# Water heater popping or rumbling: sediment is the cause, a flush is the fix, and the one noise that means stop > Popping and rumbling come from steam trapped under sediment. A.O. Smith and Rheem say flush once a year, Rheem says every 6 months on hard water. | | | |---|---| | Source | https://ownerspec.com/plumbing/guides/water-heater-popping-noise-sediment-flush/ | | Publisher | OwnerSpec (https://ownerspec.com/) | | Published | 2026-09-24 | | Updated | 2026-09-24 | | Facts verified | 2026-09-24 | | Disclosure | Contains affiliate links. OwnerSpec may earn a commission at no cost to the reader. | --- ## Short answer A tank water heater that pops, rumbles or crackles has sediment on the tank bottom, and the fix is a drain and flush. A.O. Smith's Technical Bulletin 13 names the mechanism: lime comes out of solution as water is heated, settles on the bottom, and the popping is water trapped under the lime deposits boiling to steam. Bradford White describes it on gas heaters as droplets of water bubbling through crusty formations while the burner is on. A.O. Smith says most tank water heaters require flushing at least once per year, and more often in a hard water area. Rheem says once a year, or every 6 months with hard water. Bradford White says a pro will likely recommend maintenance once a year. One noise is not a flush job: a relief valve that pops or drains is, in Rheem's words, an unacceptable condition that must be corrected, and the outlet must never be plugged. ## Fact sheet - **Answer:** Popping, rumbling and crackling in a tank water heater are water trapped under lime and sediment on the tank bottom boiling to steam, as the maker of Technical Bulletin 13 on hotwater.com describes it; that maker says flush at least once a year, and Rheem says once a year or every 6 months with hard water. A relief valve that discharges is a different problem and must be corrected, never plugged. - **Basis:** Manufacturer maintenance guidance and manuals: A.O. Smith (hotwater.com and instruction manual 185762), Rheem (rheem.com articles and Use and Care Manual AP23657) and Bradford White (For The Pro service pages). No code section number is cited; plumbing codes are adopted locally and vary. - **Assumptions:** A residential storage tank water heater, gas or electric, on a municipal or well supply. Tankless units have no tank bottom for sediment to settle on and are outside this page.; The flush interval is the manufacturer's general figure; a maker's manual for a specific model, and any local code, override it.; Gas and electric work follows the manufacturer manual. A relief valve that discharges, or a drain valve that will not open, is a call to a plumbing contractor per the makers quoted. - **Method:** Noise causes and flush intervals read from A.O. Smith's maintenance, draining and noise pages and Technical Bulletin 13, from Rheem's flushing and noise articles, and from Bradford White's Internal Noises and Draining a Water Heater service pages. Procedure and relief valve language read from A.O. Smith manual 185762 and Rheem manual AP23657 (PDFs, text extracted). All read on 24 September 2026. - **Primary source:** [Rheem, How Often To Flush Your Water Heater (And How To Do It)](https://www.rheem.com/water-heating/articles/how-often-to-flush-your-water-heater-and-how-to-do-it/) (manufacturer or vendor document) - **Facts verified:** 2026-09-24 - **Last updated:** 2026-09-24 - **Canonical URL (cite this):** https://ownerspec.com/plumbing/guides/water-heater-popping-noise-sediment-flush/ If the noise is a bang when a tap or the washing machine shuts off, it is not the heater at all: A.O. Smith calls that [water hammer](/plumbing/guides/water-hammer-banging-pipes-arrestor/), and a hammer arrestor, not a flush, is the fix. ## Which noise is it? | Sound | Likely cause | Who says so | What to do | |---|---|---|---| | Popping, rumbling, crackling | Water trapped under lime and sediment on the tank bottom boiling to steam | A.O. Smith Bulletin 13; Bradford White Internal Noises | Drain and flush; see below | | Ticking, clicking | Heat traps or pipe expansion as temperatures change | Rheem noise guide; A.O. Smith manual 185762 | Nothing: A.O. Smith says expansion noises do not necessarily represent a harmful condition | | Sizzling, hissing | Gas: water dripping onto the burner. Electric: a leaking element gasket, or lime on the element blades | Rheem noise guide; Bradford White Internal Noises | Rheem lists persistent hissing among the sounds that warrant a professional | | Screeching, whistling | Restricted flow: a partially closed inlet or outlet valve, or a faulty T&P valve | Rheem noise guide | Open a partially closed valve fully; a suspect T&P valve is a service call | | Relief valve popping or draining | Thermal expansion in a closed system | Rheem manual AP23657; A.O. Smith manual 185762 | Stop. Do not plug it. Contact the water supplier or a plumbing contractor | ## Why does sediment make the tank pop? Because lime is, in A.O. Smith's phrase, inversely soluble: the more the water is heated, the more lime comes out. Lime is calcium carbonate, the same mineral behind [white scale on faucets and kettles](/water/guides/hard-water-white-scale/). Rheem explains the cost: sediment acts like insulation at the bottom of the tank, so the