First, a 30-second reality check
Has anything changed at your place? A new baby, teenagers discovering long showers, winter water coming in colder? If usage went up or the incoming water got colder, a system that was always "just enough" will suddenly feel small. That's not a fault — that's sizing.
If nothing changed and the hot water still shrank, keep reading.
Cause 1: A failed element (electric systems)
Most electric tanks have two heating elements. When the bottom one fails, the top element keeps a small amount of water hot — so you get a few good minutes, then cold. This is the classic "it runs out fast" fault.
The good news: an element and thermostat replacement is one of the cheaper hot water repairs — usually well worth doing on a tank under 10 years old.
Cause 2: Sediment build-up
Minerals settle at the bottom of the tank over the years, forming a layer that insulates the water from the burner or element. Your 250L tank might effectively be heating 180L. Rumbling, popping or banging sounds are the giveaway.
The verdict: caught early, a flush and service helps. Heavy build-up in an older tank usually means it's near the end.
Cause 3: A stuck or failing tempering valve
The tempering valve mixes hot and cold to deliver safe water to your taps. When one drifts or fails, it can over-mix — cold water diluting your hot supply so the tank "empties" faster. If your hot water is also not as hot as it used to be, suspect this.
Cause 4: The tank is simply done
Tanks last roughly 8–12 years (less with hard water). If yours is in that window and the hot water is fading, spending on repairs is often throwing good money after bad — especially if there's any rust in the water or moisture around the base.
Repair or replace: the honest maths
- Under 8 years old + single fault (element, thermostat, valve): repair it.
- 8–12 years + significant fault: compare the repair price to a replacement — often the repair is half the cost of a new unit that would come with a fresh warranty.
- Any age + leaking tank: replace. A leaking tank shell can't be repaired, and they tend to fail properly at the worst possible moment.
Whatever you do, size the new system for your actual household — not just like-for-like. Households change; your hot water should match how you live now.