Flushing a Tank or Tankless Water Heater: Why and How Often

Plumbing · 4 min read · By Rodney Jessen · Published November 6, 2026

Every gallon of water that passes through your water heater carries a little dissolved mineral with it. Heat the water and some of that mineral drops out as sediment or hardens into scale. In a tank heater it settles on the bottom. In a tankless unit it coats the inside of the heat exchanger. Either way it builds up quietly, and the heater keeps working around it until one day it does not.

What sediment does inside a tank heater

A layer of sediment on the bottom of a gas tank heater sits right between the burner and the water. The burner has to heat the sediment before it heats the water, so the unit runs longer, uses more gas and the steel bottom gets hotter than it was designed for. That overheating is what shortens the life of the tank.

On an electric tank heater the sediment can bury the lower heating element, which overheats and burns out, leaving you with lukewarm showers.

What scale does inside a tankless heater

A tankless heater has no tank to collect sediment, but it heats water very quickly through a narrow heat exchanger. That is exactly where scale likes to form. As the passages narrow, flow drops, the unit works harder, and most models eventually show an error code and shut down to protect themselves.

Most tankless manufacturers call for a descaling flush on a regular schedule, and many tie their warranty to it.

How often to flush on the Eastside

Water in Bellevue and most Eastside cities is on the softer side, which helps. It does not mean you can skip flushing, only that the schedule can be a little more relaxed than in a hard-water area:

• Tank water heater: flush once a year. If you have a water softener or very soft water, every two years is usually fine.
• Tankless water heater: descale once a year, or follow the interval in your owner's manual if it says otherwise.
• Homes on a private well: check the manual and lean toward more often, since well water often carries more mineral.

If a heater has gone many years without a flush it is still worth doing, but heavy, hardened sediment may not all come out, and a drain valve that has not been opened in a decade can refuse to close again. That is a good reason to have a plumber do the first one.

Signs a flush is overdue

• Popping, rumbling or crackling from a tank heater as it heats. That is water boiling up through the sediment layer.
• Less hot water than you used to get, or hot water that runs out faster.
• Cloudy or gritty water from the hot taps, or a sandy residue in faucet aerators.
• A tankless unit that cycles, loses temperature, or shows a maintenance or flow error code.
• Higher gas or electric use with no change in habits.

What the job involves

For a tank heater: turn off the power or set the gas valve to pilot, shut the cold supply, connect a hose to the drain valve, open a hot tap in the house to let air in, and drain the tank. Short bursts of cold water stir up the bottom and push more sediment out. When it runs clear, close the drain, refill the tank completely, and only then restore power or gas. Turning an electric heater on with air in the tank destroys the elements in seconds.

For a tankless heater the job is a descaling loop. The unit is isolated with its service valves, a small pump circulates white vinegar or a manufacturer-approved descaler through the heat exchanger for about an hour, and the unit is rinsed with fresh water. If yours was installed without service valves, a plumber can add them.

From our job log

• Mercer Island, Apr 2026: Flushed a water heater.
• Bothell, Nov 2025: Flushed a tankless water heater.

These are real OutToday jobs from our records. No customer names or addresses are shown.

When to call a professional

A routine flush on a tank heater is a fair weekend project if you are comfortable with a hose and a shutoff valve. Call a plumber instead if the drain valve is plastic and stiff, the heater is more than ten years old and has never been flushed, you see rust in the drain water, or the relief valve drips after you refill. Those can be signs the heater is nearing the end of its life, and a professional can tell you whether a flush is worth it or replacement makes more sense.

For tankless units, having the first descaling done professionally is a good way to learn the routine, confirm the service valves are in place and have the filter checked at the same time. OutToday has serviced water heaters on the Eastside since 1989, and an annual flush is the simplest appointment we make all year. If yours is overdue, call (425) 615-5000 and we will get it back on schedule.