How often should you flush a water heater?

What sediment actually does

In a gas heater the burner sits underneath the tank, so a layer of sediment sits directly between the flame and the water. Heat has to travel through it, which means the burner runs longer for the same hot water, and the steel at the bottom of the tank runs hotter than it was designed to. That's where the rumbling and popping come from: water trapped under the sediment flashing to steam.

In an electric heater the lower element can end up buried in the sediment instead, which cooks it and shortens its life.

Either way the tank holds less usable hot water than it used to, because part of its volume is no longer water.

The signs there's some

Tank and tankless are different jobs

A tankless heater has no tank to flush, and no sediment layer sitting at the bottom of anything. What it does have is a heat exchanger, and in hard water that scales up. Clearing it is called descaling, it uses a pump and a solution rather than a hose, and the interval depends on your water and the maker's schedule. Don't read a tank flushing interval as a tankless one. They're unrelated.

Why this is often a plumber's job

Physically, a flush is not much more than opening a valve. The reasons to think twice are all about what surrounds it.

The water coming out is scalding. The drain valve on an older heater is often plastic, and a valve that hasn't been turned in a decade sometimes won't seat again afterwards, which turns a maintenance task into an emergency with the water still on. On a gas heater, the control valve and the flue are right there. And on a tank that's well into its second decade, the sediment can be part of what's holding a corroded bottom together, which is why some heaters begin leaking shortly after their first flush in years.

The part that decides how long the tank lasts

Flushing keeps a heater efficient. The anode rod is what keeps it alive. It's a length of magnesium or aluminum suspended inside the tank whose whole purpose is to corrode instead of the steel around it, and once it has been eaten away the tank is next.

Rods typically last a few years, faster on softened water, and replacing one is cheap next to replacing a heater. It's also a real job rather than a look: the rod is threaded into the top of the tank, it's often seized after a decade, and getting at it means draining down and working around the gas control or the elements. Worth asking about at the same visit as a flush, especially on a tank in its second half.

The valve worth recognizing

There's one part of a water heater that isn't about efficiency at all. The temperature and pressure relief valve is what stops a tank becoming a pressure vessel if the thermostat fails, and it has a discharge pipe running from it down toward the floor.

You don't need to touch it, and lifting the lever on an old one is how people discover it won't reseat. What's worth doing is looking. The pipe should be dry, unobstructed, and running downward the whole way, ending either a few inches above the floor or above a floor drain with a visible gap between the pipe and the drain. That gap is the point: it has to be able to dump water where somebody will see it. A capped, plugged or threaded end, a section that runs uphill, any drip, or a rust trail below it is a call rather than a note for next month.

The check that costs nothing

Whatever you decide about flushing, two things are worth knowing and take a minute. The first is the age of the heater, which is usually encoded in the serial number on the data plate. The second is where the shutoff is, both for the water and for the fuel, and whether you could find it in the dark.

Most people look at their water heater properly for the first time on the day it fails. Knowing its age and where the shutoffs are before then saves a lot of scrambling.

The job nothing else reminds you about

A water heater asks for nothing until it stops. There's no dust to see, no filter to look gray, no smell. It's exactly the kind of job that only ever happens if something remembers it for you.

A yearly task in Patina solves the part you can't: remembering. It also keeps the history, so next year you're reading a date rather than trying to recall one.