Low hot water pressure: why only the hot side
If the cold water is fine and the hot is a trickle, the problem is not your mains pressure. That single observation rules out half the possible causes before anyone opens a toolbox.
"Low hot water pressure" is two completely different complaints wearing the same words, and separating them takes about a minute.
Stand at a tap you can run both sides of independently — a laundry tub or an outdoor tap with a mixer is ideal — and run the cold on its own, then the hot on its own. What you learn from that comparison sends you to opposite ends of the house.
| What you see | What it points at | Where to look |
|---|---|---|
| Hot and cold both weak, everywhere | Whole-house supply. Nothing to do with the hot water system. | Stop tap, meter, pressure limiting valve, any whole-house filter |
| Hot weak, cold normal, everywhere | The hot side of the house, or the system itself | This page |
| Hot weak at one tap only | That tap, not the system | Aerator, mixer cartridge, the flexible hose under it, its isolation valve |
| Both weak at one tap only | That tap | Aerator or the isolation valves under the basin |
Pressure or flow? They are not the same thing
Almost every complaint about "pressure" is really about flow — the litres actually arriving at the tap. Pressure is the push behind the water; flow is how much of it gets through. A restriction anywhere in the path leaves the pressure at the meter untouched and drops the flow at the outlet to a dribble.
This matters because it changes what is worth investigating. Genuinely low pressure is a supply or a valve problem. Low flow with normal pressure is almost always a restriction, and restrictions are much more common.
Six things that weaken the hot side and leave the cold alone
1. You have a gravity-fed system, and it was always like this
Older Australian homes sometimes have a low-pressure or gravity-fed hot water system — typically a tank in the roof space, delivering hot water at whatever pressure the height of the tank provides, while the cold comes straight off the mains at full pressure. The result is exactly the symptom on this page, and nothing is broken.
The giveaway is that it has always been this way rather than appearing suddenly, and that the mixer taps feel unbalanced: turn towards hot and the flow falls away. If that describes your house, the fix is a change of system rather than a repair, which is a different conversation — start with choosing a hot water system.
2. The tempering valve is failing or partly blocked
Your bathroom outlets are fed through a tempering valve, which blends stored hot water with cold before it reaches you. It is a small device with moving parts sitting in hot water, and it is a consumable — it does not last as long as the system it is fitted to.
When it starts to fail it can restrict flow on the hot side, and the tell is usually that the bathroom is weak while the kitchen or laundry, which may not be tempered, is fine. A tempering valve on the way out often changes the temperature too: water that is suddenly lukewarm, or that swings, points the same direction.
3. A valve on the hot side is not fully open
The least interesting cause and one of the most common. There is an isolation valve on the cold inlet to the hot water system, there are usually stop taps under basins and behind the toilet, and there may be one on the hot outlet. Any of them left part-closed after work — a new tap, a leak repair, a filter change, a system service — throttles the hot side and nothing else.
If the weak hot water started after someone was in the house working on something, check this before anything else.
4. Sediment and corrosion debris have collected downstream
A storage cylinder ages from the inside. Scale, and eventually corrosion products, break loose and travel with the water, and they lodge in the narrowest things in the path: tap aerators, mixer cartridges, the mesh filters in flexible connection hoses under the vanity, and the tempering valve.
The pattern to look for is that it affects the hot side only, appears gradually, and gets worse at whichever outlet is furthest along or narrowest. Discoloured water alongside it points the same way — see the other hot water faults for how to read that.
5. A continuous flow unit is at its limit, not low on pressure
If you have a continuous flow (instantaneous) system, this one is not a fault at all. These units do not store water; they heat it as it passes, and they can only raise it by so many degrees at so many litres per minute. Ask for a bigger temperature rise — which is exactly what winter does — and the unit holds the temperature by cutting the flow.
So a continuous flow system that felt fine in summer and feels weak in July is behaving as designed. It is a sizing question rather than a repair, and it is covered in instantaneous versus storage and in what size system you need.
6. A pressure limiting or expansion control valve is failing
Most mains-pressure hot water installations have valves that manage pressure — limiting what comes in, and giving expanding hot water somewhere to go. When one of these fails it can restrict, and depending on where it sits in the plumbing that restriction can land on the hot side alone.
This is diagnosis for a plumber rather than a check for you, but it is worth knowing the category exists, because it is the usual answer when the simple causes above have all been ruled out.
The checks that are yours to make
All of these are outside the system, need no tools beyond a cloth, and none of them is licensed work:
- Compare hot and cold at the same tap, then compare that tap with another one. One tap behaving differently to the rest is a tap problem, not a system problem.
- Check whether the kitchen and laundry are fine while the bathroom is weak. That split points at the tempering valve.
- Look at the valve handles on the cold inlet to the system and under the basins. Are they fully open?
- Unscrew and rinse an aerator on an affected tap. If the flow improves, you have found a restriction, and the debris in the mesh tells you where it came from.
- Note when it started, and whether it was sudden or gradual. Sudden points at a valve or a failure; gradual points at scale and debris.
- Note the season. Weak only in winter, on a continuous flow unit, is a temperature rise limit.
What to tell the plumber
The callout goes faster, and costs less of everyone's time, if you arrive with four facts rather than "the hot water is bad":
- Hot only, or hot and cold?
- Everywhere, or particular taps? Which ones.
- Sudden or gradual, and did it follow any work in the house?
- What the system is — storage or continuous flow, gas, electric, heat pump or solar — and whether the temperature has changed as well as the flow.
How far can you take this yourself?
The comparisons, the valve handles and the aerators are yours. Everything on the pressurised side of a hot water installation — valves, the tempering valve, the system itself, the pipework — is licensed plumbing work in every Australian state and territory, and the plumbing regulator for your state or territory is who licenses and checks it. Where the line falls is set out in full on the licensed work pillar.
Frequently asked questions
Why is my hot water pressure low but the cold is fine?
Because the fault is on the hot side rather than the supply. The usual causes are a gravity-fed system that was always low pressure, a failing tempering valve, a valve left part-closed after work, sediment from the cylinder blocking aerators and hoses, or a continuous flow unit hitting its flow limit in winter.
Why is the hot water weak in the bathroom but fine in the kitchen?
That split usually points at the tempering valve, which feeds bathroom outlets and blends hot water down with cold before it reaches you. Kitchen and laundry taps are often not fed through it, so a restricted or failing tempering valve shows up in the bathroom alone.
Can I fix low hot water pressure myself?
You can rinse a tap aerator, check that isolation valves are fully open, and compare taps to work out whether it is one outlet or the whole hot side. Adjusting pressure limiting valves, replacing a tempering valve or working on the system is licensed plumbing work in Australia.
Why does my instantaneous hot water go weak in winter?
It is not losing pressure. A continuous flow unit heats water as it passes and can only raise it by so many degrees at so many litres per minute. Colder winter mains water needs a bigger temperature rise, so the unit reduces the flow to hold the temperature. That is a sizing limit rather than a fault.
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