THE SHORT VERSION
- How often: A.O. Smith’s manuals say drain and flush after the first 6 months, then at least yearly (its gas manual says every 6 months). Rheem says once a year, or every 6 months with hard water.
- The sequence: heat off, run a hot tap until the water runs cool, close the cold valve, hose to a drain, open a hot tap to let air in, open the drain valve. Flush with the cold valve open until the water runs clear, then refill until a hot tap runs a full, steady stream.
- Electric tanks: never switch the breaker back on until the tank is completely full. A.O. Smith says power on a partly empty tank burns out the upper element (“dry fire”).
- Stop if it barely drains. Sediment can clog the drain valve. A.O. Smith says to call a qualified person rather than force it; Rheem says the same for a valve that won’t open or leaks.
- Think twice about an old tank that has never been flushed. Rheem warns that hardened sediment can make a DIY flush cause leaks, and suggests a professional for heaters 8–12 years old or older.
How often to flush: what the manufacturers say
There is no single interval. Each maker publishes its own, and all of them depend on how hard your water is. The manual for your exact model (search the model number on the rating plate) has the final word.
| Manufacturer and document | What it says |
|---|---|
| A.O. Smith gas manual (Signature 100, 2021) | Drain and flush every 6 months to reduce sediment buildup |
| A.O. Smith electric and heat pump manuals | Drain and flush after the first 6 months to see how much sediment builds up; then at least once a year, more often if needed |
| Rheem flushing guide (May 2026) | Once a year for most homes; every 6 months with hard water; more often with visible sediment or heavy use |
| Rheem gas manual (2017) | Drain “a few quarts” from the tank every month to clear deposits |
| Bradford White FAQ | Tanks are not self-cleaning, even with its anti-sediment inlet; drain periodically as the service manual lists, and ask your plumber |
Sources: the manuals and pages listed under Sources, as each document words it.
Hard water is the multiplier. USGS classes water above 120 mg/L of hardness as hard and above 180 mg/L as very hard; your utility’s water quality report gives your number. A.O. Smith says the rate of buildup depends on hardness, the temperature setting and other variables, which is why it treats the first flush at six months as a test: it is there “to determine the amount of sediment build up.” If that first flush runs clear quickly, a yearly flush fits; if it runs gritty for a long time, shorten the interval. More on water hardness in hard water and your water heater.
What sediment does, and what flushing can’t fix
Heated water drops minerals. A.O. Smith describes lime as “inversely soluble”: the hotter the water and the more of it you heat, the more comes out. It settles on the bottom of the tank and, in electric tanks, crusts on the heating elements. Manufacturers tie it to four problems:
- Noise. A.O. Smith’s Technical Bulletin 13 lists rumbling, crackling and popping: water trapped under lime deposits, or next to electric elements, flashing to steam. Rheem’s and A.O. Smith’s gas troubleshooting tables both answer rumbling with draining the tank. See water heater noises.
- Less hot water and lower efficiency. A.O. Smith’s electric manual says sediment or lime “can reduce the efficiency” of the heater and heavy deposits can damage it. Rheem says sediment “acts like insulation at the bottom of the tank.”
- Water hotter than the setting on gas tanks. Rheem’s gas manual says deposits can cover the gas control’s sensors, so water at the faucet can be hotter than the thermostat setting.
- Wear and leaks. A.O. Smith says failing to drain and flush “can deteriorate the tank, heating elements, and drain valve and cause leaks.”
How big is the effect? The best measurement we found is a 2010 lab study by Battelle for the Water Quality Association, a trade group whose members sell softeners. Gas tanks on very hard, unsoftened water built up about 1.16 pounds of scale a year, and their efficiency fell from 70.4% to 67.4% over the equivalent of two years. That measures what scale costs, not what a flush wins back; we found no independent study of savings from flushing.
A quick flush under pressure, or a full drain
Manuals describe two procedures. A.O. Smith’s gas manual gives a pressure flush: the cold supply stays open and house water pressure pushes sediment out of the drain valve. Its electric and heat pump manuals, and Rheem’s guide, describe a full drain: empty the tank, then flush it with the cold valve open.
Quick pressure flush
Good
- The tank stays full, so no air gets in and nothing needs refilling
- Faster; Rheem’s monthly “few quarts” is a small version of it
Watch for
- The hose is under full house pressure; A.O. Smith says to secure it before and during the flush
- You can’t see how much sediment the tank holds
Best for: Routine flushes on a tank that is already maintained.
Full drain and flush
Good
- Empties the tank, so you see what comes out
- The procedure in A.O. Smith’s electric and heat pump manuals
Watch for
- Takes longer
- The tank must be refilled completely before the heat goes back on
Best for: The first flush, a yearly flush, or when a quick flush keeps running cloudy.
What you need before you start
Gather this first
- A garden hose long enough to reach a floor drain or the outdoors, running downhill the whole way. A.O. Smith’s bulletin calls for a hose rated for 200°F.
