A tank water heater is a steel drum with a glass lining, a burner or heating elements, and one sacrificial part whose only purpose is to rust so the tank does not. Over years, two things happen inside it that nobody can see: minerals settle out of the water and build a layer of sediment on the bottom, and the anode rod slowly dissolves. When the sediment gets thick, the heater works harder, makes noise and delivers less hot water. When the anode is gone, the tank itself starts to rust, and a rusted tank is not repairable.
A flush and an anode check are the two maintenance jobs that extend a tank's life, and many tanks are replaced years early because nobody did them. This article explains what each one does, what you can notice yourself, what a plumber does on the visit, and how often it makes sense. Draining the tank and pulling the anode involve hot water, the power or gas supply and fittings that seize; this is a plumber's job.
What sediment does inside the tank
Water carries dissolved minerals, and heating it makes them settle on the bottom of the tank as a layer of sand-like scale. On a gas heater that layer sits between the burner and the water; on an electric heater it buries the lower element, which overheats and fails early. Either way the usable volume shrinks, and water trapped under the sediment superheats and pops, which is the rumbling an old tank makes. Hard water speeds all of this up.
- Rumbling, popping or crackling from the tank while it heats.
- Hot water that runs out faster than it used to, with no change in use.
- Hot water that comes out cloudy or carries grit, especially first thing in the morning.
What the anode rod does and why it runs out
The anode rod is a long rod of magnesium or aluminum that screws into the top of the tank. Because it is a more reactive metal than steel, corrosion attacks it instead of the tank. That is the whole design: the rod is meant to be consumed. The University of Hawaii's building maintenance text recommends inspecting the rod every couple of years and replacing it roughly every five, sooner if it is badly corroded, and water softeners and some water chemistries use it up faster.
Once the rod is gone, the tank itself takes the corrosion. A new rod costs a small fraction of a new heater, but the old rod is often seized in place and needs a long breaker bar to remove, which is why it is a plumber's job.
- Rust-colored hot water while the cold water runs clear is a sign the anode is gone and the tank is corroding.
- A rotten-egg smell only in the hot water is a reaction between the magnesium rod and sulfate in the water. University of Georgia Extension and Penn State Extension both describe swapping the rod for an aluminum or zinc type as the usual fix; removing the rod entirely can void the warranty.
What you can check yourself
Everything you can do safely is done at the tap and from a few feet away from the heater. Do not open the drain valve, touch the temperature and pressure relief valve, remove access panels or turn the thermostat dial. The U.S. Consumer Product Safety Commission recommends a water heater setting of 120 degrees Fahrenheit to prevent scalds, and a plumber can confirm and adjust the setting during the visit.
- Run a hot tap into a clear glass first thing in the morning and look for cloudiness, grit or a rust tint, and compare the smell with the cold tap.
- Listen to the tank while it recovers after a shower.
- Look at the floor around the base for water, rust streaks down the side, or a drip from the relief valve pipe.
- Find the label on the tank and note the manufacture date, model and capacity for the plumber.
What a plumber does during a flush and anode check
The visit is usually an hour or so, longer if the rod is seized or the sediment is heavy. The plumber shuts off the power or gas and the cold supply, drains and flushes the tank until the water runs clear, checks the anode rod and replaces it if it is thin or mostly bare wire, tests the relief valve, and refills the tank fully before restoring power, because an electric element energized in an empty tank fails immediately.
- Drain and flush the tank until the sediment is cleared, sometimes with several fill-and-drain cycles.
- Inspect the anode rod and replace it, choosing magnesium, aluminum or aluminum-zinc depending on the water.
- Test the temperature and pressure relief valve and replace it if it does not reseat.
- Check the burner or elements and the thermostat setting, and set the temperature to about 120 degrees unless you need otherwise.
How often, and when it is too late
A flush once a year is the common recommendation, more often with hard water. The anode should be checked every couple of years and replaced when it is used up. Tankless heaters have no tank or anode, but they need descaling on the maker's schedule instead.
There is a point where maintenance stops helping. A tank that is leaking from the body or badly rusted at the fittings is a replacement, not a flush, and a plumber who finds heavy rust on a first flush of an old tank may advise against disturbing it further. That is a good reason to start the maintenance when the tank is new.
Questions people ask
Can I flush the tank myself with a garden hose?
Many guides describe it, but it involves shutting off the gas or power, dealing with scalding water, a drain valve that often will not close again, and refilling fully before restoring power. A plumber does it routinely and checks the anode and relief valve in the same visit.
My tank is ten years old and has never been flushed. Should I start now?
Ask a plumber to look first. On an older tank, flushing can dislodge sediment that was sealing small leaks, and a seized anode may not come out. Sometimes the right advice is to plan a replacement instead.
Why does my hot water smell like rotten eggs?
Usually a reaction between the magnesium anode rod and sulfate or bacteria in the water. Replacing the rod with an aluminum or zinc type usually fixes it. Removing the rod and leaving it out can void the warranty and shortens the tank's life.
Sources
- University of Hawaii (OER): Building maintenance, section 4.7 water heating (flushing and anode rods) pressbooks.oer.hawaii.edu
- University of Georgia Extension: Removal of hydrogen sulfide and sulfate (water heater anode rods) fieldreport.caes.uga.edu
- Penn State Extension: Common drinking water problems and solutions extension.psu.edu
- U.S. CPSC: Avoiding tap water scalds (Publication 5098, PDF) www.cpsc.gov