Quick Answer
Hot water running out too quickly in a Melbourne home usually points to one of five causes: undersized tank, failed heating element (electric), thermostat fault, sediment build-up reducing capacity, or high simultaneous demand. Most can be diagnosed without a plumber — but element replacement and thermostat repairs require a licensed plumber or electrician. A service call runs $120–$280; a full replacement $900–$2,800 depending on system type.
The Five Most Common Causes
1. Tank Too Small for Your Household
The most common cause in growing Melbourne households: the tank was sized for 2 people and now serves 4. As a rough guide, allow 50L of storage per person for electric continuous-storage systems. A 125L tank for 3 people will run cold on school-morning rush days regardless of how well it functions.
| Household Size | Recommended Tank Size | Notes |
|---|---|---|
| 1–2 people | 80–125L | Off-peak electric adequate for 2 |
| 3–4 people | 160–250L | 250L recommended for families with teens |
| 5+ people | 315L+ or continuous flow | Consider continuous flow or heat pump |
2. Failed Heating Element (Electric Storage)
Electric storage systems have two elements: an upper element that heats the top third of the tank quickly and a lower element that heats the full volume. When the lower element fails — which happens most often after 8–12 years — you get one shower’s worth of hot water then cold. The upper element keeps the top portion hot so it’s deceptive: the first person gets a hot shower, everyone after gets cold water.
3. Thermostat Set Too Low
Electric hot water thermostats should be set to 60–65°C — below this, Legionella bacteria can survive in the tank. More practically, a thermostat drifting below 55°C will produce water that feels barely warm when mixed at the tap. Thermostats fail gradually and are inexpensive to replace — typically $80–$150 parts and labour.
4. Sediment Build-Up Reducing Effective Capacity
Melbourne’s water supply has moderate mineral content. Over years, calcium and magnesium carbonate sediment accumulates on the bottom of the tank and around the element. In a 250L tank, 20–30L of sediment is not unusual — effectively reducing usable capacity and insulating the lower element so it heats less efficiently. Symptoms include a popping or rumbling sound from the tank during heating and reduced hot water volume.
5. High Simultaneous Demand (Continuous Flow Systems)
Continuous flow (instant) hot water systems are rated in litres per minute at a temperature rise. A 16L/min unit running two showers simultaneously may not keep both hot — particularly if the inlet water temperature is cold (Melbourne winter mains water can be 10–14°C). Check your unit’s capacity rating on the label and compare to peak demand.
Diagnosis Checklist
| Symptom | Most Likely Cause | DIY Check | Fix |
|---|---|---|---|
| Cold after one shower | Failed lower element or undersized tank | Check tank age and size label | Plumber/electrician: element replacement |
| Warm but never truly hot | Thermostat set low or faulty | Check thermostat dial setting | Plumber: thermostat replacement |
| Popping/rumbling when heating | Sediment build-up | Listen during heating cycle | Flush tank or replace if old |
| Hot then cold mid-shower | Pilot light fault (gas) or element cycling | Check pilot light status | Gasfitter or plumber |
| Cold water only, no recovery | Element failed completely or no power to unit | Check circuit breaker for HWS circuit | Electrician + plumber |
| Adequate for small household, poor for full household | Tank undersized | Count household members vs tank size | Upgrade tank or switch to continuous flow |
When to Call a Licensed Plumber
In Victoria, all hot water system work — including element replacement, thermostat replacement, pressure valve replacement and new installations — must be performed by a VBA-licensed plumber. Electric element work additionally requires a licensed electrician for any internal wiring.
Call a plumber if: the tank is over 10 years old (it may be more economical to replace than repair), the pressure relief valve is weeping constantly, there is rust in the hot water, the tank is making loud clanging noises, or you can see water pooling around the base of the tank.
Repair vs Replace: Cost Guide
| Issue | Repair Cost | Replacement Cost | Recommendation |
|---|---|---|---|
| Failed element (tank <8 yrs) | $150–$220 | $900–$1,500 | Repair |
| Thermostat replacement | $80–$150 | $900–$1,500 | Repair |
| Failed element (tank 8–12 yrs) | $150–$220 | $900–$1,500 | Consider replacing |
| Tank over 12 years old, any fault | $150–$300 | $900–$2,800 | Replace — tank at end of life |
| Tank leaking at base | Not repairable | $900–$2,800 | Replace immediately |
Top 8 Tips and Gotchas
- Off-peak tariff matters — electric storage systems on an off-peak tariff only heat overnight. If someone uses a lot of hot water in the evening, the tank may not recover until the next night cycle.
- The two-element test — if you get one mediocre-length shower then cold water, assume the lower element has failed. If you get zero hot water, assume the upper element has also failed or there is a power supply issue.
- Check the age sticker — it’s usually on the top or side of the tank. Most electric storage tanks have a 5–10 year warranty; useful life is 10–15 years. Gas storage 8–12 years. Continuous flow up to 20 years.
- Flush the tank annually — attach a hose to the drain valve at the tank base and let 20–30L run out. This removes sediment and extends tank life. Turn off power/gas first.
- Test the T&P valve annually — lift the lever briefly and confirm water flows then stops cleanly. A valve that drips after release is faulty and must be replaced by a plumber.
- Cold winter mains water — Melbourne mains water in winter (June–August) can be 10–12°C. Your continuous flow unit rated at a 25°C rise will only deliver 35–37°C water — not comfortable for a shower. This is a capacity issue, not a fault.
- Check for a split system fault code — heat pump hot water systems display fault codes on a small panel. Look these up in your manual before calling a technician.
- Never reduce the thermostat below 60°C — Legionella bacteria can colonise tanks stored below this temperature. The Victorian health guidelines are clear on this.
Frequently Asked Questions
Why do I only get one shower of hot water?
This is almost always a failed lower heating element in an electric storage system. The upper element keeps the top 30–40L hot, which is enough for one shower, but the lower element that heats the full tank has failed. A licensed plumber or electrician can replace it for $150–$220 — book before winter as demand spikes in June.
How long should a hot water system last in Melbourne?
Electric storage systems typically last 10–15 years. Gas storage 8–12 years. Continuous flow gas systems 15–20 years. Heat pump systems 10–15 years with proper maintenance. Melbourne’s water quality is moderate — not as hard as some regional areas — so sediment build-up is manageable with annual flushing.
Can I adjust the thermostat myself?
The thermostat access panel on most electric storage systems requires removing a cover with a screwdriver — the thermostat dial is accessible without electrical work. However, if the thermostat needs replacement (not just adjustment), that is plumbing and electrical work requiring a licensed tradesperson under Victorian law.
What is the VIC rebate for hot water systems?
Solar Victoria offers rebates for heat pump hot water systems of $700–$1,400 depending on eligibility. You must use an approved installer and the system must meet minimum efficiency requirements. Check current eligibility at solar.vic.gov.au — the scheme is subject to annual budget limits.
Why does my hot water smell like rotten eggs?
Sulphur (rotten egg) smell in hot water is caused by a reaction between the sacrificial anode rod (a magnesium rod inside the tank that protects against corrosion) and naturally occurring bacteria. It is common in tanks over 5 years old when the anode has partially depleted. A plumber can replace the anode rod ($150–$280) — this typically resolves the smell permanently.
Melbourne Local Resources
- Reece Plumbing — hot water systems, elements, thermostats, T&P valves, trade counter in Dandenong, Frankston and Berwick
- Solar Victoria — current heat pump hot water rebate details and approved installers
- Bunnings — basic plumbing fittings, hoses for tank flushing
- Victorian Building Authority — find a licensed plumber in your Melbourne postcode