USA Expert Plumbing

How to flush a water heater

By USA Expert Plumbing Editorial Team. Updated . 2 min read.

Short answer

Turn off the power or set the gas to pilot, close the cold water supply, attach a hose to the drain valve, open a hot water tap and drain the tank. Briefly open the cold supply to stir up sediment, drain again until the water runs clear, then refill completely before turning the heat back on.

In this guide
  1. Before you start
  2. Step-by-step
  3. If the drain valve clogs
  4. How often to flush
  5. When flushing will not help

Minerals in your water settle to the bottom of the tank as sediment. Over time it makes the heater noisy, slows heating and can shorten the life of the tank. Flushing once a year keeps it under control.

You need a garden hose, a bucket and about an hour. The steps are the same for gas and electric tanks except for how you turn the heat off.

The water is hot. Let the tank cool for a few hours, or route the hose somewhere hot water cannot hurt anyone or damage plants.

Before you start

  • Electric: turn off the breaker. Never power an electric heater while the tank is empty, because the elements burn out in seconds without water around them.
  • Gas: turn the gas control to the pilot or vacation setting so the burner does not fire.
  • Check the drain valve. Plastic drain valves on older tanks can crack when opened. Have a replacement brass valve on hand if yours looks brittle.

Step-by-step

  1. Close the cold water shutoff valve on the pipe entering the top of the heater.
  2. Connect a garden hose to the drain valve and run it to a floor drain, a utility sink or outside, lower than the valve.
  3. Open a hot water faucet somewhere in the house. This lets air in so the tank drains.
  4. Open the drain valve and let the tank drain.
  5. When the flow slows, open the cold water valve for a few seconds to stir up sediment on the bottom, then close it again. Repeat until the water from the hose runs clear.
  6. Close the drain valve and remove the hose.
  7. Open the cold water valve and let the tank refill. Leave the hot faucet open until water flows from it steadily with no sputtering. That means the tank is full.
  8. Turn the breaker back on, or return the gas control to its normal setting.

If the drain valve clogs

Heavy sediment can plug the drain valve. Close the valve, disconnect the hose and briefly open the cold supply to push water back through. If it stays clogged, a plumber can remove the valve and flush the tank through the larger opening.

How often to flush

Once a year works for most homes. With hard water, every six months helps. If you have never flushed an older tank, be aware that disturbing years of sediment can sometimes cause a weak drain valve to leak afterward.

When flushing will not help

If the tank keeps rumbling after a thorough flush, or the water stays rusty, the tank may be near the end of its life. Check its age and compare the cost of a replacement.

Common questions

How long does it take to flush a water heater?

About an hour for most 40 to 50 gallon tanks, plus time to let the water cool first.

Do I need to flush a new water heater?

Yes, starting in the first year. Sediment builds up from day one, and regular flushing keeps it from hardening.

Can flushing a water heater cause a leak?

On an old tank, the drain valve can fail to seal after being opened. The tank itself should not start leaking because of a flush.

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