How to flush a tank water heater

Why and how often

Water carries minerals, and a tank heater lets them settle to the bottom as sediment. Over time that layer can make the heater noisy, cut how much hot water you get and lower its efficiency. Flushing rinses it out.

A. O. Smith says most tank water heaters need flushing at least once a year, more often in hard-water areas or if you notice signs of buildup, and that your model's manual has the final word. Rheem also lists flushing as yearly upkeep and suggests about every six months where the water is hard. If your water is hard, check your manual and watch for the signs below.

• Rumbling or poppingSediment at the bottom of the tank is a common cause.
• Less hot water than beforeSediment takes up room and gets between the burner or element and the water.
• Rusty or smelly water, or leaksThese can have other causes too, so a plumber should confirm what's going on.

Before you start: safety

The water in the tank is hot enough to scald. A. O. Smith recommends turning off the heat source and, ideally, waiting a few hours for the tank to cool before you drain it, and wearing thick gloves and safety goggles. Rheem suggests a thermostat setting of 120 to 125 degrees F to limit scald risk in everyday use, which also makes this job safer.

Know which kind of heater you have. An electric heater has a cord or a wire running to a junction box and no flue pipe on top. A gas heater has a gas line, a control valve on the front and a vent pipe on top. Rheem notes that gas lines, burners and electrical parts are best left to a licensed pro, and that some work done yourself can affect the warranty, so read your manual first.

How to flush a tank water heater

1. 1. Turn off the heatElectric: switch off the water heater's breaker. Gas: turn the control dial to OFF. Then wait, ideally a few hours, for the water to cool.
2. 2. Shut the cold water supplyClose the valve on the cold water pipe going into the heater by turning it clockwise.
3. 3. Hook up a hoseScrew a garden hose onto the drain valve near the bottom of the tank and run the other end to a floor drain, outside below the level of the valve, or into a bucket.
4. 4. Open a hot water faucetOpen a hot tap somewhere in the house so air can get in and the tank drains.
5. 5. Open the drain valveOpen it by hand or with a flathead screwdriver. To flush, briefly open the cold supply to stir up sediment, and keep going until the water runs clear with no grit in it.
6. 6. Close the valve and refillClose the drain valve, remove the hose and open the cold supply. Leave the hot tap open until water flows steadily, then run it about 3 minutes more to clear the air.
7. 7. Turn the heat back on lastOnly once the tank is full: switch the breaker back on for electric, or turn the gas control back on and relight the pilot exactly as the label on your heater says. Check the drain valve for drips over the next day.

Test the relief valve while you're there

Every tank has a temperature and pressure (T&P) relief valve, usually on the top or upper side, with a pipe running down toward the floor. Rheem's check is to lift the valve's handle briefly and make sure water flows freely out of the discharge pipe. Hot water comes out, so keep clear of the pipe's end. If the valve doesn't release water, or keeps dripping after you let go, A. O. Smith and Rheem both say to replace it. That's a good job for a plumber. Follow the test steps in your own manual if they differ.

When to call a plumber instead

• The drain valve is cloggedA. O. Smith says to call a plumber if the drain valve won't flow.
• The drain valve drips after you close itOlder drain valves can wear out. Cap the hose thread and call to have it replaced.
• Water at the base of the tankA leak from the tank itself usually can't be fixed. See our guide on when to replace a water heater.
• You smell gas, or the pilot won't relightLeave the gas off and call. Don't keep trying.

Tankless heaters are different

A tankless heater has no tank to drain. It needs descaling instead: a pump circulates a cleaning solution through the heat exchanger to dissolve mineral scale. Rheem says to do it at least once a year, often by a professional, and about every six months with hard water. A. O. Smith notes many kits run for about 45 minutes and that some newer models must be put in a service mode first. Use the kit and the steps your heater's manual calls for, and hire a pro if you're unsure.

Questions people ask