How often should you flush a water heater?

Short answer

Flush the water heater once a year. If you have hard water, do it every 6 months, because the minerals that settle out of hard water pile up on the bottom of the tank faster. Flushing means draining water out through a garden hose until it runs clear, which takes the sediment with it. Expect it to take 30 to 60 minutes, and most of that is waiting for the tank to drain.

Why it matters

Every gallon that enters the tank carries dissolved minerals. When the water sits and heats, some of those minerals fall out and settle as a layer of sand and flakes on the bottom. That layer grows a little every day.

On a gas heater, the burner sits under the tank, so it has to heat through the sediment before it heats the water. The sediment acts like a blanket. The burner runs longer, you pay for fuel that mostly warms the sediment, and the steel under it gets hotter than it was designed for. That is how a tank cracks early. On an electric heater, the lower heating element ends up buried in sediment and burns out. Either way, the sediment also takes up space that used to hold hot water, so showers run cold sooner. A yearly flush clears it before any of that happens.

When to do it more often

Hard water is the main reason to flush every 6 months instead of once a year. You probably have hard water if you see a white crust around faucets and showerheads, spots on glasses out of the dishwasher, or soap that won't lather well.

If you just moved in and nobody can tell you when the tank was last flushed, do it now rather than waiting for the yearly date. And if the tank starts rumbling before its next flush is due, that is the sediment telling you to move the date up.

What you need

A garden hose, long enough to reach a floor drain, a utility sink or the driveway
A bucket, to catch the first draw and check the water
A flathead screwdriver, because many drain valves have a slotted stem instead of a handle

Steps at a glance

Turn the heater off: set a gas control to the pilot or off position, or switch off the breaker for an electric unit.
Close the cold water supply valve on top of the tank.
Screw the hose onto the drain valve near the bottom and run it to a drain or outside.
Open a hot tap somewhere in the house so air can get into the tank as it empties.
Open the drain valve and let the tank empty, then open the cold supply in short bursts to stir up what's left, until the water from the hose runs clear.
Close the drain valve, take the hose off, and let the tank refill until the open hot tap runs in a steady stream with no spitting.
Turn the heater back on.

You're done when the hose runs clear and the tap upstairs gives a steady, air-free stream of water.

Signs it's overdue

Rumbling or popping from the tank: water trapped under the sediment boils and bursts through it.
Cloudy hot water: sediment stirred up and carried to the taps.
Less hot water than before: the sediment is taking up room that used to hold hot water.
Sediment at the end of the hose: when you do drain it, sand or flakes come out instead of clear water.

DIY or pro

This is a moderate job that most people can do with the tools above. There is no plumbing to take apart, just valves to turn and a hose to connect. The hard part is patience while the tank drains.

Call a plumber if the drain valve is stuck or crumbles when you try it, if the water keeps running brown long after it should have cleared, or if you would rather not relight a gas pilot. A plumber charges $100 to $200 and will usually look over the relief valve and the anode rod while the tank is open.

Safety

Gas: turn the gas control to pilot or off before you drain, and don't turn the burner back on until the tank is full. If you smell gas at any point, stop, leave the room, and call your gas company.
Hot water under pressure: the water that comes out of the drain valve is hot enough to scald. Let the tank cool for a while first, or run hot taps in the house to use up the hottest water. Open a hot tap before you open the drain so pressure can't build in the tank, and don't lift the relief valve as a shortcut for letting air in.
Electric: switch off the breaker before you start, and don't switch it back on until the tank is full. An element that heats in an empty tank burns out in moments.

FAQ

Can I flush a water heater myself?

Yes, if you can turn a valve and connect a garden hose. The job is more about waiting than skill, and the steps above are the whole of it.

What happens if you don't flush a water heater?

Sediment keeps building until the burner or element overheats and the tank fails early. Along the way you get less hot water and higher bills.

Does a tankless water heater need flushing?

Not in the same way. A tankless unit has no tank to hold sediment, but scale builds up inside its heat exchanger, so it needs descaling on its own schedule instead.

How long does it take to flush a water heater?

Plan on 30 to 60 minutes. Most of that is the tank draining and refilling, so you can do other things while you wait.

Is it safe to flush a water heater that hasn't been flushed in years?

Usually, but go gently. A thick layer of old sediment can clog the drain valve, and on a tank near the end of its life, the sediment may be plugging small leaks. If the tank is old and has gone years without a flush, have a plumber do the first one.