- A bucket, to check how cloudy the water is and to catch drips
- Work gloves, eye protection and a flashlight
- Your heater’s manual, or at least the lighting and operating label on the tank
- The location of the cold water valve, the gas control or the breaker (see the shutoff diagrams in our emergency guide)
- Time: A.O. Smith says letting the hot water run cool can take 10 minutes or longer, and an electric tank can take an hour or more to reheat afterward
Step by step: draining and flushing a gas or electric tank
This sequence follows A.O. Smith’s gas, electric and heat pump manuals and its deliming bulletin, with the safety warnings from Rheem’s manual. Where your own manual differs, follow your manual.
Turn the heat off
Gas: turn the control knob to PILOT, as A.O. Smith’s bulletin says, or to OFF, as Rheem’s manual says (OFF means you will relight the pilot afterward). Heaters with an electronic control follow their manual. Electric or heat pump: switch off the heater’s breaker, usually a double breaker, or remove its fuses.
Cool the water
With the cold valve still open, run a nearby hot tap until the water is no longer hot. Fresh cold water replaces the hot water in the tank. A.O. Smith’s electric manual: “Be sure the water runs cool before draining the tank.”
Close the cold valve and attach the hose
Close the cold water valve on the pipe above the heater. Screw the hose onto the drain valve near the bottom and run it downhill to a floor drain or outdoors, below the level of the valve, where no one will walk into the stream.
Open a hot tap, then the drain valve
The open hot tap lets air into the tank so it can drain (Bulletin 13 calls it relieving the pressure). Use a tap, not the relief valve lever; see below. Then open the drain valve, keeping your hands and face clear. If you get only a trickle or nothing, close the drain valve and stop (see when the drain valve won’t cooperate). Otherwise let the tank drain, checking the water in a bucket now and then.
Flush
Close the hot tap and open the cold valve. A.O. Smith says to let the water run until no more sediment comes out, or “until it runs clean.”
Close the drain and refill
Close the drain valve (clockwise, per A.O. Smith) and take off the hose. With the cold valve fully open, open a hot tap and let it run a full, steady stream for at least 3 minutes to push out the air. Then close the tap.
Heat back on, then check
Electric: breaker back on. Gas: knob back to your setting; if you turned it to OFF, relight by following the label on the tank (see pilot light problems if it won’t stay lit). A.O. Smith says to let the heater complete several heating cycles; check the drain valve for drips over the next day.
Refill completely before the heat goes back on
A.O. Smith calls it a common installation mistake: power applied before the tank is completely full burns out the upper heating element, because it sits in air instead of water. This is dry fire, and A.O. Smith says a dry-fired upper element is not covered by its warranty. Some newer models have dry-fire protection, but A.O. Smith still says to fill the tank first.
The test: a hot tap running a full, steady stream, without sputtering, for 3 minutes. Gas tanks need the same care; A.O. Smith’s gas manual says water must flow from a hot tap before the gas goes back on.
When the drain valve won’t cooperate
The drain valve is the weak point of a flush. Some tanks come with a plastic one; others, such as Bradford White’s atmospheric gas models, list a brass valve on the spec sheet. Mr. Rooter, a national plumbing franchise, says plastic drain valves can become brittle over the years and crack when used, and that their rubber gaskets wear out.
| Problem | Likely cause | What to do |
|---|---|---|
| Only a trickle, or nothing | Sediment clogging the valve (A.O. Smith) | Close the valve. Don’t poke wire into it or force it. A.O. Smith says to call a qualified person, who may use a deliming agent. |
| Handle won’t turn | Stuck or brittle valve | Don’t put pliers on a plastic handle. Rheem lists a drain valve that won’t open among reasons to call a pro. |
| Drips after you close it | Not fully closed, or grit on the seat | Close it firmly by hand. A.O. Smith’s fix is to back-flush it briefly to clear sediment, and replace it if that fails. A garden-hose cap is a common stopgap. |
| Leaks where it meets the tank, or the body is cracked | Worn gasket or threads, cracked plastic | Call a plumber. Replacing the valve means draining the tank; Mr. Rooter says replacing the whole valve is the most reliable fix. |
Sources: A.O. Smith gas, power-vent gas and electric manuals; Rheem flushing guide (2026); Mr. Rooter, “Water heater leaking from drain valve.”
When to leave it to a pro, or not flush at all
Rheem’s own flushing guide lists times to call a professional instead: the drain valve won’t open or is leaking, there is heavy rust or corrosion around fittings, noise continues after flushing, the unit is 8–12 years old or older, or you’re unsure about the gas or electrical parts. Its reason for older tanks: “Older systems or tanks that haven’t been flushed in a while may have hardened sediment that’s difficult to remove without specialized tools, and attempting to flush these units yourself could lead to leaks or further damage.”
- An old tank that has never been flushed: the risk is that the drain valve won’t open, won’t close or won’t reseal once grit has passed through it, and you’re left with a leak on a tank near the end of its life. Rheem and ENERGY STAR both put the planning point for a replacement around year 10; see how long water heaters last.
- Rusty hot water: Rheem lists rusty or cloudy water as a sign a tank needs flushing, but ENERGY STAR lists rust in the water as a red flag for failure. If it stays rusty after a flush, see smelly or discolored water.