heater runs longer heating cycles. A.O. Smith's manual lists "sediment or lime in tank" as a cause of both insufficient hot water and high operating costs, and its remedy is to drain and flush, then provide water treatment if needed. That last clause is the reason the noise comes back. Bradford White says flushing may not remove all the lime formations from the tank bottom, and the heater may become noisy again once reconnected. In hard water areas it names a water softener as the best solution. The [hardness unit converter](/water/calculators/water-hardness-unit-converter/) turns the number on your utility report into grains per gallon. ## How often should the tank be flushed? Once a year is where three makers meet, and hard water shortens it. | Maker | Interval | Where it says so | |---|---|---| | A.O. Smith | At least once per year; more often in a hard water area | Water Heater Maintenance Guide, hotwater.com | | Rheem | Once a year; every 6 months with hard water | How Often To Flush Your Water Heater, rheem.com | | Rheem (hybrid manual AP23657) | Drain a few quarts from the tank every month to clear deposits | Routine Preventative Maintenance page | | Bradford White | Maintenance once a year by a plumbing professional, who may drain and flush the tank | Spring maintenance guide, bradfordwhite.com | Rheem lists rumbling or popping noises from the tank first among the signs that flushing is overdue. The popping is the schedule telling you it slipped. ## How do you flush the tank? Follow the manual for your model. This is the shape A.O. Smith's draining page, its manual 185762 and Bradford White's For The Pro page share. 1. Turn off the power at the breaker (electric) or set the gas control to OFF, as the manual directs. A.O. Smith says ideally the heater is off for a few hours first, so the tank cools. 2. Close the cold water inlet valve. 3. Open a hot tap in the house, or the T&P valve lever per Bradford White, to admit air. 4. Connect a garden hose to the drain valve and run it to a drain. Bradford White says to put the hose end where scalding hot water cannot cause harm. 5. Open the drain valve and let the tank drain completely. 6. Close the drain valve, remove the hose, close the T&P valve if you opened it, open the cold inlet, and purge air at the hot taps before restoring power or gas. A.O. Smith says to wear thick gloves and safety goggles, and Rheem's manual warns the drained water may be hot enough to present a scald hazard. Both say to call a plumber if the drain valve will not open or sediment has clogged it. Bradford White highly recommends a plumbing professional's help. For packed sediment, A.O. Smith's Bulletin 13 describes dissolving it with household cider vinegar left in the heater for at least six hours: a service procedure, not a Saturday flush. **Before you buy a flush kit** A plain garden hose does the annual flush on most tanks. A flush kit with a sediment tool earns its place when the drain runs slow or the popping survives a plain flush. If the valve will not seal afterwards, A.O. Smith says replacement is usually the answer: the House Mods full-port brass valve lists a 3/4 inch MNPT tank thread and a 2.5 inch shank, so check your manual's drain valve size first. The valve listing says lead-free compliant brass; the flush kit prints no potable-water certification, so none is claimed for it. ## When does a noise mean stop and call someone? When it comes from the relief valve. The temperature and pressure relief valve (T&P valve) is the safety valve on the tank. Watts lists its 100XL residential valve at 150 psi and 210 degrees F, and A.O. Smith's manual requires a set pressure no higher than the tank's 150 psi working pressure. Rheem's manual lists a relief valve producing popping noise or draining as an unacceptable condition that must be corrected, attributes it to thermal expansion in a closed system, and says to contact the water supplier or a plumbing contractor, and in capitals: do not plug the relief valve outlet. A.O. Smith's manual agrees, points to a check valve or water meter on the supply, and says do not plug or remove it. The fix both describe is an expansion tank, covered in [when a thermal expansion tank is required](/plumbing/guides/thermal-expansion-tank-when-required/). The other stop is a valve that will not reset: A.O. Smith says if the T&P valve continues to release water after its yearly manual test, close the cold inlet immediately, drain, and fit a new valve. ## What should you do first? 1. Match the sound to the table above. Ticking needs nothing; a discharging relief valve needs a plumber. 2. Find the flush interval in your model's manual, and shorten it if your hardness is high. 3. Flush per the manual, with the power or gas off and the hose end somewhere hot water can do no harm. 4. While the tank is drained, check the anode: A.O. Smith says after a maximum of three years and annually thereafter, or annually on softened water. See [anode rod by model](/plumbing/parts/water-heater-anode-rod-by-model/). 