5 min read · Updated August 18, 2026
The failure everybody pictures is a tank splitting open. What actually happens first is a performance change, and it usually shows up months earlier.
1. Hot water runs out faster than it used to
Sediment accumulating in the bottom of a tank displaces water and insulates the burner or element from it. The usable volume drops and the recovery time climbs. Same household, same usage, less hot water.
2. It takes longer to recover between uses
Related, and often the first thing a household notices. Two showers back to back used to work and now they do not.
3. Rumbling, popping or kettling
Those noises are water flashing to steam underneath a sediment layer. It is the sound of a tank working much harder than it should, and it accelerates everything else.
4. Rust or cloudiness on the hot side only
If the cold side is clear and the hot side is not, the discolouration is coming from inside the tank. That generally means the sacrificial anode has been consumed and the tank lining is now doing the corroding.
5. Moisture, staining or mineral crust at the fittings
Small weeps at the connections, the relief valve, or the drain valve. A stain on the pan or the floor that was not there last year. These do not reverse.
6. The relief valve discharges
The temperature-and-pressure relief valve is a safety device. If it is opening, something is wrong — excessive temperature, excessive pressure, or a failed expansion control. Do not cap, plug or tie off a discharging relief valve. Have the cause diagnosed.
7. A tankless unit short-cycling or error-coding
On tankless systems the equivalent signal is inconsistent outlet temperature, short-cycling, or repeated error codes. Scale accumulation in the heat exchanger is the usual cause, and descaling is maintenance rather than repair.
What to do about it
If the unit is over finished living space, in a condominium, or above a tenant, the calculus changes: the cost of a failure is not the water heater, it is the ceiling below it. That is the case for replacing on a schedule rather than on failure.
For portfolios, the highest-value version of this is simply knowing the age and condition of every unit you own. Once that record exists, replacements become a budget line instead of an emergency.
Safety
If you smell gas, leave the building and call the gas utility from outside. Do not relight a gas appliance, operate switches, or use a phone inside the building.
This article is general information about how plumbing systems behave. It is not a diagnosis of your building, and it is not instructions for performing regulated work. Gas, electrical, pressurised systems, sewage contamination and code-regulated work belong with the appropriate licensed trade.