How to flush a water heater

Minerals in the water settle at the bottom of the tank as sediment. Draining it once a year keeps the heater quieter and more efficient, and it can add years to the tank’s life. It takes about half an hour and a garden hose.

Written by the developer of Gable · Published 13 September 2026

Short answer

Once a year: turn the heater off (breaker off for electric, pilot or vacation for gas), close the cold-water supply, attach a hose to the drain valve and run it outside or to a floor drain, open a hot tap, then open the drain valve and let it run until the water comes out clear. Close the valve, refill the tank completely — until a hot tap runs without spitting air — and only then turn the power back on.

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Why sediment is a problem

Hard water carries calcium and magnesium. When it’s heated, some of it drops out and settles on the bottom of the tank. That layer sits between the burner or element and the water, so the heater burns more energy to do the same job. Water trapped under the scale boils in small bursts, which is the popping or rumbling sound old heaters make. Over the years the overheated bottom of the tank corrodes. When it finally leaks, the heater has to be replaced, often by flooding the room first.

Gable’s library puts it as “30 minutes vs a $1,200–2,500 replacement several years early, plus the flood when it fails.”

Before you start

The water is hot enough to scald. Turning the heater off a few hours earlier, or the night before, lets it cool.
You need a garden hose that reaches a floor drain or the outside, and ideally a bucket to check the water.
This guide is for tank heaters. Tankless heaters aren’t flushed this way. They’re descaled with a pump and vinegar or a descaling solution through their service valves, following the manual.

Step by step

Turn off the heat source. Electric: switch off the breaker. Heating elements burn out in seconds if they come on in an empty tank, so this step isn’t optional. Gas: turn the control knob to Pilot or Vacation.
Close the cold-water supply valve on the pipe going into the top of the tank.
Connect the hose to the drain valve near the bottom of the tank and lay the other end somewhere lower than the valve: a floor drain, or outside down a slope.
Open a hot tap somewhere in the house, ideally on an upper floor. This lets air in so the tank drains instead of holding a vacuum.
Open the drain valve. The first water will be cloudy or carry grit. Let the tank empty.
Stir up what’s left. With the drain still open, open the cold supply for a few seconds, then close it again, and repeat a couple of times. The bursts of water loosen the sediment. Drain into a bucket to check: when it comes out clear, you’re done.
Close the drain valve and take the hose off.
Refill completely. Open the cold supply fully and leave the hot tap you opened running. When it gives a steady flow with no sputtering air, the tank is full. Close the tap.
Turn the heat back on: breaker on, or the gas knob back to its temperature setting. Hot water will take an hour or so to return.

If something goes wrong

Water barely trickles out. Sediment is blocking the valve. Close it, remove the hose, and open the valve again in short bursts to clear it. If that doesn’t work, stop and call a plumber rather than forcing it.
The drain valve drips after closing. Plastic drain valves on older tanks often don’t reseal. A brass hose cap on the outlet stops the drip for now. Replacing the valve is a small plumber’s job.
The tank has never been flushed in ten-plus years. Flushing a very old tank can dislodge scale that was plugging a weak spot, and the tank can start leaking. Many plumbers suggest leaving very old, never-flushed tanks alone and budgeting for a replacement.

Two more checks while you’re there

Test the pressure-relief valve (yearly)

The temperature-and-pressure relief (T&P) valve is the lever near the top of the tank with a pipe running down toward the floor. It’s the tank’s last safety device. With a bucket under the end of the pipe, lift the lever for a few seconds: hot water should rush out and stop when you let go. If nothing comes out, or it keeps dripping afterwards, have the valve replaced.

Check the anode rod (every few years)

The anode is a metal rod screwed into the top of the tank. It corrodes on purpose so the steel tank doesn’t. Gable’s library schedules a check every three years: once the rod is worn thin or coated in scale, a new one costs a small fraction of a new heater. It takes a large socket and some force, and plenty of people hand this one to a plumber.

How often?

Once a year suits most homes. With very hard water, or if you already hear rumbling, every six months is reasonable. Always follow what the heater’s manual says: some manufacturers make yearly flushing a condition of the warranty.

Know when the tank was last flushed

Gable adds the flush, the pressure-relief valve test and the anode check to your plan when your house has a tank water heater, then reminds you and keeps the log. No account needed.