5. If the popping returns within months, the water is the problem. Get a hardness number before buying anything. ## The picks - **Sediment Gone Water Heater Flush Kit and Sediment Removal Tool**: A tank that drains slowly or keeps popping after a plain hose flush (Listed as a flush kit and sediment removal tool for the drain valve; the listing does not print a potable-water certification, and none is claimed here) Amazon: https://www.amazon.com/dp/B0G674JLLB/?tag=ownerspec-20 - **House Mods HMP012 Full Port Water Heater Straight Drain Valve, 3/4 in MNPT, 2.5 in shank**: A plastic drain valve that clogs or will not close after the flush (Listing states lead-free compliant brass, a 3/4 inch MNPT tank thread, a 2.5 inch shank and a full-port bore; confirm the tank's drain thread in its manual before ordering) Amazon: https://www.amazon.com/dp/B0DNNBZJZX/?tag=ownerspec-20 ## Frequently asked questions ### Is a popping water heater about to explode? No. A.O. Smith explains that popping is sediment at the bottom of the tank trapping steam bubbles that have to fight to escape, and its Technical Bulletin 13 describes water trapped under lime deposits boiling to steam. The manufacturers treat it as a maintenance problem, fixed by draining and flushing. Pressure problems announce themselves at the relief valve, not as popping. ### How often should a water heater be flushed? A.O. Smith says most types of tank water heater require flushing at least once per year, and more often in a hard water area. Rheem says once a year for most households and every 6 months with hard water. Bradford White recommends annual maintenance by a plumbing professional that includes draining and flushing the tank. Your model's manual overrides all three. ### Will flushing stop the noise for good? Not always. Bradford White's service page says flushing can reduce the noise, but it may not be possible to remove all the lime formations from the tank bottom, and the heater may become noisy again once reconnected. In hard water areas Bradford White names a water softener as the best solution, because it inhibits the scale building up in the first place. ### What does a sizzling or hissing water heater mean? It depends on the fuel. Rheem lists water dripping onto a hot burner on gas models, a leaking heating element gasket on electric models, and internal condensation meeting heated components. Bradford White says electric heaters develop singing or hissing when lime accumulates on the element blades. Rheem lists persistent hissing or sizzling among the sounds that warrant a professional. ### My relief valve drips after the tank heats. Is that part of the sediment problem? No, and it is the one noise that is not a flush job. Rheem's manual calls a relief valve that pops or drains an unacceptable condition that must be corrected, attributes it to thermal expansion in a closed system, and says do not plug the relief valve outlet. A.O. Smith's manual says the same and points to a check valve or water meter on the supply. ## Sources - [A.O. Smith, Water Heater Maintenance Guide (hotwater.com)](https://www.hotwater.com/info-center/water-heater-maintenance.html) (manufacturer or vendor document) - [A.O. Smith, How to Drain and Flush a Water Heater (hotwater.com)](https://www.hotwater.com/info-center/how-to-drain-a-water-heater.html) (manufacturer or vendor document) - [A.O. Smith, Hot Water Heater Making Noises (hotwater.com)](https://www.hotwater.com/info-center/water-heater-making-a-noise.html) (manufacturer or vendor document) - [A.O. Smith, Technical Bulletin 13, Mineral Build-Up](https://www.hotwater.com/info-center/technical-bulletins/bulletin-13.html) (manufacturer or vendor document) - [A.O. Smith, Instruction Manual, Residential Electric Water Heaters, document 185762 (PDF)](https://assets.hotwater.com/damroot/Original/10001/185762.pdf) (manufacturer or vendor document) - [Rheem, How Often To Flush Your Water Heater (And How To Do It)](https://www.rheem.com/water-heating/articles/how-often-to-flush-your-water-heater-and-how-to-do-it/) (manufacturer or vendor document) - [Rheem, The Ultimate Guide to Water Heater Noises: What's Normal (and What Isn't)](https://www.rheem.com/water-heating/articles/the-ultimate-guide-to-water-heater-noises-whats-normal-and-what-isnt/) (manufacturer or vendor document) - [Rheem, Electric Residential Hybrid Water Heater Use and Care Manual AP23657 Rev 04 (PDF)](https://media.rheem.com/blobazrheem/wp-content/uploads/sites/36/2025/01/AP23657-Use-Care-Manual.pdf) (manufacturer or vendor document) - [Bradford White For The Pro, Internal Noises](https://forthepro.bradfordwhite.com/internal-noises/) (manufacturer or vendor document) - [Bradford White For The Pro, Draining a Water Heater](https://forthepro.bradfordwhite.com/draining-a-water-heater/) (manufacturer or vendor document) - [Bradford White, Spring Cleaning for Your Water Heater: A Maintenance Guide](https://www.bradfordwhite.com/blogs/spring-water-heater-maintenance-tips/) (manufacturer or vendor document) - [Watts, 100XL-150 3/4 temperature and pressure relief valve product page](https://www.watts.com/products/plumbing-flow-control-solutions/relief-valves/temperature-and-pressure-relief-valves/100xl/100xl-150-34) (manufacturer or vendor document) --- Citation: OwnerSpec, "Water heater popping or rumbling: sediment is the cause, a flush is the fix, and the one noise that means stop", https://ownerspec.com/plumbing/guides/water-heater-popping-noise-sediment-flush/, facts verified 24 September 2026. 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