No hot water in the shower only: it's the mixer, not the system
If the kitchen tap runs hot and the shower does not, your hot water system is working. The fault is in the last two metres of pipe, and that changes both the diagnosis and who you need to call.
This is one of the few plumbing complaints where a thirty-second test gives you almost the whole answer.
Go to the kitchen sink and run the hot on its own. Then do the same at the bathroom basin, and at the laundry tub. What you find there decides everything that follows — and in most cases it rules the hot water system out entirely.
| What you find | What it means | Where the fault is |
|---|---|---|
| No hot anywhere in the house | The system has stopped heating or is not being fed | The system itself — see no hot water at all |
| Hot at the kitchen, none in the shower | The system is fine. One outlet is not getting it | The shower mixer, its valves, or the pipe feeding it |
| Hot at the kitchen, weak or cool everywhere in the bathroom | Something feeding the bathroom as a group | The tempering or thermostatic mixing valve |
| Hot at first, then cold after a few minutes | The system is heating but running out | Capacity or a failed element — see what size system you need |
The rest of this page is about the second row, which is by far the most common version of the question.
Why one outlet can go cold while the rest of the house is fine
A shower is not a tap. A tap has a hot side and a cold side that you open independently, so if hot water arrives at the wall it arrives at the spout. A shower has a mixer — one handle, one cartridge, and a single outlet — which means hot and cold water are brought together inside a device before they reach you.
That device is the difference. It is the only place in your plumbing where hot and cold meet under pressure inside a moving part, and it is the part that fails.
Six things that cause it, roughly in order of likelihood
1. The mixer cartridge has failed
Far and away the most common cause. The cartridge is the ceramic or thermostatic core inside the mixer body that decides how much hot and how much cold to let through. It is a wearing part with a finite life, and the way it usually dies is gradually: the handle travels further and further towards hot to get the same result, until one day full hot gives you cold.
The giveaways are that it came on over weeks rather than overnight, that the handle feels different — stiffer, gritty, or looser than it used to — and that the shower still runs at normal flow. Water is arriving; it is just all coming from one side.
2. The anti-scald limiter has moved
Many mixers have a temperature limit stop built into the handle assembly, which caps how far towards hot the handle can go. If someone has had the handle off — during a repair, a repaint, a clean — the stop can be reassembled in the wrong position, and the shower is then capped well below where it used to be.
This one has a distinctive signature: the change was sudden, it happened around the time someone touched the tapware, and the shower is not cold so much as stuck at a fixed lukewarm.
3. The tempering or thermostatic mixing valve has failed
Bathroom outlets in Australia are fed through a valve that blends stored hot water down to a safe delivery temperature before it reaches you. It is a safety device, it is set deliberately, and like the cartridge it is a consumable sitting in hot water.
When it drifts or fails it can drop the temperature at every outlet it serves. The tell is the pattern: the shower and the basin go cool together while the kitchen and laundry, which are often not fed through it, stay hot. If that matches what you are seeing, stop looking at the shower.
4. An isolation valve on the hot side is part-closed
The least interesting cause and one that catches people regularly. Showers frequently have small isolation valves on the hot and cold feeds behind the wall plate or in the adjacent cupboard, and any plumbing work in the house — a new mixer, a leak repair, a system service — is an opportunity for one to be left part-closed.
If the problem began the day after somebody worked in the house, check this before you check anything else. The signature is that hot is not absent so much as overwhelmed: there is not enough of it to hold its own against the cold.
5. A diverter or bath spout is bleeding the hot away
On a bath-and-shower combination fed by one mixer, the diverter is what sends water up to the rose instead of out of the spout. A worn diverter lets part of the flow escape at the spout, and because that lost flow comes from the mixed water, the shower ends up weaker and cooler than the handle position suggests.
Look at the bath spout while the shower is running. Anything more than a slow drip points here — and it is a much cheaper fault than the ones above.
6. The mixer's inlet filters are blocked
Most mixers have small mesh filters on the hot and cold inlets. Scale and corrosion debris travelling out of an ageing cylinder collect there, and because the debris comes from the hot water system it blocks the hot inlet first.
This shows up as a gradual loss of hot alongside a gradual loss of flow, and it often arrives with other symptoms elsewhere in the house — discoloured water, aerators that keep clogging. If that is the picture, the mixer is the victim rather than the culprit, and the cylinder is worth a look.
The checks that are yours to make
None of these needs a tool or opens anything pressurised:
- Run the hot on its own at the kitchen, the basin and the laundry. This is the whole diagnosis in one step. If hot arrives anywhere else, the system is working.
- Compare the shower with the basin in the same bathroom. Both cool means a tempering valve. Only the shower means the mixer.
- Note whether it came on suddenly or gradually. Sudden means a limit stop or a valve position; gradual means a cartridge or a filter.
- Ask whether anyone has worked on the plumbing or the tapware recently, including you. This finds causes 2 and 4 more often than any amount of investigation.
- Watch the bath spout while the shower runs, if it is a combination unit.
- Check the flow, not just the temperature. Normal flow with no heat is a mixing fault. Poor flow as well points at a restriction — see low hot water pressure.
What to tell the plumber
Turn up to the call with these four facts and the visit gets shorter:
- Which outlets are affected, and which are fine. Name them.
- Whether it is the shower alone or the whole bathroom.
- Sudden or gradual, and whether it followed any work in the house.
- Whether the flow is normal while the temperature is not.
Those four answers are usually enough for a plumber to arrive with the right cartridge or valve on the van, instead of diagnosing on your time and coming back.
How far can you take this yourself?
The comparisons above are yours, and so is everything you can do without a tool. Replacing a mixer cartridge, working on a tempering valve, or opening anything behind a wall plate is licensed plumbing work in every Australian state and territory, and the plumbing regulator for your state or territory is who licenses and inspects it. Where that line falls is set out in full on the licensed work pillar.
Frequently asked questions
Why is there no hot water in my shower but the taps are fine?
Because the hot water system is working and something between it and that one outlet is not. A shower mixes hot and cold inside a single cartridge, so a failed cartridge, a misplaced anti-scald limit stop, a part-closed isolation valve, a worn diverter or a blocked inlet filter can all leave you with cold water at the rose while every tap in the house runs hot.
Is no hot water in the shower a problem with the hot water system?
Almost never, if the rest of the house has hot water. Run the hot on its own at the kitchen sink: if it comes out hot, the system is heating and delivering, and the fault is at the shower. Only when hot is missing everywhere should you start looking at the system itself.
Why did my shower suddenly go cold when nothing else changed?
A sudden change points at something being moved rather than something wearing out. The usual causes are a temperature limit stop reassembled in the wrong position after the handle was off, or an isolation valve on the hot feed left part-closed after plumbing work. A cartridge failing normally gets worse over weeks, not overnight.
Can I replace a shower mixer cartridge myself?
No. The mixer body sits in the wall on mains-pressure hot water, and work on it is licensed plumbing work in every Australian state and territory. You can compare outlets, look at the bath spout and note when the problem started, which is the diagnosis. The repair needs a licensed plumber.
Sources
What this page explains is based on these official sources. If you check one and it does not match what you read here, send it to us and the article gets corrected.
- Australian Government, energy.gov.au. Hot water systems — types, installation and energy performance
- NSW Fair Trading. Plumbers, drainers and gasfitters — licensing