Water Heater Hissing Noise, Including After a Shower
On this page (6 sections)
The Short Answer: Start with your nose, then your ears. A hiss with a rotten-egg smell near a gas heater may be a gas leak: leave and call the utility from outside. Without that smell, a hiss is usually sediment sizzling as the heater runs, the relief valve letting water out, or a small leak dripping onto a hot surface. Locating where the hiss comes from tells you which.
Locate the hiss
| Where the hiss is | Likely cause | How urgent |
|---|---|---|
| Gas pipe, flexible connector, or gas control, with or without a smell | Possible gas leak | Leave now and call from outside |
| Relief valve or its discharge pipe, which may be warm or dripping | Relief valve opening from high pressure or temperature | Prompt service |
| Inside the tank, only while the burner or elements run | Sediment boiling under scale | Not urgent; schedule service |
| Burner area of a gas heater, with sizzling and drips | Water dripping onto the flame | Turn off; see gas water heater dripping on the burner |
| A pipe fitting or valve on top, with dampness | Small water leak spraying or dripping onto hot metal | Repair soon |
| Behind an electric heater’s access panel, with a burning smell | Electrical fault or water reaching wiring | Switch off the breaker now |
Why it often starts after a shower
A shower pulls cold water into the tank and the heater starts reheating, so many hisses are loudest right after heavy use, when the burner or elements run longest.
- Sediment. Hard-water minerals settle on the bottom of a gas tank and coat the lower element of an electric tank. Water seeping into that layer boils in small pockets, which you hear as sizzling, popping, or rumbling while the heater recovers. Water heater sounds like rocks covers the louder version of the same problem.
- Thermal expansion. Reheating also expands the water. In a home with a check valve or pressure-reducing valve and no working expansion tank, pressure climbs during recovery and the relief valve may hiss or drip. A. O. Smith’s thermal-expansion bulletin (opens in a new tab) explains that pressure rise.
- Drips on hot surfaces. On a gas heater, condensation or a leak dripping onto the burner sizzles while it runs; see water dripping on the pilot light.
A sizzle that starts within minutes of a shower and stops when the heater finishes, with a dry floor and a dry relief-valve pipe, is almost always sediment.
The relief-valve hiss
A temperature and pressure relief valve hisses when it opens slightly. It is responding to pressure from thermal expansion in a closed system, unusually high street pressure, or water that has become too hot. The fixes are on the pressure or temperature side: an expansion tank, a pressure-reducing valve, or a thermostat or gas control repair. Never cap or plug the valve or its pipe. If the valve releases a steady stream, see pressure relief valve stuck open.
What you can check safely
- Smell near the gas line and control, from a normal standing position. Any gas odor means leave.
- Look at the relief-valve discharge pipe outlet for drips, and touch the pipe lightly; a warm pipe means hot water is passing.
- Dry the top fittings and watch for moisture returning.
- Check the hot water temperature at a tap. Water far hotter than your setting means switch the heater off and call for service.
- Note whether the hiss tracks the heater running or happens at random times, and whether it starts after showers and stops when the heater finishes.
- If your owner’s manual includes periodic draining or flushing as owner maintenance, follow it exactly, including turning off the gas control or the breaker first. On an older tank that has never been flushed, a technician may be the better choice, because a drain valve can clog or fail to reseal.
What the technician will check
For gas concerns, the utility or a gas technician checks the fittings and control with leak-detection equipment. For a relief-valve hiss, a plumber measures house pressure, checks the expansion tank and pressure-reducing valve, and verifies the heater is shutting off at its set temperature. For sediment, they flush the tank if it is in reasonable condition and may inspect or replace the lower element on electric models. For small leaks on top, they repair or replace the fitting.
Frequently Asked Questions
Q: Does a hissing water heater mean the tank will burst? A: No. Tanks have a relief valve so that pressure and temperature cannot climb without release. A hissing relief valve shows that protection working, and the underlying cause should be fixed. Never block the valve.
Q: Is it normal for a water heater to hiss a little while heating? A: A soft sizzle from sediment is common in older tanks, especially in hard-water areas. It is a maintenance sign rather than an emergency. A hiss that is new, loud, or comes with water, steam, or an odor is not normal.
Q: Will flushing the tank stop the noise after showers? A: Often it reduces it, if the noise is sediment and the tank is not too old. Heavy, hardened scale may not come out, and on an older tank the noise can continue even after a flush.
Manuals and supporting references
These links distinguish general safety information from model-specific documentation. A manual directory helps you locate your unit’s instructions; it does not establish that every model uses the same parts, codes, or test values.
- PG&E: recognizing and responding to a gas leak (opens in a new tab)Emergency actions for suspected fuel-gas leaks.
- A. O. Smith technical bulletin 51: puddles around a heater (opens in a new tab)Common water-leak sources and the distinction between fittings and a failed tank.
- A. O. Smith technical bulletin 45: thermal expansion (opens in a new tab)Background on pressure rise in a closed water system. Pressure testing and repairs are professional work.
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