely expansion-related.
Step 2 — Correlate the noise with pressure peaks using a max-needle gauge
- Attach a max-needle pressure gauge to an outdoor hose bib or a house bib near the water entrance. A max-needle gauge records the highest pressure reached while you’re testing.
- Start the heater recovery cycle (after a typical draw) and watch the gauge. If the gauge’s maximum needle rises noticeably above the static line at the same time you hear banging, you’ve confirmed pressure spikes from expansion.
- Record the static and peak readings so you have numbers to work with before changing the system.
Step 3 — Fix expansion tank sizing and charge first
- Turn off the water supply to the house and relieve pressure per the tank manufacturer’s instructions before touching the expansion tank.
- Check the tank pre-charge pressure using a tire gauge on the Schrader (air) valve. The tank should be pre-charged to match your home’s static water pressure.
- If the tank is waterlogged or the pre-charge is wrong, either recharge it to the correct pressure or replace the tank with the correct size for your water heater and household water volume.
- If you find signs of a faulty Schrader valve or weeping air valve, see Expansion tank leaking from air valve for troubleshooting and repair options.
- If the tank is mounted or piped incorrectly, or you suspect an installation error, see Expansion tank installed wrong for what to check and change.
Step 4 — If noise remains, add hammer arrestors at the worst branches
- Only add hammer arrestors after you have stabilized the system pressure by properly sizing/servicing the expansion tank and confirming lower peak readings on the max-needle gauge.
- Install arrestors close to the fixtures or branch connections that still transmit the noise. Arrestors give a local cushion that stops vibration or resonance in short pipe runs.
- Do not rely on arrestors to fix a systemic pressure spike — they are a local mitigation for persistent noise after the main pressure issue is addressed.
Step 5 — Re-check and document
- Repeat the max-needle gauge test after any change. Confirm that peak pressures are lower and the banging is gone or reduced.
- Keep a short note of static pressure, peak pressure, tank model/charge, and any arrestors added. That helps future troubleshooting.
What Not to Do
- Don’t install hammer arrestors as the primary fix — control the pressure peak first by servicing or resizing the expansion tank and correcting pre-charge.
- Don’t ignore expansion tank symptoms (continuous waterlogged tank, visible leaks, or a constantly rising peak on the gauge). Address the tank before adding local devices.
- Don’t make major plumbing changes without relieving system pressure and following safety practices.
When to Call a Professional
- If you don’t have a max-needle gauge or you’re uncomfortable working on the water heater or tank.
- If the expansion tank needs replacement and you’re unsure what size or type is correct for your system.
- When pressure spikes remain after replacing or recharging the tank, so a pro can check piping layout, check valves, and the water heater’s internal components.
- If incoming water pressure is very high, or you suspect a failed pressure regulator or check valve—these are best handled by a licensed plumber.
Safety Notes
- Always turn off and isolate water and power to the heater before working on the tank or piping. Follow the heater manufacturer’s shutdown steps.
- Release pressure slowly and carefully before disconnecting any tank or pipe fittings. Hot water and steam can cause scalds.
- If soldering or using tools near the heater, protect the tank and follow safe hot-work practices or hire a pro.
Common Homeowner Questions
- Is this the same as normal water hammer?
No. Normal water hammer happens when valves shut quickly while fixtures are used. Expansion hammering happens during heater recovery, from pressure spikes as water heats. - How long will a fix take?
Simple checks and gauge tests take an hour or less. Recharging or replacing an expansion tank and retesting can be a few hours; more complex fixes may take longer. - Can I do this myself?
You can confirm timing and use a max-needle gauge yourself. Servicing a tank or installing arrestors is doable for a confident DIYer; call a licensed plumber if you’re unsure.
Related Articles
If you’re troubleshooting a similar symptom, these guides may help:
For the full directory, see Thermal Expansion Pressure Behavior.