Quick Answer

If your hot water keeps running out, the most common causes are an undersized tank, a failed heating element, sediment build-up, or a faulty thermostat. Tank systems typically need 50–60 litres per person; if yours is smaller or older than 10 years, it may simply be unable to keep up. Most fixes cost $150–$600 — but a system over 10 years old may be worth replacing rather than repairing.

The Most Common Reasons Hot Water Runs Out

1. Undersized Tank for Your Household

The most overlooked cause. The standard rule of thumb is 50 litres of tank capacity per person for electric storage systems. A growing household, extra guests, or an Airbnb listing can push a previously adequate 160-litre tank beyond its limits. Gas systems recover faster, but continuous flow systems can still be overtaxed if multiple outlets run simultaneously.

Household Size Recommended Tank Size Minimum Acceptable
1–2 people 80–125 litres (electric) / 135L (gas) 80 litres
3–4 people 160–250 litres (electric) / 170L (gas) 125 litres
5–6 people 250–315 litres (electric) / 170L continuous flow 160 litres
6+ people 315+ litres or continuous flow system 250 litres
Pro tip: Off-peak electric systems take 6–8 hours to reheat. If your tank is on off-peak tariff (E31/E33 in Victoria) and your household is using hot water heavily in the morning, the tank may not have fully reheated from the overnight off-peak cycle. Ask your electrician to check your tariff setting.

2. Failed Heating Element (Electric Systems)

Electric storage heaters have one or two heating elements — a top element for quick top-up heating and a lower element for bulk heating. When the lower element fails, the tank can only heat the top 20–30% of its volume, meaning a 250-litre tank effectively functions as a 60-litre tank. A replacement element costs $200–$400 installed and is one of the most common hot water repairs in Melbourne homes.

3. Sediment Build-Up

In Melbourne’s SE suburbs, the water supply contains dissolved minerals that settle as sediment at the bottom of the tank over time. This sediment insulates the lower element from the water, reducing efficiency. On a gas system, it forces the burner to work harder and longer. Symptoms: rumbling or popping sounds when heating, gradually worsening performance over 3–5 years.

4. Faulty Thermostat

Most electric systems have two thermostats — one for each element. A failed thermostat may cause the element to stop heating at 40°C instead of the correct 60°C (the legal minimum for Legionella prevention). A thermostat replacement costs $150–$300 installed and can restore full capacity instantly.

5. Temperature Pressure Relief (TPR) Valve Weeping

If your TPR valve — the safety relief valve with a small pipe running to a drain — is slowly releasing hot water, you’re losing hot water continuously without knowing it. Check the drainpipe near the base of your system: if it’s warm or dripping, the TPR valve needs replacement ($150–$280).

6. Continuous Flow System Flow Rate Issues

Continuous flow (instantaneous) systems don’t run out of hot water in the traditional sense, but they have a maximum flow rate — typically 16–26 litres per minute. Running multiple showers simultaneously may exceed this rate, resulting in cooler water. The fix may be upgrading to a higher-capacity unit or adding a second system.

Repair Costs at a Glance

Problem Repair Cost DIY Possible?
Lower heating element replacement $200–$400 No — requires licensed plumber/electrician
Thermostat replacement $150–$300 No — live electrical components
TPR valve replacement $150–$280 No — plumbing licence required
Anode rod replacement (sediment prevention) $180–$350 No — specialist task
Tank flush (sediment removal) $100–$200 Partial — draining is DIY, flushing needs care
Gas system thermocouple replacement $150–$280 No — gas licence required
Full system replacement (electric) $900–$2,000 installed No
Full system replacement (heat pump) $2,500–$4,500 installed No — VIC rebate available
Pro tip: Before spending on repairs, check your system’s age. The anode sticker or serial number typically encodes the manufacture year. Electric systems last 8–12 years; gas systems 10–15 years. If yours is approaching end-of-life, a repair might buy only 1–2 seasons before the next failure.

When to Replace Instead of Repair

Use the 50% rule: if a single repair costs more than 50% of a new system’s installed price, replacement is the better investment. For electric storage systems (new cost ~$900–$1,400 installed), repairs over $600 are generally not worth doing on a system over 8 years old.

