Why is my water heater hissing?

Start with a look and a listen. Keep your hands off the relief valve outlet, because the water that comes out of it can be extremely hot.

1. 1Check for steam or very hot water. A. O. Smith's manual says not to operate the heater until a qualified technician has fully checked it out if it "Produces steam or unusually hot water."
2. 2Shut it off if the hiss is loud, steady or paired with water. Rheem's noise guide says that for hissing, sizzling or dripping you should turn off power or gas to the unit, avoid using it, and schedule a professional inspection. On an electric heater, that means the breaker. On a gas heater, that means the manual gas control valve. A. O. Smith says to turn that valve off if overheating occurs or the gas supply fails to shut off.
3. 3Look at the relief valve pipe. If water or steam is coming out of the discharge pipe beside the tank, stay clear and leave the heater off. More on that below.
4. 4Look for wet spots. Check the burner area, the fittings on top, the base and the drain pan.

If none of that shows up and the hiss is brief, keep reading. It may be normal.

Give it time before you decide. A hiss that shows up right after a long shower or a fresh install is a different situation from one that runs all day. Note when it starts, how long it lasts, and whether it stops once the burner or element cycles off. Those details help whoever looks at it.

Three causes cover most cases, and the first is often harmless.

### Is hissing from condensation on a gas water heater normal?

Often, yes. Gas burning produces water vapor, and that vapor condenses on a cold tank. A. O. Smith says the drops can fall onto the burner or other hot surfaces and make a sizzling or frying noise. It happens most often when:

• A new heater is filled with cold water for the first time.
• A lot of hot water is used in a short time, so the refill water is very cold.
• The heater is undersized for the household.

A. O. Smith adds that condensation causes sizzling and popping within the burner area during heating and cooling and should be considered normal. Rheem's power vent manual says drops falling on the burner can produce a sizzling or pinging sound, and that it will disappear after the water becomes heated. The timing matters. A. O. Smith says the condition should disappear in about one to two hours once the tank warms, and "Do not assume the water heater is leaking until there has been enough time for the water in the tank to warm up."

Here's the deal. If the sizzle fades on its own, you're fine. If it keeps going, Rheem's power vent manual says to examine the piping and fittings for possible leaks.

### Can sediment make a water heater hiss?

It can make a sizzling noise, and Phoenix water gives it plenty to work with. A. O. Smith's troubleshooting chart lists "sizzling or rumbling" with the cause "Sediment or calcium in bottom of heater tank" and the fix of cleaning the sediment out. Rheem calls sediment the most common cause of popping or rumbling in tank-style units, and says flushing often stops it.

The City of Phoenix's 2025 water quality report lists total hardness of 10 to 17.6 grains per gallon. The city's report says hard water can cause scaling in pipes and water heaters. Our page on whether hard water will ruin a hot water heater covers what scale does to each type, and how often to flush a water heater in Phoenix covers timing. For the popping side of the same problem, see why a water heater pops or rumbles.

### Could the hiss be a leak on a hot part?

Yes, and that's the one not to ignore. Rheem's guide lists water dripping onto a hot burner on gas models, a leaking heating element gasket on electric models, and internal condensation meeting heated components as causes of hissing, sizzling and dripping. It warns that leaks can worsen quickly.

Here's how to tell the difference. A condensation drip should stop as the tank warms. A leak keeps going. Our pages on why a water heater leaks from the bottom and the drain pan help you trace the source. The fix depends on where the water is coming from.

It can, so treat it seriously. The temperature and pressure (T&P) relief valve is the safety valve that opens when the tank gets too hot or the pressure climbs too high. A. O. Smith says it must not be plugged or removed, and that the water it releases may be extremely hot. Rheem lists a faulty T&P valve among the causes of whistling or screeching.

Occasional weeping can come from thermal expansion in a closed water system, but A. O. Smith is clear that "The Temperature-Pressure Relief Valve is not intended for the constant relief of thermal expansion." A valve that keeps running, or a heater that also makes steam or very hot water, is a stop-point. Turn off the gas or power, keep people away from the discharge pipe, and read our page on a leaking T&P valve. Never cap the pipe.

The scale story is different. Navien says mineral deposits build up in a tankless unit's waterways, and that buildup can reduce flow and interfere with heating. It also says a heat exchanger could malfunction and overheat the water. It recommends descaling at least once a year, more often with harder water. The manufacturer material we checked doesn't tie tankless scale to a specific hiss, so treat a hissing tankless unit like a tank: look for a leak, shut it off if it's wet or hot, and get it checked.

Call a licensed plumber if the hiss doesn't fade as the tank warms, if you find water on or under the heater, or if the relief valve is discharging. Rheem's list of warning signs includes hissing or sizzling, visible leaks or moisture, noise that continues after a flush, and a unit that's 8 to 12 years old or more. If yours is in that age range, our page on whether to repair or replace a water heater helps you decide.

### What should I tell the technician?

A few details speed up the visit. Say when the hiss starts and how long it lasts, whether the heater is gas, electric or tankless, and whether you saw steam, water at the relief valve pipe or wet spots. Mention how old the heater is, whether the relief valve has ever run, and whether the household uses a water softener. A photo of the rating plate and of any wet area helps too. Shut-off first, questions second.

HQ Plumbing and Air is licensed and insured and takes emergency calls 24/7. Free estimates, and you'll get an upfront price before we start.

See water-heater warning signs to distinguish symptoms needing service from an active leak, a gas smell or relief-valve discharge that needs prompt action.