The 7 Reasons Why Your Hot Water Is Running Out Quickly

Hot water running out quickly is one of those things that might be labelled a “21st century problem” – right? But we all know this kind of scenario, too:

Everyone’s showered, the dishes are done, and the person who went last – usually the one who’s been working hardest all day and got home the latest – gets a pathetic lukewarm trickle or perhaps even a freezing cold blast … even though they cranked the hot tap on as far as it will go.

Sometimes, running out of hot water is just a matter of demand outpacing supply on a particularly heavy day. But if it’s happening regularly, something in the system isn’t working the way it should.

Here’s what’s probably going wrong at your place:

1. The tank is too small for the household

Why is my hot water running out quickly? This is the most common and most straightforward explanation in homes that have never had the problem diagnosed:

A storage tank sized for two people will consistently fail a household of 5 – regardless of how efficiently the system runs.

• Has your family grown?
• Have teenagers arrived with their legendary shower durations?
• Have your usage patterns simply changed?

If so, the original tank sizing has probably just become inadequate.

The rough guide for electric storage systems is around 50 litres per person per day. A family of 4 typically needs 250-315 litres, so a tank below that threshold will run out under normal daily demand.

THE FIX: If your tank is genuinely undersized, the long-term solution is replacement with an appropriately sized hot water unit. A licensed plumber can assess your household’s demand and recommend the right capacity.

In the short term, staggering shower times and running the dishwasher later in the evening allow the tank to recover between uses.

2. The thermostat is set too low

A storage tank thermostat set below the required hot water temperature of 60 °C doesn’t just create a Legionella risk – it reduces the effective volume of usable hot water in the tank. When hot water is stored at 60 °C and mixed with cold water at the tap, you get more usable litres from the same tank than if it’s stored at 50 °C.

A thermostat that’s drifted down or been set incorrectly is a surprisingly common cause of a hot water tank running out quickly – and it’s not something most homeowners would ever think to investigate.

THE FIX: Thermostat access on most modern storage systems requires a licensed plumber – it’s behind a panel inside the unit, not a dial you can turn yourself.

If your system hasn’t been serviced in some time, a thermostat check is worth including in the next hot water service visit. It’s a quick assessment that can make a meaningful difference to how much usable hot water your tank delivers.

3. Sediment buildup in the tank

Over time, mineral sediment from the water supply settles at the bottom of a storage tank. In Brisbane’s water supply, this is a gradual but consistent process. Sediment accumulation reduces the effective storage capacity of the tank – the space sediment occupies is space that can’t hold hot water – and it insulates the heating element or gas burner from the water above it, reducing heating efficiency.

Why is my hot water running out so quickly in older systems that haven’t been serviced lately? The answer is in the question itself!

THE FIX: Flushing the tank annually removes sediment buildup. This is a job for a licensed plumber – it involves connecting a hose to the drain valve and flushing until the water runs clear, with the system properly isolated first.

4. A failing heating element or burner

If your electric hot water system has a faulty lower heating element, the tank is effectively only heating from the top down – which means the volume of properly heated water is a fraction of what the tank should be producing.

A gas hot water heater running out quickly despite being the right size for the household often has a burner issue, or:

• Insufficient gas pressure
• A partially blocked burner
• Or an ignition problem that causes the system to heat intermittently rather than consistently.

THE FIX: This requires a licensed plumber or gasfitter. A faulty heating element is a straightforward replacement. Gas burner issues involve the gas system and must be handled by a licensed gas fitter. Don’t attempt either yourself!

5. The system can’t keep up with simultaneous demand

This one applies specifically to instant hot water systems, which heat on demand rather than storing a volume of pre-heated water.

A quality instant system at an adequate flow rate handles one shower comfortably. But two showers running simultaneously, or a shower and a dishwasher running at the same time, can exceed the system’s maximum output – resulting in water that’s warm rather than hot, or hot water is running out quickly from one outlet while another is drawing heavily.

THE FIX: Check the flow rate specification of your instant system. If simultaneous demand is consistently exceeding it, upgrading to a higher-capacity unit or installing a second system to serve different zones of the home is the right solution.

A plumber can assess whether your current unit is matched to your household’s peak demand.

6. A tempering valve or TMV that needs servicing

A tempering valve blends cold water into the hot supply to deliver a safe outlet temperature. If the valve is failing and mixing in too much cold water, the hot water supply will feel depleted faster than it should – because you’re receiving a higher proportion of cold water than intended at every outlet.

A hot water tank running out of hot water quickly in a system that’s the right size and appears to be heating correctly is sometimes a tempering valve problem rather than a tank or heating issue.

THE FIX: A licensed plumber can test and replace a failing tempering valve quickly. It’s a relatively straightforward job and can make a significant difference to the perceived hot water supply.

7. Hot water running out quickly in winter

Cold water entering the tank in winter is significantly colder than in summer, sometimes by 10 degrees or more in Brisbane overnight temperatures. This means the system has to work harder and longer to bring incoming cold water up to storage temperature, and recovery time between draws increases.

Hot water running out quickly in winter when the system performs fine in summer is often a seasonal demand issue rather than a fault.

THE FIX: In the short term, stagger usage and allow recovery time between showers. If the problem is severe enough to affect daily life consistently through winter, the tank may be undersized for year-round demand and replacement with a larger or more efficient unit – such as a heat pump hot water system – is worth considering.

Still running out?

If you’ve worked through this list and the problem persists, the system needs a proper assessment. S&J Hot Water Brisbane‘s licensed plumbers diagnose and fix hot water system faults across Brisbane all the time – with fixed upfront pricing and a workmanship guarantee on every job. Book your obligation-free quote today.