How to flush a tank water heater

Minerals in your water settle to the bottom of a tank water heater as sediment. Over time that layer insulates the water from the burner or lower element, wastes energy, causes popping sounds and can shorten the life of the tank. Flushing removes much of it. Most manufacturers recommend doing it about once a year, and more often if you have hard water.

Before you start

• Water in the tank can be scalding. Either wait a few hours after shutting off the heat, or be careful where the hose discharges and wear gloves.
• Use a hose that can handle hot water, and run it to a floor drain, a utility sink or outside to a spot that can take the water.
• Look at the drain valve. Many tanks have a plastic valve at the bottom. If it has never been opened, it may be stiff, and plastic valves can crack. Have a replacement brass valve handy if yours looks fragile, or plan to call a plumber.
• Find your manual, or the label on the tank. It may include model-specific steps that override anything here.

What you need

• Garden hose (hot-water rated if you won't let the tank cool)
• Flat screwdriver or pliers for the drain valve
• Bucket and towels
• Gloves

Step-by-step

1. Turn off the heat. On an electric heater, switch off its breaker at the panel. This matters: if the elements turn on while the tank is empty, they can burn out in seconds. On a gas heater, turn the control knob to "Pilot" or "Vacation" (or "Off" if you're comfortable relighting it).
2. Close the cold water supply valve on the pipe that feeds the top of the tank.
3. Open a hot water faucet somewhere in the house, ideally on a floor above the heater. This lets air in so the tank drains freely instead of glugging.
4. Attach the hose to the drain valve near the bottom of the tank and run it to your drain point.
5. Open the drain valve and let the tank empty. Expect cloudy or gritty water at first. A 40- or 50-gallon tank can take 20 minutes or more.
6. Flush. With the drain valve still open, open the cold supply valve for a minute or two. The incoming water stirs up sediment that the drain alone left behind. Close it, let it drain, and repeat until the water runs clear.
7. Close the drain valve and remove the hose. Check that the valve isn't dripping.
8. Refill. Open the cold supply valve fully. Leave the hot faucet open until water flows steadily without sputtering. That means the air is out and the tank is full.
9. Turn the heat back on. Restore the breaker on an electric unit only after the tank is full. On a gas unit, return the knob to its normal setting (relight the pilot first if you turned it off).
10. Check after an hour that the water is heating and nothing is leaking around the drain valve.

If the drain valve clogs

Large flakes of sediment can block the valve. Closing and reopening it a few times sometimes clears it. Briefly opening the cold supply can push water back through the valve and dislodge a clog. If the tank hasn't been flushed in many years and the flow stops entirely, don't force it. A plumber can replace the valve with a full-port version that passes sediment more easily.

While you're there: the anode rod

Most tanks have a sacrificial anode rod that corrodes so the steel tank doesn't. Checking it every few years and replacing it when it is mostly eaten away can add years to the tank. Removing it takes a large socket and some force, and on some models clearance above the heater is tight, so many homeowners leave this part to a plumber.

What about tankless heaters?

Tankless water heaters don't hold sediment the same way, but they do build up scale in the heat exchanger. They are "descaled" by circulating a mild acid (often white vinegar or a manufacturer's solution) through service valves with a small pump. Follow your model's manual, since the procedure varies.