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hot water goes lukewarm after 5 minutes

The question, put plainly: Why is my hot water not hot enough?

The short answer

Running lukewarm quickly usually means the system is undersized, the element or thermostat has failed, or a tempering valve has drifted. Never hot at all points at the heat source rather than the storage.

TradieAnswers editorial · written in-house and sourced to the authorities listed below

Checked against 4 published sources, listed in full at the end of this page. How we source this

Separate two symptoms, because they have different causes. Hot at first and then lukewarm is a storage or recovery problem. Never properly hot at any point is a heat source or a mixing problem.

Hot then lukewarm

  • The system is too small for current use, which is what happens when a household grows or a bathroom is added.
  • On an electric storage unit, the lower of two elements has failed, so only the top portion of the tank heats.
  • Sediment has reduced the effective volume, which is common in hard water areas and in tanks over about ten years old.
  • On an off-peak tariff, the system may simply have run out before its next heating window.

Never properly hot

  • A failed thermostat, which is usually inexpensive.
  • A tempering valve at end of life. These are required to limit the temperature at bathroom outlets and they fail gradually.
  • On gas, a pilot, ignition or gas supply fault.
  • On a heat pump, refrigerant or fan faults, or operation in conditions below its rated range.41

Australian requirements are specific here: stored water is generally kept hot enough to control legionella, while the temperature delivered to bathroom outlets is limited to guard against scalding. That is why the tank can be hot and the shower still not be.

Who does this work

You need a licensed plumber

Because hot water units are connected to the water supply and to either a gas or an electrical supply, and all three connections are licensed work.

Depending on the cause it can also involve electrician or gasfitter.

This is licensed work in every Australian state and territory. Check a licence in your state

Before you call anyone

  1. Find the compliance plate on the unit and photograph it. The brand, model and date of manufacture decide whether this is a repair or a replacement conversation.
  2. Work out what it runs on: gas, electric, solar with a booster, or a heat pump. It changes which trade you need.
  3. Check whether the problem is no hot water at all, or hot water that runs out quickly. Those point at different faults.
  4. Check one tap you have not used, in case the problem is a single mixer rather than the system.

Sources

Every source below was fetched and its response recorded on the date shown. Where a government site blocks automated clients, that is noted rather than reported as a successful check.

  1. NCC 2022 Volume Three: Plumbing Code of Australia · Australian Building Codes BoardChecked 2026-08-11 · responded 200 OK
  2. Hot water systems · energy.gov.auChecked 2026-08-10 · responded 200 OK
  3. Hot water rebate · Solar VictoriaChecked 2026-08-10 · responded 200 OK
  4. National Construction Code · Australian Building Codes BoardChecked 2026-08-10 · responded 200 OK

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