Key takeaways
- Hard water leaves mineral scale that settles at the bottom of a tank as sediment.
- Many plumbers suggest flushing about once a year, and more often in hard water areas.
- Hot water can scald, and electric elements can burn out if powered while exposed, so shut off power or gas first.
- An old heater that has never been flushed is best checked by a plumber before the drain valve is opened.
- Flushing removes sediment but does not soften your water.
What sediment is and why it builds up here
Much of Southwest Ohio has hard water, which carries dissolved calcium and magnesium. When water is heated, those minerals come out of solution and settle as scale. In a tank heater the scale falls to the bottom and forms a layer of sediment. It also coats electric heating elements.
That layer causes real problems. In a gas heater the burner sits under the tank, and sediment acts as insulation between the flame and the water. The heater runs longer, and the bottom of the steel gets hotter. In an electric heater, scale on the elements makes them work harder and shortens their life. You may hear popping or rumbling as water trapped under the sediment boils up through it. Sediment also takes up room in the tank, so you have less hot water, and it can clog the drain valve.
How hard your water is depends on your water supplier and whether you use municipal water or a private well. Your water utility can tell you.
What flushing does and does not do
Flushing means draining water from the bottom of the tank so loose sediment flows out with it. Done regularly, it can reduce noise, keep more of the tank available for hot water, and ease the strain on the burner or elements.
It has limits. Flushing removes only what has already settled. It does not soften the water or stop new scale from forming. It also cannot remove sediment that has hardened into a crust on the bottom of an older tank. If hard water is causing scale on fixtures and in appliances throughout the house, a water softener or other treatment deals with the cause. Our water treatment page explains what a plumber can test and install.
How often to flush
Many plumbers suggest flushing a tank heater about once a year. In hard water areas it is reasonable to flush more often, especially if you hear rumbling or notice less hot water than before. If your water has been softened, once a year is usually enough, and you can check the water coming out to see how much sediment there is.
The most important exception is an older heater that has never been flushed. Sediment can be packed hard, and opening a drain valve that has not moved in years can cause it to clog, snap or leak afterward. If your heater is well past the middle of its life and has never been drained, have a plumber look at it first.
A tankless heater is different. It does not have a tank to drain. Instead the heat exchanger is flushed with a pump and a descaling solution, following the manufacturer's instructions. Most homeowners leave that to a plumber.
How a flush is done
The following is a general outline, not a replacement for your heater's manual. Hot water can scald, so take your time.
- Turn off power at the breaker for an electric heater. For a gas heater, set the gas control to pilot or off.
- Let the water cool for a while so the water you drain is not scalding.
- Connect a garden hose to the drain valve near the bottom of the tank. Run the hose to a floor drain or outdoors where the water will not harm people, plants or a driveway surface.
- Close the cold water supply valve to the heater and open a hot tap somewhere in the house so air can enter.
- Open the drain valve and let the tank drain. If the flow slows or stops, sediment may be blocking the valve.
- Briefly open the cold supply valve to stir the bottom and flush again until the water runs clear.
- Close the drain valve, remove the hose, and open the cold supply valve with the hot tap still open. Let the tank fill until water runs steadily from the tap, then close it.
Only after the tank is full should you restore power or gas. An electric element that is switched on while exposed to air can burn out quickly.
Some tanks have a plastic drain valve that can crack or fail to close fully after use. If the valve drips or will not shut, stop and call a plumber.
When to call a plumber
Call a plumber if the heater is old and has never been drained, if the drain valve is clogged, stuck or leaking, or if you are not comfortable working near gas or electrical connections. Also call if you flush the tank and the water keeps coming out full of sediment, if rumbling returns soon after, or if you see signs of wear such as rust in the hot water or moisture at the base of the tank. The signs of a failing water heater can help you judge what you are seeing.
A plumber can flush the tank, check the anode rod, test the relief valve and advise on whether repair or replacement makes sense. See our water heater service page, or contact us to set up a visit.
Frequently asked questions
Can flushing damage an old water heater?
It can. On an old heater that has never been drained, the drain valve may clog, break or leak after being opened, and loosened sediment can cause new problems. A plumber can assess the heater first and tell you whether flushing is a good idea.
Do I need to flush a tankless water heater?
A tankless heater has no tank to drain, but its heat exchanger collects scale in hard water areas. It is cleaned with a pump and descaling solution, and the manufacturer's instructions set the schedule. Many homeowners have a plumber do this.
Will flushing fix a rumbling water heater?
It can if the sediment is still loose. If the sediment has hardened into a crust, flushing will not remove it and the noise will return. A plumber can tell you whether the heater is worth keeping.
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