Victoria offers rebates of up to $1,000 for upgrading to a heat pump hot water system through the Victorian Energy Upgrades (VEU) program — making this the right time to replace rather than repair an aging electric resistance system.

Top 10 Tips and Gotchas

  1. Check off-peak tariff timing. If your system is on off-peak power, it reheats between midnight and 6am. Heavy morning use may exhaust the tank before the next off-peak cycle. Ask your energy retailer if you can switch to continuous tariff.
  2. Flush the tank annually. Attach a garden hose to the drain valve at the base and release 10–15 litres. This removes settled sediment and extends element and tank life.
  3. 60°C is the legal minimum. Under AS/NZS 3500.4, storage systems must be set to at least 60°C to prevent Legionella bacteria growth. Don’t reduce the setting to “save power” — it creates a health risk.
  4. Check the pressure reducing valve (PRV). If water hammer or fluctuating pressure precedes the hot water issue, a faulty PRV (the valve near your meter) may be sending inconsistent flow to the system.
  5. Heat pump systems take longer to recover. Heat pump hot water systems are highly efficient but slower to reheat than gas. A 270-litre heat pump tank takes 2–3 hours to recover vs 45–60 minutes for a gas unit.
  6. Cold water inlet dip tube failure. In older tanks, the plastic dip tube that directs cold water to the bottom of the tank can collapse, causing cold and hot water to mix. Symptom: water runs lukewarm almost immediately. Replacement is a plumber job.
  7. Check for a mixing valve. Modern installations include a tempering valve that mixes cold water into the hot water outlet for scalding prevention. If this valve is set too low or fails, you’ll get lukewarm water even from a fully heated tank.
  8. Insulate your pipes. Melbourne winters cause significant heat loss in uninsulated pipes, especially in SE suburb homes with long pipe runs from external systems. Pre-formed pipe insulation foam is $15–$25 per metre from hardware stores.
  9. Two-person households on off-peak often have oversized tanks. A 250-litre tank designed for 4 people may develop sediment faster if only 2 people are using it, as the full tank volume rarely turns over completely.
  10. Get a written quote before authorising repairs. Under the Australian Consumer Law, a tradesperson must provide a written quote if the repair cost will exceed $50. Don’t authorise work verbally without a written estimate.
Safety warning: Never attempt to replace a heating element, thermostat, TPR valve or any plumbing connection yourself. Hot water systems operate at mains pressure and high temperatures — incorrect repairs can result in scalding, flooding or Legionella contamination. All hot water system work in Victoria requires a licensed plumber.

Local Melbourne Resources

Frequently Asked Questions

Why does my electric hot water keep running out even though the tank is full?

If the tank is full but water runs cold quickly, the most likely cause is a failed lower heating element. The tank holds water but isn’t heating it properly — so only the top portion (heated by the upper element) is usable. A licensed plumber can test both elements with a multimeter in about 15 minutes and quote for replacement.

How long does it take for a hot water system to reheat after it runs out?

Electric storage systems on continuous tariff: 1–2 hours for a 160-litre tank. Gas storage systems: 30–60 minutes. Heat pump systems: 2–3 hours. Off-peak electric systems only reheat during off-peak periods (typically midnight to 6am in Victoria), so if you run out during the day, you may wait until the next overnight cycle.

Is it worth repairing a hot water system that is 10 years old?

For repairs under $300, usually yes — even a 10-year-old system may have 2–4 years of life remaining. For repairs over $500, do the maths: a new heat pump system installed under the VEU rebate can cost as little as $1,500–$2,000 net. With energy savings of $300–$500/year versus electric resistance, a new system pays for itself within 4–6 years.

What temperature should my hot water system be set to?

Storage systems must be set to a minimum 60°C to prevent Legionella bacteria growth (AS/NZS 3500.4). Continuous flow systems should be set to 50°C at the outlet (where a tempering valve is installed). Never lower the storage temperature below 60°C to “save electricity” — the health risk outweighs any energy saving.

Can I get a rebate for replacing my old hot water system in Victoria?

Yes — the Victorian Energy Upgrades (VEU) program offers point-of-sale rebates when you upgrade from electric resistance or gas storage to a heat pump hot water system. The rebate amount depends on your existing system type and the new model’s energy rating, but can reduce the installed price by $500–$1,000. Ask your plumber about current VEU rebate amounts when getting a quote.