Hot tap not working but the cold is: it is the tap, not the system
If hot runs at the kitchen sink and not at this tap, your hot water system is working. The fault sits in the last metre of pipe, and that changes who you need to call.
The commonest mistake here is to blame the hot water system. If cold runs from a tap and hot does not, check one thing first: does hot run from other taps?
Open the hot at the kitchen sink, the laundry tub and the bathroom basin. The pattern you find decides the rest, and it often rules the system out in under a minute.
| What you find | What it means | Where the fault is |
|---|---|---|
| Hot runs at other taps, not at this one | The system is fine. This outlet is not getting it | The tap itself, its flexible hose or its isolation valve |
| No hot at any tap | The system has stopped heating or delivering | See no hot water at all |
| Hot at the kitchen, none at the bathroom basin and shower | Something feeds the bathroom as a group | Often the tempering valve. See what a tempering valve does |
| Weak or lukewarm hot at this tap only | Partial restriction on the hot side | Blocked aerator, filter or part-closed valve |
The rest of this page is about the first row, which is by far the most common version of the question.
Why one tap can lose its hot while the house keeps it
Each tap is the end of its own short branch of pipe. Hot water reaches the wall, passes through a valve, then travels along a flexible hose or a short copper tail to the tap body. A fault anywhere on that last stretch cuts the hot at one tap and leaves every other outlet alone.
That is good news. A branch fault is cheaper to find than a system fault, and it rarely means the hot water system is ageing.
Five causes, roughly in order of likelihood
1. A mixer cartridge has worn out
On a single-lever mixer tap, a ceramic cartridge blends hot and cold. It is a wearing part. When it fails, the lever moves but the hot side no longer opens, or opens only a little.
The giveaways: it got worse over weeks, the lever feels stiff or gritty, and cold still runs at normal flow. A dripping tap often comes first. See which part has actually failed.
2. The isolation valve under the tap is part-closed
Most modern taps have a small isolation valve on each feed under the sink or basin. Any recent plumbing work is a chance for the hot one to be left shut or part-shut. If the problem began the day after someone worked in the house, look here before anywhere else.
Looking is all you do. A valve left in the wrong position is a job for the person who moved it, or for a licensed plumber.
3. The aerator or inlet filter is blocked
Scale and corrosion flakes from an ageing cylinder travel down the hot pipe and collect in the fine mesh at the tap. Because the debris comes from the hot side, the hot clogs first. You see a thin trickle of hot while the cold runs strongly.
If several taps in the house are slowly losing hot flow, the tap is the victim and the cylinder is worth a look. See brown hot water for the signs.
4. The flexible hose is kinked or crushed
The braided hoses under a sink get pushed around by stored cleaning products and bins. A kinked hose passes cold and starves hot, or the other way round. You can see a bend in a hose without touching anything.
5. The hot pipe to that outlet is restricted
Older houses may still have galvanised steel pipe that corrodes from the inside and narrows over the years. The sign is hot flow that fell slowly at one end of the house while cold stayed strong. This is not a DIY diagnosis, and it is rarely cheap to fix.
The checks that are yours to make
- Run the hot on its own at three other taps. If hot arrives anywhere else, the system is working.
- Compare the hot and cold flow at the problem tap. Normal cold with a trickle of hot points at a restriction.
- Note sudden or gradual. Sudden points at a valve position. Gradual points at a cartridge or a blocked filter.
- Ask about recent work in the house, including your own. It finds the shut valve faster than anything else.
- Look under the sink for a kinked hose or a valve handle turned crosswise to the pipe. Look only.
What to tell the plumber
- Which tap is affected, and which taps run hot normally.
- Sudden or gradual, and whether it followed any work in the house.
- Whether cold flow is normal while the hot is weak or absent.
- Whether other taps have lost hot flow over the last few months.
Those four answers usually let a plumber arrive with the right cartridge or fitting. That saves you a second visit.
How far can you take this yourself?
The comparisons above are yours. Opening the tap, replacing a cartridge or working on the pipe is licensed work in most states and territories. 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 does my cold tap work but the hot does not?
Usually because the hot water system is fine and something between it and that one tap is not. A worn mixer cartridge, a part-closed isolation valve, a blocked aerator or a kinked hose can each cut the hot at one outlet. Run the hot at other taps first: if it arrives there, the system is working.
Is a hot tap not working a problem with the hot water system?
Rarely, if other taps in the house run hot. Only when hot is missing everywhere should you look at the system itself. A single dead hot tap points at the tap, its hose or its valve.
Why is only a trickle of hot water coming from one tap?
A trickle of hot with strong cold usually means a restriction on the hot side: a blocked aerator or inlet filter, a part-closed isolation valve or a kinked hose. Debris from an ageing cylinder blocks the hot side first, because it travels down the hot pipe.
Can I fix a hot tap that is not working myself?
You can compare taps, look under the sink for a kinked hose and note when it started. Replacing a cartridge or working on the pipework is licensed plumbing work in most Australian states and territories, so call a licensed plumber for the repair.
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