How to flush a tank water heater

Updated October 10, 2026. By the OakTend team.

Flushing means running water out of the drain valve at the bottom of the tank until it comes out clear. It washes out the mineral sediment that settles on the tank floor.

How often: At least once a year; the Irvine Ranch Water District recommends yearly. A.O. Smith's manuals call for every 6 months. Follow your own heater's manual.
What you need: Garden hose, Bucket, Work gloves, A flat screwdriver, if the drain valve has a slot

Why it matters

Heating water makes its minerals drop out, and Orange County's water is hard, so tanks here collect sediment faster. Our hard water guide shows how hard your supplier's water is and what scale does to a heater.

Steps

Electric: switch off the heater's breaker. Gas: turn the gas control to OFF and close the gas shutoff valve on the supply line.
Close the cold water supply valve on the pipe that feeds the top of the tank. It stays closed until the tank is drained.
Open a hot water tap in the house and let it run until the water is no longer hot, or let the tank sit a few hours to cool before draining. Leave the tap open so the tank can drain.
Screw a garden hose onto the drain valve near the bottom of the tank and run it to a floor drain or outside, lower than the valve.
Secure the hose so it cannot whip loose.
Open the drain valve and let the tank empty. If the water is still cloudy at the end, open the cold water valve for a minute to stir out the rest, then close it.
Close the drain valve and take off the hose.
Reopen the cold water supply valve to refill the tank. Keep the hot tap open until water flows from it steadily with no air, then close it. The tank is full.
Only with the tank full, turn the breaker back on. Gas: open the gas shutoff and relight by following the lighting instructions on the label on the heater. If you are not comfortable relighting it, call SoCalGas or a plumber. If you smell gas at any point, stop, leave the house and call SoCalGas at 1-800-427-2200 from outside.

When to call a pro

Call a plumber if the drain valve will not open, will not close or keeps dripping afterward, or if the tank itself is leaking. On an old tank that has never been flushed, the valve can clog or fail to reseal, so it can be wiser to leave it and plan for a replacement.

Safety

The water coming out can be hot enough to burn, so keep hands and pets clear of the hose end. Do not turn the power or gas back on until the tank is full again: water must flow from the hot tap first.

Go deeper

General information, not professional advice. Your equipment's manual comes first when it says something different.

Sources

A.O. Smith: electric water heater instruction manual, draining and flushing
Drain and flush every 6 months; secure the hose; run a hot tap until it is no longer hot; flush until the water runs clean; refill with a hot tap open.
A.O. Smith: FVIR gas water heater manual, draining and flushing (hosted by ManualsLib)
On a gas heater, close the manual gas shutoff and set the gas control to OFF before draining.
A.O. Smith: water heater maintenance guide
Shut off the cold water inlet and open a hot faucet before draining; refill and purge the air by running a faucet; make sure the tank is completely full before returning it to service; relight following the lighting instructions. Checked 2026-10-09.
SoCalGas: natural gas leaks
If you suspect a gas leak, leave the area and call SoCalGas at 1-800-427-2200 or 911 from a safe location. Checked 2026-10-09.
Irvine Ranch Water District: water quality questions and answers
Imported Colorado River and Northern California water is typically hard, and the district recommends flushing the water heater once a year. Checked 2026-09-21.