Guides › Repair

How to flush a water heater: step-by-step sediment removal

Minerals in your water settle to the bottom of a tank water heater as sediment. Over time that layer can cause popping or rumbling noises, slower recovery, less usable hot water, and extra strain on the tank. Flushing the tank washes much of it out through the drain valve. It's one of the most useful bits of maintenance a homeowner can do, and most manufacturers recommend it in the owner's manual.

Safety first. The water in the tank can be hot enough to scald. Turn the heater off and let it cool for a few hours (or overnight) before draining, or drain it into a spot where hot water can't splash anyone. Never turn on power to an electric heater while the tank is empty or partly empty: the elements can burn out in seconds without water around them.

What you'll need

A garden hose long enough to reach a floor drain, utility sink, or outside to a spot lower than the drain valve
A flat screwdriver or channel-lock pliers (some drain valves have a slotted stem instead of a handle)
A bucket and towels
Gloves, since the drain valve and hose fitting may get warm

Step-by-step

1. Turn off the heat source

Electric: switch off the heater's breaker at the panel. Gas: turn the gas control knob to OFF or PILOT (check your manual; PILOT keeps the pilot lit on many standing-pilot models). Power-vent and some newer gas models also have a plug or switch to turn off.

2. Shut off the cold water supply

Close the valve on the cold water line going into the top of the heater. If it's a gate valve with a round handle, turn it clockwise until it stops. A ball valve with a lever is closed when the lever is crosswise to the pipe.

3. Connect the hose and open a hot faucet

Screw the garden hose onto the drain valve near the bottom of the tank and run the other end to your drain spot. Then open a hot water faucet somewhere in the house, ideally an upstairs or nearby tub. This lets air in so the tank can drain instead of holding the water in with a vacuum.

4. Open the drain valve and let it drain

Open the drain valve and let the tank empty. Watch the water coming out of the hose: early on it may look cloudy or carry grit and flakes. A full tank takes a while to drain through a garden hose, so be patient.

5. Flush with fresh water

Once the tank is mostly empty, open the cold supply valve for a short burst (with the drain still open). The incoming water stirs up sediment on the bottom and carries it out. Close it, let it drain, and repeat a few times until the water running out of the hose looks clear. Catching some in a bucket makes it easier to see when it's clean.

6. Close up and refill

Close the drain valve and remove the hose. Open the cold water supply fully. Leave the hot faucet you opened running: at first it will sputter air, and once it runs a steady stream with no spitting, the tank is full. Then close the faucet. Check the drain valve for drips.

7. Turn the heat back on

Electric: only after the tank is completely full and the faucet runs steadily, switch the breaker back on. Gas: turn the gas control back to ON (relight the pilot first if it went out, following the instructions on the heater's label). Give it time to reheat a full tank before judging the results.

If the drain valve clogs

Heavy sediment can plug the drain valve so the flow slows to a trickle or stops. A few things to try:

Close the drain, then briefly open the cold supply to build a little pressure, and open the drain again. Short bursts can push the clog through.
With the cold supply off and the tank mostly drained, remove the hose and gently probe the valve opening with a stiff wire or straightened coat hanger to break up flakes.
Many factory drain valves are plastic and can crack if forced. If yours won't open, won't close, or keeps dripping afterward, it can be replaced with a full-port brass drain valve, which also makes future flushing easier. That job requires fully draining the tank.

How often to flush

Check your owner's manual first, since recommendations vary by manufacturer. Many recommend draining or flushing roughly once a year, and more often if you have hard water. If you've never flushed an older tank, it's worth knowing that years of buildup can be hard to clear, and disturbing it occasionally leads to a clogged or leaking drain valve. That's not a reason to skip it, but have a plan (and a replacement drain valve) in case.

When flushing won't fix it

Rusty hot water that doesn't clear up, or water seeping from the tank body itself, usually means the tank is corroding. See water heater leaking.
A sulfur or rotten egg smell often involves the anode rod, not just sediment. See hot water smells like rotten eggs.
Little or no hot water on an electric heater often comes down to a failed element or thermostat. See hot water runs out fast.
Tankless heaters don't get drained this way. They're descaled by circulating a cleaning solution through service valves, following the manufacturer's procedure.

When to call a plumber

The drain valve breaks, won't close, or leaks after flushing.
You're not comfortable working with gas controls or relighting a pilot.
The noises or performance problems come back quickly after a flush.
The heater is older and showing several problems at once. See how long water heaters last before putting money into it.

Tip: Write the date you flushed on a piece of tape on the tank. It makes it easy to remember next time, and it helps a plumber know the maintenance history.