Start with the label, not the symptoms
Before you go hunting for signs, find out how old the tank is. The rating label is a sticker on the side near the top, and the serial number usually hides the month and year of manufacture. Most tanks last 8 to 12 years. A tank at 11 that is making a noise is a different animal from a tank at 5 that is making the same one.
Once you know the age, the symptoms make more sense. Some are early warnings you can act on at your own pace. A couple mean the tank has already given up, and the clock is running.
Water where it shouldn't be
If there's water at the base of the tank, that's the one sign not to put off. Check where it's coming from. A drip from the pipe fittings on top, or from the relief valve, can be a small fix. Water seeping from the bottom or the seam of the tank means the steel has corroded through from the inside, and a tank can't be patched once that happens. Turn off the cold-water valve above it and call.
A heater in the garage will leak onto a slab and you may notice a damp patch and not much else. A heater in an attic or an upstairs closet does a lot more damage, because the water comes down through the ceiling. That's why an old tank above a living space deserves a replacement date even if it still works. The attic installation page explains the pan and drain line that stop this becoming a ceiling repair.
Noises
A rumble or popping sound while the heater runs is sediment. Minerals settle on the bottom, water trapped under the layer boils into steam bubbles, and the bubbles pop on their way out. It means the tank is working harder than it should. If the tank is young, a flush can fix it. If it's old and has never been flushed, the layer may have set hard, and the steel under it has already taken a beating.
A ticking sound that fades is usually just pipes expanding as they warm and isn't a problem. A hiss from the tank, or steam from the relief valve pipe, is something else: switch the heater off and call. The post on popping and rumbling noises goes through them in more detail.
A five-minute test with a white bucket
You can learn a lot from a clean white bucket. Run a few gallons of hot water into it from a tap near the heater, then a few gallons of cold into a second one, and look at both in good light.
If the hot bucket has brown or orange tint and the cold one is clear, the discoloration is coming from the tank. Let the bucket stand for a few minutes. Grit or flakes on the bottom are sediment or scale. A metallic smell points to rust. A rotten egg smell in the hot water only usually points to bacteria and the anode rod, and a flush and a new rod often fix it.
If both buckets are brown, the problem is in the supply pipes or the water main, and the heater isn't to blame. That's worth knowing before you spend any money.
What the water looks like
Rust-colored hot water, when the cold side is clear, points at the tank. If both are discolored, the problem may be in the supply pipes instead, so test both before drawing conclusions. Hot water that smells of rotten eggs often comes from bacteria reacting with the sacrificial anode rod, and it can be fixed without replacing the tank, but a tank that is both smelly and old is worth another thought.
Water that is never quite hot enough, or that runs out faster than it used to, can mean a failed element on an electric tank, a gas valve or thermocouple trouble on a gas one, or a thick layer of scale insulating the water from the heat. A plumber can usually tell which in a few minutes.
Gas and electric signs that are easy to miss
On a gas heater, look at the flame. It should be steady and blue. A yellow, lazy flame, a pilot that keeps going out, or soot around the vent opening are all reasons to have it looked at, and the soot in particular should not be ignored. If you ever smell gas, leave the house and call your gas utility or 911 from outside.
On an electric heater, a breaker that trips when the heater runs points to a failing element or a wiring fault. It's not something to reset five times and forget.
On either type, a relief valve that drips after the heater cycles points at high pressure, which is often a failed or missing expansion tank. That one is a smaller repair, and you can read why in the post on expansion tanks.
A monthly look that takes two minutes
You don't need to be a plumber to catch most trouble early. Once a month, walk by the heater and look at it.
- Is the floor around the base dry, with no rust ring or stain?
- Is the relief valve pipe dry, with no drip or mineral crust at its end?
- Are the pipes at the top free of green or white deposits?
- Does the heater sound the way it always has?
- On a gas heater, is the flame steady and blue, and the vent free of soot?
- Does anything smell off?
If an answer changes from one month to the next, look closer. A heater that you know well will tell you when something is different, and it's rarely a shock.
Repair or replace
A simple rule of thumb that holds up well. If the tank is under about eight years old and the problem is a part, such as an element, a thermostat or a valve, a repair is often sensible. If the tank is leaking, replace it. If it's past ten years and showing more than one of the signs above, put the money toward a new one, because you'd be paying to keep a failing tank alive.
There's no shame in being told to repair a young tank, and a plumber who will only ever sell you a new heater isn't giving you the full picture.
What not to do
Don't turn the temperature up to fix lukewarm water. It won't help if the tank is full of sediment or an element has failed, and it raises the scald risk. Don't ignore a relief valve that drips, or cap its discharge pipe, because that's the valve's way of releasing pressure and blocking it is dangerous. Don't keep resetting a tripped breaker. And don't put off a leak at the base of the tank because it's small: it never gets smaller.
Why it's worth acting before it fails
A tank that fails without warning can release a full 40 or 50 gallons in minutes. In a garage that's a mop and a bad afternoon. Above a finished room it's drywall, flooring and mold, none of which is cheap. Replacing on your own schedule also means you get to choose the size and the type, instead of taking whatever fits on the day.
If you want to see what a swap involves and costs, the water heater replacement page lays it out, and the cost guide covers the estimated ranges.
A traffic-light guide
Here's a simple way to decide how fast to act.
- Red, act today: water at the base or seams of the tank, steam or a hiss from the relief valve pipe, a smell of gas, soot at a gas heater's vent, or a breaker that keeps tripping. Turn off the water and the power or gas and call.
- Amber, act within a few weeks: rust-colored hot water, a relief valve that drips, rumbling that's getting louder, or hot water that runs out much sooner than it used to. Book a look and ask for a price.
- Green, keep an eye on it: a tank over eight years with no symptoms, or a tank that's a little noisy but flushes clean. Start a yearly routine and write down the age.
The colors are guidance, not a rule. A tank above a ceiling moves up a color for any of these, because the cost of being wrong is so much greater.
A word on gas heaters and carbon monoxide
Gas heaters make carbon monoxide when they don't burn cleanly, and the gas has no smell. A heater that has started to soot up, with a yellow flame or black marks around the vent opening, deserves attention for that reason alone. A carbon monoxide alarm in the hallway near the heater and bedrooms is cheap protection, and it should be checked when you change the batteries.
If the alarm sounds, get everyone out of the house and call for help from outside. Don't try to find the cause first. A failing gas heater is a repair-or-replace question most days, and on the rare day it's a safety one, it's worth knowing which.