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Water heater anode rod: how to check and replace it

Most tank water heaters are steel tanks with a glass lining on the inside. The lining is never perfect, so manufacturers add an anode rod: a metal rod that corrodes on purpose so the exposed steel of the tank doesn't. Once the rod is used up, the tank itself starts to corrode, and a corroded tank eventually leaks. Checking the anode every few years and replacing it when it's spent is one of the few things that can actually extend a tank's life.

Safety first. Turn off the heat source before you start, and remember the water inside can be hot enough to scald. Never turn power back on to an electric heater until the tank is completely full again, or the elements can burn out. If you have a gas heater and aren't comfortable with the gas control, call a plumber.

Where the anode rod is

Look at the top of the heater. On many models the anode has its own hex head (often under a plastic cap or a little insulation) on top of the tank. On others it's built into the hot water outlet fitting, called a combination or outlet anode, which means you have to disconnect the hot water line to reach it. Some tanks have two rods. Your owner's manual or the manufacturer's website will show which layout you have; search the model number on the label.

Signs the anode may be worn out

The heater is several years old and the rod has never been checked.
Rusty or discolored hot water (cold water is clear).
A rotten egg or sulfur smell in the hot water only. That's often a reaction between the anode and bacteria in the water; see hot water smells like rotten eggs.
You use a water softener. Softened water tends to eat through anodes faster, so check more often.

What you'll need

A breaker bar and the right socket for the hex head (commonly 1-1/16 inch, but check yours)
Thread sealant tape or pipe dope rated for potable water
A replacement anode that fits your heater (see below)
A garden hose and bucket for partially draining the tank
A helper, or a strap, to keep the heater from twisting while you loosen the rod

Step-by-step: inspect and replace

1. Turn off the heat and the cold water

Electric: switch off the breaker. Gas: set the control to OFF or PILOT. Then close the cold water supply valve to the heater.

2. Relieve pressure and drain a few gallons

Open a hot water faucet in the house to relieve pressure. Hook a hose to the drain valve and let out a few gallons so the water level drops below the top of the tank. You don't need to drain it completely just to change the anode.

3. Break the rod loose

Put the socket on the hex head and turn counterclockwise. Anodes are often very tight after years in place, so a long breaker bar helps. Brace the tank so it doesn't twist, since twisting can stress the water and gas connections. If it truly won't budge, stop before you damage the tank or fittings; a plumber can use an impact tool.

4. Pull it out and look at it

Lift the rod straight up. A healthy rod still has most of its metal. A spent rod is thin, pitted, crusted, or worn down to the bare steel core wire in places. A common rule of thumb is to replace it when the core wire is showing along a good part of its length or the rod is much thinner than a new one. If it still looks solid, you can reinstall it with fresh sealant and check again in a year or two.

5. Install the new rod

Wrap the threads of the new rod with sealant tape or apply pipe dope, lower it in, and tighten it snugly. If there isn't enough ceiling clearance to drop a rigid rod in, use a flexible segmented anode, which bends as it goes in.

6. Refill and restart

Close the drain valve, open the cold supply, and leave the hot faucet open until it runs a steady stream with no sputtering. Check the anode fitting for leaks. Then turn the breaker back on, or turn the gas control back to ON (relighting the pilot if needed per the label instructions).

Choosing a replacement anode

Magnesium: the most common original rod. It protects the tank well, but it's the type most likely to contribute to a sulfur smell with certain water.
Aluminum or aluminum/zinc: often recommended for hard water, and the zinc blend is a common fix for rotten egg odor.
Powered (impressed current): uses a small electrical current instead of a sacrificial metal, so it doesn't wear away. It costs more up front and needs an outlet nearby, but it's a popular choice when smell is a recurring problem.

Match the thread size and length to your heater, and check your warranty terms before swapping types. Don't remove the anode and leave it out to stop a smell: without it the tank will corrode much faster.

When to call a plumber

The rod is seized and won't turn without the tank moving.
Your heater uses a combination anode in the hot outlet and you're not comfortable disconnecting the plumbing.
Water is leaking from the tank body itself. A new anode won't save a tank that's already rusted through; see water heater leaking and how long water heaters last.

Tip: While the tank is partly drained, it's a good time to flush out sediment too. See how to flush a water heater.