1. No hot water, or not enough of it
This is the most obvious sign, and usually the one that finally prompts a call. On a gas water heater, a total loss of hot water is often traced to a pilot light that's gone out or a failed thermocouple, which is the small safety sensor that keeps the gas valve open. On an electric unit, it's more often a blown upper heating element or a tripped high-limit switch. If you're only running out of hot water faster than you used to rather than losing it entirely, that's usually a sign of sediment buildup reducing the tank's effective capacity, or a failing lower element that isn't finishing the job.
2. Rusty, brown, or discolored hot water
Discolored water coming specifically from the hot tap, and not the cold, usually points to the tank rather than the plumbing. The most common cause is a worn-out anode rod. This sacrificial rod is designed to corrode instead of the tank, and once it's used up, the tank itself starts to rust from the inside. Caught early, replacing the anode rod is a straightforward fix. Left alone, it can lead to actual tank corrosion, which isn't repairable and generally means replacement.
3. Popping, rumbling, or banging noises
A water heater that's suddenly noisier than it used to be is almost always dealing with sediment. Minerals in the water settle at the bottom of the tank over time, and as the burner or heating element heats the water underneath that layer, small pockets of steam form and collapse, creating the popping or rumbling sound. It's not dangerous in the short term, but it does mean the unit is working harder than it should be, and a tank flush usually resolves it.
4. Water pooling around the base of the unit
Any standing water near a water heater deserves a closer look. Sometimes it's something simple, like a loose drain valve or a fitting that's worked itself loose over time. Other times it's condensation from a nearby AC line or a cold snap. But if the leak is coming from the tank itself, it means the inner tank has cracked or corroded through, and that's not something that gets patched — it means the unit needs to be replaced. Because it's not always obvious which of these is happening, an active leak is worth treating as a same-day issue rather than something to watch.
5. Inconsistent or fluctuating water temperature
Water that runs hot, then lukewarm, then hot again in the same shower usually means a component isn't doing its job consistently. On a tank unit, that's often a failing thermostat or a heating element on its way out. On a tankless unit, it's more often a flow sensor issue or mineral scale building up on the heat exchanger and restricting how evenly the unit can heat water on demand. Either way, this kind of inconsistency tends to get worse rather than resolve on its own.
6. A pilot light that won't stay lit, or an error code
On gas units, a pilot that keeps going out usually points to a failing thermocouple, a draft issue, or a dirty pilot assembly. On tankless units, most modern models display an error code when something's wrong, whether that's a flame sensor issue, a flow problem, or a venting fault. These codes are genuinely useful for diagnosis, so if your unit displays one, it's worth writing it down before calling for service — it can speed up the visit considerably.
7. The smell of gas near the unit
This is the one sign that should never wait. A gas odor near a water heater, faint or strong, should be treated as an emergency. If you smell gas, don't operate switches or open flames near the unit, shut off the gas supply at the valve if you can reach it safely, ventilate the area, and call for emergency service immediately. This is the one item on this list where the right move is to act first and ask questions later.
