How often should I flush my water heater?

Most tank water heaters need a flush once a year to clear sediment from the bottom. Homes on well water or in hard-water regions should flush every six months. Tankless heaters need an annual vinegar descale instead. Skipping it shortens the heater’s life and cuts efficiency.

How do I know it’s time?

The first sign is sound. A heater that pops, rumbles, or kettles when it runs has steam bubbles escaping through a sediment layer at the bottom. The bigger the layer, the louder the noise.

The second sign is recovery time. The fourth shower in a back-to-back round shouldn’t be cold. If it is, the burner can’t keep up because sediment is insulating it from the water.

Rusty or cloudy water at the hot tap is the third sign and the most urgent. That’s the anode rod failing, the tank lining corroding, or both. At that point a flush is still worth doing, but a plumber should look at the anode rod the same week.

If none of those are happening but you also can’t remember the last flush, the answer is “now.” Most water heaters get flushed exactly never until something goes wrong.

How do I flush a tank water heater?

Plan on 30 to 45 minutes. The water coming out is hot enough to burn, so route the hose to a floor drain or outside, not into a bathtub.

First, turn off the heat. On a gas heater, set the dial on the front to “Pilot” or “Vacation.” On an electric heater, flip the dedicated breaker in the panel. Heating elements that fire dry will burn out in seconds.

Next, relieve the pressure. Close the cold-water supply, the valve on the cold-side pipe at the top of the heater (usually the right-hand inlet). Then open a hot tap somewhere in the house, any sink or tub. That breaks the vacuum so the tank can drain.

Now drain it. Hook a garden hose to the drain valve, the spigot near the floor on the front of the tank. Run it to a floor drain, a utility sink, or out a basement door to the yard. Open the drain valve. Water comes out hot and cloudy at first; five to fifteen minutes later it runs clear. That’s done. If the drain valve clogs (common on the plastic ones), close the cold supply and use a wet/dry vacuum at the valve, or call a plumber.

Last, refill. Close the drain, remove the hose, and reopen the cold supply. Leave the hot tap open until water flows steady (no air sputter). Then close the tap and restart the heat.

While the tank is empty is also the right window to check the anode rod, the magnesium or aluminum rod that screws into the top and corrodes in place of the tank lining. If the wire core is exposed for more than 6 inches, replace it. A new rod runs $20 to $40; a plumber doing the swap runs $200 to $300.

What about a tankless water heater?

Tankless units don’t store water, so there’s no sediment layer to drain. But the heat exchanger inside scales up with the same minerals over time, and that scale chokes flow and shortens the unit’s life. The annual job is a vinegar descale, not a flush.

The setup needs a small submersible pump, two short washing-machine hoses, a 5-gallon bucket, and three to four gallons of plain white vinegar. Most tankless units installed after 2010 have a pair of isolation valves (one red, one blue) on the cold and hot service ports for exactly this job.

The process: shut off the gas or breaker, close both isolation valves, attach the hoses (pump out to cold service port; hot service port to bucket), fill the bucket with vinegar, and run the pump for 45 minutes. Drain the vinegar, refill the bucket with fresh water, and rinse for 5 to 10 minutes. Close the service ports, reopen the isolation valves, restart power.

Manufacturer instructions vary by unit. Rheem, Rinnai, and Navien each publish flush procedures specific to their models, and the manual that came with the unit is more authoritative than any general write-up.

What goes wrong if I skip it?

A tank water heater on InterNACHI’s life-expectancy chart runs 8 to 12 years. Sediment cuts that short. The bottom inch of the tank running hotter than the rest accelerates anode-rod failure, which accelerates lining corrosion, which ends in a leak. A leaky 50-gallon tank in a finished basement is a plumber, a water-mitigation crew, and weeks of drywall and floor repair before the new heater is even installed. The first move when you find the leak is closing the main water shutoff.

Efficiency drops too. Water heating is about 18% of a typical home’s energy bill according to the Department of Energy. A heater fighting through a thick sediment layer runs longer per cycle to deliver the same hot water. Over a year, that’s real money on the gas or electric bill.

Tankless skipping is faster and meaner. A scaled-up heat exchanger can fail in 4 to 6 years instead of 20+. The replacement runs $1,500 to $3,500. The annual descale is $0 in DIY parts past the first one, or about $150 from a plumber.

A note from Kempt

Kempt remembers when the water heater was installed, where the cold-side valve and the drain spigot are, and how long since the last flush. When the year is up, the Sunday Brief on your phone names the date, the location, and the size of the tank. That’s the part homeowners forget. Kempt doesn’t.