- Water already on the floor: don’t flush. Find the source with water heater leaking, and if it is the tank, follow the emergency steps.
Tankless and heat pump models are different
Tankless: descale, don’t flush
A tankless heater has no tank for sediment to settle in. Rheem says tankless units need periodic descaling instead of flushing, and Rinnai says to flush mineral deposits out of the heat exchanger at least once a year and clean the inlet filter. That job uses a pump and a descaling solution, not a garden hose and gravity; see descaling a tankless water heater.
Heat pump: an electric tank, plus a filter and a drain line
A heat pump water heater drains and flushes like an electric tank: breaker off, run the hot water until it is cool, then the same steps, including the full refill before power goes back on, since it has backup elements that can dry fire. A.O. Smith’s hybrid manual pairs the flush with cleaning the air filter and the condensate pan, after the first six months and at least yearly. Use the breaker for flushing, not the vacation mode, which only lowers the setting.
What a professional flush costs and includes
HomeAdvisor, a contractor-referral site, puts a professional flush at $110–$200, about $160 on average, and $75–$150 for a tank specifically, based on its own cost data (updated June 2026). That is one site’s range; ask for a flat price in writing, and consider bundling it with other small jobs.
What a service visit should cover (Rheem’s list)
- Shut down the gas or electric supply safely
- Fully drain and flush the tank
- Inspect the temperature and pressure relief valve
- Check that the drain valve works and seals
- Evaluate the anode rod
- Test water pressure and look for leaks
- Restart the heater and confirm it heats
For the full yearly routine, including anode checks and temperature settings, see water heater maintenance.
Common questions
Can I flush a water heater without turning off the water?
Yes, for a quick flush. A.O. Smith’s gas manual flushes with the cold supply open, so house pressure pushes sediment out through the drain hose; secure the hose first. To drain the tank completely you have to close the cold valve.
How long does it take to flush a water heater?
Longer than people expect. A.O. Smith says letting the hot water run cool can take 10 minutes or more, and an electric tank can take an hour or more to reheat after refilling. Draining time depends on tank size, the hose and how much sediment is in the valve.
What happens if I never flush my water heater?
Rheem says sediment buildup can reduce efficiency, raise energy bills, cause noise and shorten the heater’s life. A.O. Smith says it can deteriorate the tank, elements and drain valve and cause leaks. How fast depends mostly on your water hardness.
Should I flush an old water heater that has never been flushed?
Rheem recommends a professional for heaters 8–12 years old or older, because hardened sediment can make a DIY flush cause leaks. On a tank that age, it is also worth pricing a replacement; see our guide to how long water heaters last.
Why is my water heater still noisy after flushing?
Hardened scale doesn’t wash out with water. A.O. Smith’s fix for a “limed-up” tank is a deliming acid, which means disconnecting fittings, and Rheem lists noise after flushing as a reason to call a professional.
Do I need to relight the pilot after flushing?
Only if you turned the gas control to OFF. If you turned it to PILOT, the pilot stays lit. To relight, follow the lighting instructions on the label on the tank, and only after the tank is full.
Can I open the relief valve to help the tank drain?
Rheem’s instructions allow it, but we recommend a hot tap instead. A.O. Smith says a relief valve that has been operated may not reset and keep releasing water, and then it needs replacing.
Sources
Sources checked October 4, 2026. Prices, codes and rebates change. Check current details with your installer and local permit office.
- Signature 100 gas water heater installation instructions and use & care guide 100341468 (2021) ↗ · A.O. Smith
- Residential electric water heater installation instructions and use & care guide ↗ · A.O. Smith, hosted by Lowe’s
- Hybrid electric heat pump water heater installation instructions and use & care guide ↗ · A.O. Smith, hosted by Lowe’s
- Residential power vent gas water heater manual 100366722 (2023), troubleshooting table ↗ · A.O. Smith, hosted by Lowe’s
- Technical Bulletin 13: Mineral build-up ↗ · A.O. Smith
- How Often to Flush Your Water Heater (And How to Do It) (May 29, 2026) ↗ · Rheem
- Low NOx gas water heater use & care manual AP16412-1 (2017) ↗ · Rheem, hosted by InspectAPedia
- FAQs: Are your water heaters self-cleaning? ↗ · Bradford White
- Residential atmospheric vent gas water heater spec sheet (brass drain valve) ↗ · Bradford White, hosted by Young Supply
- Residential tankless water heater FAQ ↗ · Rinnai
- Hardness of Water ↗ · U.S. Geological Survey, Water Science School
- Battelle finds water softeners aid energy efficiency, appliance life (2010; study funded by the Water Quality Association) ↗ · Contractor magazine
- When Should You Replace Your Water Heater? (2024) ↗ · ENERGY STAR
- Water heater leaking from drain valve: what should I do? ↗ · Mr. Rooter Plumbing
- How Much Does It Cost to Flush a Water Heater? (updated June 19, 2026) ↗ · HomeAdvisor (contractor-referral site)