How to Flush a Water Heater: Steps From the Manuals

The Short Answer: Turn off the gas or the breaker, let the hot water cool, close the cold-water inlet, open a hot tap, and drain the tank through a hose from the drain valve. Then run cold water through until it comes out clear, and refill completely before turning the heater back on. A. O. Smith suggests a first flush at six months, then at least once a year.

How often to flush

The manuals disagree on how often, and most tie it to how hard your water is:

A. O. Smith’s gas manual (opens in a new tab) explains why: the amount of sediment “depends on the hardness of your tap water, the temperature settings, and other variables,” and “Draining sediment extends the life of the water heater.” The first flush at six months shows how much your water leaves behind. Follow the manual for your own heater if it differs.

Before you start

• Check the heater first. Don’t flush a tank that shows rust at the base or the fittings, or one that is already leaking; see water heater leaking. Rheem’s flushing article (opens in a new tab) suggests calling a professional for units 8–12 years old or more, or tanks that haven’t been flushed in a long time, because hardened sediment can be hard to remove and flushing could lead to leaks.
• Look at the drain valve. If it’s a plastic valve that’s cracked, or the valve already drips, have a plumber replace it rather than opening it.
• Get a hose. State’s manual says the drain hose should be rated for at least 200°F; otherwise, cool the water first. If there’s no floor drain nearby, see how to drain a water heater with no floor drain.

Steps for a gas tank

These follow A. O. Smith’s gas manual, which addresses the steps to the owner and suggests contacting a qualified person if you can’t do them yourself.

1. Turn the gas control (or its power switch) to OFF, and turn off the manual gas valve on the supply line.
2. Open a hot water faucet and let it run until the water is cool. This may take 10 minutes or longer.
3. Connect a garden hose to the drain valve and run it to a drain, outside, or into buckets.
4. Close the cold water supply valve to the heater.
5. Open the drain valve, and open a hot faucet to let air in so the tank drains faster.
6. If sediment came out while the tank drained, flush it: open the cold water supply with the drain valve still open, and let water run until no more sediment comes out. Then close the drain valve.

A. O. Smith’s manual also includes removing and inspecting the anode rod while the tank is empty, after the first six months and then annually. Removing the anode takes a 1-1/16-inch socket, and on some A. O. Smith models it sits under foam insulation; under our guidance that’s technician work; see water heater anode rod.

Steps for an electric tank

The A. O. Smith electric manual (opens in a new tab) gives the same sequence, starting differently:

1. Turn the water heater’s circuit breaker OFF (or remove its fuses).
2. Run a hot tap until the water is cool.
3. Attach the hose, close the cold supply, open the drain valve and a hot tap.
4. Flush with the cold supply open until no more sediment drains, then close the drain valve.

Don’t turn the power back on until the tank is completely full. An element powered in an empty or partly empty tank burns out, and A. O. Smith and Bradford White both say that isn’t covered by warranty.

Refill and restart

1. Make sure the drain valve is closed and a hot faucet is open.
2. Open the cold water supply.
3. Let the hot faucet run full for at least three minutes, as A. O. Smith specifies, so all the air is out and the tank is full.
4. Close the faucet and check the drain valve for drips.
5. Restore power at the breaker, or relight the gas following the label on the heater.

A. O. Smith says a gas tank may take an hour or more to reheat, and an electric tank about two hours. See how long a water heater takes to heat up. If there’s still no hot water, see no hot water after flushing.

If it won’t drain

Sediment can clog the drain valve. A. O. Smith’s manual says that if you can’t get the tank to drain, contact a qualified person. Its electric manual adds that sediment can form large masses, and a qualified person can use a de-liming agent suitable for drinking water. A. O. Smith’s bulletin 13 (opens in a new tab) explains that lime comes out of solution faster as water is heated, which is why heavily used tanks and hard water lime up the most. More in water won’t drain from water heater.

If the drain valve keeps dripping after you close it, leave the cold supply on, put a bucket under it and call a plumber; see water heater plastic drain valve dripping.

Heat pump water heaters

The tank on a heat pump water heater drains and flushes like an electric tank, with the power off first. Heat pump models also have an air filter on top. Bradford White’s AeroTherm manual (opens in a new tab) assigns it to the owner: a fault code shows when the filter needs cleaning, and you leave the power on, pull the filter, and vacuum, wipe or rinse it. Other brands’ steps differ, so follow your manual.

Does flushing fix a rumbling tank?

Often. Rumbling, popping and crackling are usually sediment on the bottom of the tank, and the manuals’ first remedy is to drain and flush. If the noise continues after a good flush, or the sediment has hardened into lime, a plumber may need to de-lime the tank. See water heater sounds like rocks. Tankless heaters scale up differently; see how to flush a tankless water heater.

Frequently Asked Questions

Q: Do I really need to flush my water heater? A: The manufacturers recommend it. American Water Heater’s manual (opens in a new tab) warns that failure to drain and flush sediment can deteriorate the tank and drain valve and cause leaks. How much sediment collects depends on your water’s hardness, which is why A. O. Smith uses the first flush at six months as a test.

Q: Is draining a few quarts every month enough? A: That’s what Rheem’s manuals suggest, and it keeps loose sediment from building up. A full drain and flush removes more. If a monthly drain still brings out a lot of sediment, move to full flushes.

Q: Should I put a gas water heater on PILOT or VACATION before draining? A: The manuals we checked all shut the gas off before draining. A. O. Smith and Bradford White turn the gas control to OFF and close the supply valve; State shuts it at the supply valve, and Rheem allows either the control or the supply valve. None says to leave the heater on PILOT or VACATION while draining.