Quick Answer:
If you hear a hammering noise when the water heater refills after a long hot‑water draw, it’s usually a pressure jolt from thermal expansion or a one‑way valve that traps that extra volume. After heavy use the tank refills and that quick inrush can produce a knock. A plumber can check expansion‑tank sizing and charge and inspect any check/backflow devices. If the pattern fits, also see Banging only on hot water.
Why This Happens
- Hot water expands. When the heater refills after a long draw, the replacement cold water can be forced quickly into the system. If the system has no room for the extra volume (or the expansion tank is waterlogged or undersized), you’ll hear a hammering or knocking sound.
- Check valves or backflow preventers on the incoming line stop the expanded water from moving back into the supply. That trapped expansion shows up as a pressure pulse and noise during the refill event.
- The noise happening only during refill events points to thermal expansion or a trapped pressure pulse rather than loose pipes or valves that make noise constantly or under other conditions. If the problem is seasonal, compare it with issues described under Pipes bang only in winter.
Step-by-Step What to Do
Step 1 — Reproduce the situation safely
- After a long hot‑water draw (for example, a shower or laundry cycle), listen as the heater begins to refill. Note whether the hammering starts right when cold water service resumes to the tank.
Step 2 — Feel the cold inlet line
- Carefully touch the cold water inlet pipe (it should feel cool). Feel for a quick jolt or vibration when the heater refills. That jolt is a strong clue that the refill action is causing the pressure pulse.
Step 3 — Verify whether an expansion tank exists
- Look near the water heater above the cold inlet for a small pressurized tank (typically a short cylindrical tank). It might also be installed near the house water‑meter or main shutoff.
- If you find one, note its size and whether it feels waterlogged (a very heavy tank that doesn’t sound hollow when tapped can be waterlogged and not doing its job).
Step 4 — Note the pattern
- Keep a short log for a day or two: when the noise happens, what hot fixtures were used, and whether the hammering only happens during heater refill. This helps a pro diagnose the problem faster.
Step 5 — Get a professional check if needed
- If the inlet gives a jolt, the expansion tank is missing or seems waterlogged, or the noise persists, contact a plumber. They can check expansion‑tank charge, confirm correct tank sizing for your heater and water pressure, and inspect any check/backflow valves.
What Not to Do
- Don’t adjust gas or electric controls on the heater in an attempt to stop the noise.
- Don’t open the temperature‑and‑pressure relief valve as a “test.” That valve is a safety device and should not be operated except per manufacturer guidance or by a professional.
- If you’re unsure about expansion devices or inlet check valves, don’t attempt to re‑charge or modify the expansion tank yourself—contact a plumber who can inspect and set the expansion tank correctly.
When to Call a Professional
- Call a plumber if the hammering continues after the checks above, if you can’t find or identify an expansion tank, or if you notice leaks, repeated relief‑valve discharge, or pressure‑related problems elsewhere in the house.
- A pro can measure system pressure, check expansion‑tank precharge and sizing, and confirm whether a check valve or backflow preventer is creating trapped expansion that needs correcting.
Safety Notes
- Do not touch hot pipes or the heater jacket—only feel the cold inlet pipe and do so carefully.
- Do not work on gas or electrical components of the heater unless you are qualified; unsafe adjustments can create hazards.
- If you find a leak or suspect a relief valve is discharging, shut off the water at the main and call a plumber. Avoid attempting risky repairs yourself.
Common Homeowner Questions
- Is my water heater over‑pressurizing? Not usually—hammering at refill more often indicates thermal expansion or a trapped pressure pulse, not an immediate overpressure failure. A plumber can verify actual system pressure.
- Can I fix this myself? You can do the listening and visual checks described above, but sizing or recharging an expansion tank and adjusting check valves is best left to a plumber.
- Will adding an expansion tank stop the noise? Often yes—if the cause is thermal expansion and the system lacks a properly charged tank. A professional should confirm sizing and charge for a reliable fix.
For more related articles, see the Pipes Banging After Fixtures Turn Off hub.