How to Flush a Tank Water Heater, Step by Step

Every tank water heater slowly fills with sediment: minerals that drop out of the water and settle on the bottom of the tank, where they insulate the burner or element from the water, make the heater rumble and pop, and shorten its life. Sediment is also one reason hot water runs out fast. Manufacturers such as Rheem call for flushing the tank once a year to clear it. It takes about an hour, most of it waiting, and the only tool you need is a garden hose.

It is also a job that goes wrong in two specific ways, both avoidable: draining an electric heater with the power on burns out the heating elements, and opening a drain valve that has not moved in a decade can leave you with a valve that will not close. This guide walks through the flush in order, with those traps called out, and tells you when to hand the job to a plumber. If the heater gives no hot water afterward, see no hot water. Our maintenance calendar schedules it in May; fall works just as well.

Before You Start

• A garden hose long enough to reach a floor drain, a utility sink, or outdoors — and the path has to run downhill, because the tank drains by gravity.
• A bucket, in case the drain valve drips after you close it.
• A flathead screwdriver if your drain valve has a slotted stem instead of a handle.
• A flashlight, and a towel for the floor.
• Optional: a brass garden-hose cap to put on the drain valve afterward, which is cheap insurance against a valve that weeps.

Step-by-Step

1. Turn off the heat. Electric heater: switch it off at the breaker panel (usually a double breaker labeled "water heater"). The elements burn out if they run in an empty tank, so this step is not optional. Gas heater: turn the control knob on the gas valve to "off" and close the gas supply valve to the heater, as the label and manual direct, so the burner cannot fire while the tank is low. "Vacation" is only a low temperature setting, and the burner can still fire.
2. Close the cold-water supply. The valve is on the cold pipe entering the top of the tank, usually a lever or a round handle. Turn it off so the tank stops refilling as you drain.
3. Let the tank cool. A tank holds water at 120°F or more, hot enough to scald. Wait a couple of hours, or run hot water at a tap for several minutes to draw the hottest water out and replace it with cold before you close the supply.
4. Connect the hose. Thread the garden hose onto the drain valve at the bottom of the tank and run the other end to the drain, sink, or outdoors, sloping downhill the whole way. Sediment-laden water can stain a lawn; a driveway or floor drain is better.
5. Open a hot tap upstairs. Turn on a hot faucet somewhere in the house and leave it open. It lets air into the system so the tank can drain instead of gurgling and holding a vacuum.
6. Open the drain valve. Turn the handle or the slotted stem counterclockwise. Water flows, then slows as the tank empties. If the flow trickles and stops early, sediment is blocking the valve: briefly open the cold supply to stir it and push it through, then close it again. If nothing moves the clog, stop and call a plumber rather than forcing the valve.
7. Flush until clear. With the drain still open, open the cold supply for thirty seconds to a minute to stir up and push out the remaining sediment, then close it. Repeat until the water running out of the hose is clear.
8. Close the drain valve and remove the hose. Turn it firmly clockwise. Watch it for a minute: an old plastic valve sometimes will not reseal, which is what the brass cap and the bucket are for. A valve that keeps dripping needs a plumber to replace it, but the cap will hold until then.
9. Refill the tank. Open the cold supply fully and leave the upstairs hot tap open. Air sputters out of the tap first; when water runs steadily with no spitting, the tank is full. Close the tap.
10. Restore the heat, in that order. Only after the tank is full: switch the breaker back on for an electric heater, or open the gas supply, relight the pilot per the label on the heater (if it won't stay lit, see pilot won't stay lit), and turn the gas control back to its normal setting. Give it an hour to reheat, then check the drain valve one more time for drips.

Testing the T&P valve while you are there

The temperature-and-pressure relief valve is the safety valve on the top or side of the tank with a lever and a pipe running down from it. Testing it is a separate task with its own rules: the water that comes out is scalding, so first confirm the discharge pipe runs down to a drain or a bucket and keep your hands clear. Lift the lever briefly; water should flow, then stop when you release it. If there is no discharge pipe, skip the test and call a plumber. If the valve is old and has never been tested, have a plumber do the first test — a long-seized lever can start a drip that will not stop. A valve that does not release water, or does not stop, needs replacing.

What Normal Looks Like

Cloudy, sandy, or rust-tinted water in the first minutes of draining is normal; that is the sediment you came for. A tank that takes twenty to forty minutes to drain is normal. What is not normal: water that stays rust-colored through the whole flush (the tank is corroding inside), a drain valve that will not close, water appearing at the base of the tank afterward, or a heater that rumbles just as loudly after the flush as before, which means the sediment has hardened into a layer the flush cannot move. Each of those is a plumber call and a signal to start pricing a replacement; tank water heaters typically last 8 to 12 years, and our budget guide treats the replacement as a scheduled expense.

How Often

Once a year for most homes; twice a year with very hard water. Our calendar puts it in May, between heating and cooling season. While you are in the utility room, confirm the thermostat is set to 120°F (the Department of Energy's recommendation), check the anode rod (A.O. Smith suggests once a year; our anode rod guide covers how, and when it is a plumber's job), and put a leak alarm at the base of the tank.

On a home inspection report? See what it means when an inspector lists a missing expansion tank or a missing sediment trap on a gas water heater.

Frequently Asked Questions

Do I need to turn off the water to the whole house?

No, only the cold supply valve on top of the heater. The rest of the house keeps its cold water. If that valve does not turn or is missing, use the main shutoff — our shutoff guide shows where it is — and have the heater's valve replaced when a plumber is next in the house.

What if the drain valve is plastic?

Many builder-grade heaters ship with a plastic drain valve that gets brittle with age. Open it gently, do not force it, and have the brass cap ready. If it cracks or will not reseal, close the cold supply, cap it, and call a plumber; replacing it with a brass valve is a routine job.

Can I flush a tankless water heater the same way?

No. Tankless units are descaled by circulating a vinegar or descaling solution through them with a small pump and hoses connected to their service valves, usually once a year. Follow the manufacturer's procedure or have it done at the annual service.

Is it worth doing on a heater that is already old?

If the heater is past about 12 years and has never been flushed, the honest answer is that flushing may open a leak that the sediment was plugging. Have a plumber assess it, and put the money toward the replacement fund rather than a risky first flush.

The Bottom Line

Power or gas off, cold supply closed, tank cooled, hose to a drain, hot tap open, drain until clear, valve closed, tank refilled, then and only then the power back on. An hour a year keeps the heater quieter, cheaper to run, and around for its full life. If the tank is old, leaking, or the valve will not move, that hour belongs to a plumber. When you are done, add the date to your calendar so next year's flush does not slip.

Sources

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