How to Flush a Water Heater (and Why Hard Water Makes It Essential)

Quick answer: To flush a tank water heater, turn off the gas or power, close the cold water supply, attach a hose to the drain valve, open a hot tap, drain the tank, then briefly open the cold supply to stir up and rinse out the remaining sediment before refilling. Flush about once a year, or every six months with hard or well water. Tankless heaters need descaling instead, usually once a year. If the drain valve is old or you're unsure about gas controls, let a professional handle it.

Sediment is the quiet killer of water heaters. Minerals dissolved in your water, mostly calcium and magnesium, settle to the bottom of the tank as the water heats. Over time that layer hardens, takes up space and insulates the water from the burner. Homes on wells and in hard-water areas, common in inland and rural towns like Plumsted, Millstone, Colts Neck and Shamong, see it build up fastest.

Signs your water heater needs flushing

• Rumbling, popping or crackling sounds while it heats
• Running out of hot water sooner than you used to
• Cloudy or gritty hot water
• Higher energy bills for the same hot water use
• A drain valve that drips sediment

What you'll need

• A garden hose long enough to reach a floor drain or outside
• A bucket
• A flathead screwdriver (some drain valves need one)
• Gloves and eye protection (the water is hot)

How to flush a tank water heater, step by step

1. Turn off the heat source. On a gas heater, set the control to "Pilot" or "Off." On an electric heater, switch off the breaker. Never heat an empty tank.
2. Let it cool if you can. Waiting an hour or two (or running hot water first) reduces the scald risk.
3. Close the cold water supply valve on the pipe entering the top of the tank.
4. Attach the hose to the drain valve near the bottom and run it to a floor drain, sump or outside below the level of the valve.
5. Open a hot water faucet somewhere in the house. This lets air in so the tank drains.
6. Open the drain valve. Let the tank drain. The first water out is often the dirtiest.
7. Stir it up. When the tank is mostly empty, briefly open the cold supply valve a few times. The rush of water stirs up sediment so it flushes out. Repeat until the water runs clear.
8. Close the drain valve and remove the hose.
9. Refill. Open the cold supply valve fully. Leave the hot faucet open until water flows steadily without sputtering, which means the tank is full and air is out.
10. Turn the heat back on. Relight or set the gas control back to the temperature setting, or switch the breaker on.
11. Check for leaks at the drain valve over the next few hours.

Tips and cautions

• If the drain valve clogs, sediment may be blocking it. Gently opening and closing it can help, but don't force it.
• Plastic drain valves on many heaters are fragile. If yours is old or cracked, consider having it replaced with a brass valve.
• Old tanks that have never been flushed may have heavy, hardened sediment. Flushing can disturb it and occasionally reveal a weak tank or drain valve. Have a professional look first.
• Check the anode rod every few years. It's a sacrificial rod that protects the tank; once it's used up, the tank starts to corrode.

Descaling a tankless water heater

Tankless units don't collect sediment the same way, but minerals form scale inside the heat exchanger, reducing flow and efficiency and triggering error codes. Descaling typically involves:

1. Turning off the gas and power
2. Closing the isolation valves on the service ports
3. Connecting a small pump and hoses to circulate a descaling solution (often food-grade vinegar or a manufacturer-approved solution) through the heat exchanger for about an hour
4. Flushing with clean water
5. Cleaning the inlet water filter screen
6. Restoring water, gas and power, and checking for codes

Most units have isolation valves for this purpose. If yours doesn't, a plumber or technician can add them.

How often should you flush?

Other ways to deal with hard water

• A water softener reduces scale and sediment throughout the house, protecting the water heater, boiler, fixtures and appliances.
• Scale-reduction filters for tankless units help in moderate cases.
• Lower water temperature: scale forms faster at higher temperatures, so around 120°F is both safer and easier on the heater.

When to call a professional

• The heater is old and has never been flushed
• The drain valve leaks, won't open or won't close
• You smell gas or the pilot won't relight
• Rumbling continues after flushing
• The tankless unit shows scale or flow error codes

Flushing doesn't fix a corroded tank. If you see rust in the hot water or moisture at the base, read water heater leaking and water heater lifespan.

We'll take care of it

We flush tanks, descale tankless units, replace anode rods and drain valves, and repair or replace water heaters across Ocean, Monmouth, Burlington and Middlesex counties. Book online or learn more about our water heater services.