Warning one — it starts making noise. Rumbling, popping, or a sound like something boiling. What it is: sediment. Minerals precipitate out of heated water and settle at the bottom of the tank, hardening over the years. Water trapped beneath that layer superheats and escapes as steam, which is the popping.
Why it matters: on a gas unit, that sediment sits between the burner and the water, so the burner runs longer and the tank bottom runs hotter than designed. On an electric unit, it buries the lower element. Either way it wastes energy every cycle and shortens the tank's life.
What it costs to act: a flush. Cheapest possible intervention, and it works best if you've been doing it all along — sediment hardened over a decade often can't be fully removed.
Warning two — the hot water doesn't last like it used to. Showers that used to be comfortable now run cool at the end, or recovery between them takes noticeably longer.
What it is: heavy sediment occupying volume that should be water, a failed lower element on an electric unit, or a failed dip tube — the internal tube delivering cold water to the bottom of the tank. When a dip tube fails, cold enters at the top and mixes into the outgoing hot supply, so you never get the tank's full capacity.
What it costs to act: all three are inexpensive repairs, and the dip tube in particular is a cheap part with a dramatic symptom that gets mistaken for a dying tank.
Warning three — the hot water runs rusty. Discoloured water on the hot side, or staining at fixtures and on laundry. If both hot and cold are discoloured, look at the supply or your pipework instead.
What it is: almost certainly the anode rod is exhausted. It's a sacrificial metal rod inside the tank, deliberately corroding so the tank doesn't. When it's used up, the tank becomes the anode — and that's the beginning of internal corrosion.
What it costs to act: an anode replacement is inexpensive relative to a heater and it's the single most effective way to extend one. It's also the most commonly skipped item in the trade, because nobody markets a service around a part the customer has never heard of.
Warning four — rust streaks, damp, or weeping at fittings. A stain running down the tank, dampness underneath, corrosion at the drain valve, or moisture at the connections.
What it is: this is where the important distinction lives. Water from a fitting, valve, or connection is a repair. Water from the tank body is not.
What it costs to act: a drain valve or a connection is a small job. If it's the tank body, you're at the end of the road — a corroded tank can't be patched, sealed, or economically saved, and it only gets worse.
Warning five — the relief valve discharges. Periodic dripping or discharge from the temperature and pressure relief valve.
What it is: the valve doing its job. The question is why — excessive temperature, excessive supply pressure, thermal expansion in a closed system, or a failed valve. Three of those four are conditions to correct, not parts to swap.
What it costs to act: modest, and it may be a pressure reducing valve or an expansion tank rather than the heater at all. What it must never cost you is a cap on that valve — it's discharging because something needs to discharge, and blocking it is not an option under any circumstances.