Hot water runs out too quickly? It might not be a fault
The third person in the shower gets cold water, and everyone assumes the system is broken. Often it is working perfectly and is simply the wrong size, or on a tariff nobody explained. Here is how to tell which.
Start with one question: has it changed?
This single question sorts most of it. If the hot water has always been marginal in this house, you almost certainly have a sizing or tariff problem, and no repair will fix it. If it has noticeably got worse over recent weeks or months, that is a fault, and a repair probably will.
| What you are seeing | Likely cause | Repair or replace? |
|---|---|---|
| Always been marginal, never got better | Tank undersized for the household | Resize |
| Fine in the morning, gone by evening | Controlled load tariff, tank too small to carry the day | Resize or change tariff |
| Noticeably worse than it used to be | One element failed on a twin-element system | Repair |
| Gradual decline over years, rumbling tank | Sediment build-up reducing usable volume | Repair or replace |
| Lukewarm rather than running out | Failed tempering valve or thermostat | Repair |
| Runs out only when several showers run | Flow rate limit, common on continuous flow | Resize |
The tariff nobody explains
A large share of Illawarra homes run electric storage on a controlled load tariff, often still called off-peak. The element only heats during set overnight hours, when electricity is cheapest.
That works well when the tank is big enough to carry the household through the day. It stops working the moment demand outgrows the tank, because there is no way to force a reheat — you simply wait until the small hours. Hot showers in the morning, nothing by dinner, and a system that is not faulty in any way.
Households grow into this problem. A tank sized for a couple gets a very different workout once there are teenagers in the house. Nothing changed with the plumbing; the demand changed.
Three ways out of a tariff problem
Bigger tank. Usually the cheapest option if the existing system is otherwise healthy and there is room.
Change tariff. Moving to a continuous or boosted supply lets the system reheat during the day. Costs more per unit of electricity, but no new hardware. Worth pricing against a bigger tank before deciding.
Change system type. A gas continuous flow system removes the concept of running out entirely. A heat pump on a continuous supply reheats through the day and has the lowest running cost of anything.
Sizing, roughly
For storage systems, as a starting point: 125 to 160 litres for one or two people, 250 litres for three or four, 315 litres and up for five or more. On a controlled load tariff, size up rather than down, because the tank is carrying a full day.
Habits matter as much as headcount. Two people who both take long showers back to back put more through a system than four people who are quick about it. So does a bath, a dishwasher and a washing machine running in the same window.
Continuous flow systems work differently — they are rated by flow rate, so the question is not how much hot water you have but how many outlets can run at once. Undersize one and you get the shower going cold the moment someone starts the kitchen tap.
When it genuinely is a fault
One element gone on a twin-element system
Many electric storage systems have two heating elements. Lose the lower one and you are only heating the top portion of the tank, which feels exactly like the tank shrank overnight. This is the most common genuine fault behind “it used to be fine”, and it is a straightforward repair.
Sediment in the base
Mineral sediment settles in the bottom of a storage tank over the years, taking up volume that used to hold water and insulating the lower element from what it is meant to be heating. It comes on gradually, and the tell-tale is rumbling or banging from the tank as it heats.
A failed tempering valve
Worth separating out, because it presents differently. If the water is lukewarm rather than running out, the tempering valve — which blends stored water down to a safe temperature before it reaches your bathroom — may have failed. The tank is fine; what is coming out of it is being over-blended.
Do not turn the thermostat up to compensate
It will not give you more hot water in any useful sense, and stored temperature is a safety setting rather than a volume control. Storage above 60°C controls Legionella; a tempering valve then brings it down to a safe temperature at the tap. Interfering with either to solve a capacity problem is the wrong lever.
Working out which one you have
We can usually narrow it down over the phone. The useful details are: system type and rough age, tank size if it is on the compliance plate, how many people are in the house, whether it has always been like this, and whether the tank makes any noise while heating.
From there it is either a repair, a resize, or a conversation about tariffs — and we will tell you if the answer is a phone call to your electricity retailer rather than a job for us. If it does turn out to be worth replacing, the cost comparison by system type is here, and the NSW rebates on heat pumps are here.
Hot water repairs and installation across Wollongong, Kiama, Shellharbour, Gerringong and Berry.
Questions we get asked
Why does my hot water run out so quickly?
Four common reasons. The tank is undersized for the household. The system is on a controlled load tariff so it only reheats overnight. One of two heating elements has failed, so the tank is only heating its top half. Or sediment has built up in the base, reducing the usable volume. The first two are not faults at all, which is why it is worth working out which one you have before calling anyone.
What size hot water system do I need?
As a rough guide for storage systems: 125 to 160 litres suits one or two people, 250 litres suits three or four, and 315 litres and above suits five or more. On a controlled load tariff, size up, because the tank has to carry a full day. Continuous flow systems are sized by flow rate rather than volume, so the question becomes how many showers can run at once.
What is a controlled load or off-peak tariff?
It is a cheaper electricity tariff where your hot water element only runs during set overnight hours. It saves money and works well if the tank is big enough to carry the household through the day. If it is not, you get hot water in the morning and nothing by evening, with no way to force a reheat. Nothing is broken — the system is doing what it was set up to do.
Will a bigger tank fix it?
Often, yes, and it is usually the cheapest fix if the existing system is otherwise healthy. But check the alternatives first: moving off controlled load to a continuous or boosted tariff can solve it without new hardware, and a continuous flow system removes the concept of running out altogether. Which is right depends on your household and whether you have gas.
Could it be a fault rather than sizing?
Yes. If the amount of hot water you get has dropped noticeably rather than always having been marginal, that points to a fault — most commonly one failed element on a twin-element electric system, which leaves you heating only the top of the tank. Sediment build-up does the same thing more gradually. A sudden change is a fault; a long-standing annoyance is usually sizing or tariff.
Need it looked at?
We do same-day hot water repairs across Wollongong, Kiama, Shellharbour and down through Gerringong and Berry. Licensed plumber and gas fitter, 24/7 for emergencies.