When hot water is not working, but cold is, the problem is inside your hot water system, not your general plumbing. The most common causes are a tripped circuit breaker, a failed heating element, a gas pilot light that has gone out, a faulty thermostat, or a failed tempering valve. The fix depends on your system type.
Older Sydney homes with ageing electric storage or gas hot water systems are especially prone to unexpected hot water failures.
Hello Plumbing diagnoses and repairs hot water problems across Sydney and the Northern Beaches. This guide covers how to diagnose the issue, what you can check yourself, and when to call a licensed plumber.
Why Does Cold Water Work but Not Hot?
Cold water flows directly from the mains into your home. Hot water takes a detour through your hot water system, where it is heated before reaching the taps. When cold water works but hot water doesn’t, it tells you the mains supply is fine, and the problem sits somewhere between the hot water system and your fixtures.
According to Sydney Water, property owners are responsible for maintaining all private plumbing fixtures, including hot water systems. A hot water failure is not a mains issue, and Sydney Water will not attend. You need a licensed plumber. The key to diagnosing the problem quickly is determining whether the issue affects one fixture or every fixture, and which type of hot water system you have.
Is It One Tap or Every Tap?
This is the first question to answer because it tells you where the fault sits.
Every Tap in the House Has No Hot Water
The hot water system itself has failed. The heating source (electric element, gas burner, heat pump compressor, or solar collector) has stopped producing hot water. Skip to the system-by-system causes below.
Only One Tap or Shower Has No Hot Water
The hot water system is working, but something is blocking hot water from reaching that specific fixture. The most common causes are a failed mixer cartridge inside the tap or shower valve, a seized hot water isolation valve under the sink, or a localised blockage in the hot water pipe feeding that fixture.
Hot Water Comes Out Lukewarm But Never Fully Hot
This often points to a failed tempering valve. Under Australian Standard AS/NZS 3500, a tempering valve is required on all hot water systems to blend cold water in and deliver a safe temperature (50°C maximum) at bathroom taps. When it fails, it may over-blend and deliver lukewarm water, or under-blend and deliver scalding water.
Hot Water Runs Out Faster Than Usual
The system is heating but not maintaining temperature. In electric storage systems, the lower heating element may have failed, leaving only the upper element to heat a smaller volume. In gas storage systems, sediment buildup at the bottom of the tank insulates the water from the burner, reducing capacity.
Common Causes by Hot Water System Type
The troubleshooting steps depend entirely on which system you have. Here is what fails in each type.
Electric Storage
Electric storage hot water systems are the most common type in older Sydney homes, particularly across Dee Why, Balgowlah, and Brookvale, where many homes still have original units from the 1980s and 1990s.
Common causes of no hot water include a tripped circuit breaker or blown fuse, a failed upper or lower heating element, a faulty thermostat, a failed off-peak relay or timer (many Sydney electric systems run on controlled load tariffs and heat overnight only), and sediment buildup insulating the element from the water.
Gas Storage
Gas storage systems are common in North Shore homes built from the 1990s onward. Common causes include a pilot light that has gone out (check the viewing window at the base of the unit), a faulty thermocouple that shuts off gas to the pilot, a blocked or corroded flue preventing proper combustion, a failed gas valve, and sediment buildup in the tank, reducing heating efficiency. If you smell gas, do not attempt any troubleshooting. Evacuate and call Jemena on 131 909.
Continuous Flow (Gas Instant)
Continuous flow systems are common in modern builds across Warriewood, Narrabeen, and newer townhouse developments. They heat water on demand as it flows through the unit.
Common causes of no hot water include a clogged inlet filter (the most common and easiest fix), insufficient water flow to trigger ignition, a failed ignition module or flame sensor, a blocked exhaust flue, and an error code displayed on the unit’s control panel.
Heat Pump
Heat pump systems extract heat from the surrounding air to heat water. They are increasingly common in energy-efficient homes and renovations. Common causes of failure include compressor failure in cold weather (units lose efficiency below 5°C), a tripped circuit breaker, a failed fan motor, refrigerant leaks, and a frozen evaporator coil in winter.
Solar with Electric or Gas Boost
Solar hot water systems use roof-mounted collectors to heat water, with an electric or gas booster for cloudy days. If there is no hot water, the booster may have failed (check the booster’s circuit breaker or pilot light), the solar collector may be damaged, dirty, or shaded by a new tree, or the circulation pump between the collector and tank may have failed. The booster is the most common failure point and is diagnosed the same way as a standalone electric or gas system.
Safe Checks You Can Do Before Calling a Plumber
Before calling a professional, try these checks. They resolve the most common causes and cost nothing.
- Check the circuit breaker: Go to your switchboard and look for a tripped breaker. Electric hot water systems often have a dedicated breaker, sometimes on a separate controlled load meter for off-peak tariffs. Flip it fully off, then back on. If it trips again immediately, stop and call a plumber.
- Check the pilot light (gas systems): Look through the viewing window at the base of your gas storage system. If you can see a small flame, the pilot is lit, and the issue is elsewhere. If there is no flame, follow the manufacturer’s relight instructions on the unit’s label. If the pilot won’t stay lit, the thermocouple is likely faulty.
- Check the tempering valve: If water from the hot tap is lukewarm everywhere in the house, the tempering valve may be over-blending. The valve is usually located near the base of the hot water system. It is not a DIY adjustment. A licensed plumber needs to test and recalibrate or replace it.
- Check the inlet filter (continuous flow): Turn off the water supply to the unit. Unscrew the inlet filter housing on the cold water inlet. Remove the mesh screen, rinse it under a tap, and replace it. A clogged filter restricts flow and prevents the unit from igniting.
- Run the hot tap for two minutes: If you have a storage system on an off-peak tariff, it may have used its stored hot water and not yet been reheated. Run the hot tap and wait. If hot water returns after a few minutes, the system is working, but may be undersized for your household.
When to Call a Licensed Plumber
If the basic checks above don’t restore your hot water, the problem is internal and needs professional diagnosis.
Emergency Signs
If you smell gas near the hot water system, evacuate and call Jemena on 131 909. If the system is leaking significantly from the tank or base, turn off the power or gas and the water supply to the unit, then call an emergency plumber. If your circuit breaker keeps tripping after resetting, stop resetting it and call a plumber or electrician.
What Your Plumber Will Actually Do
In over 10 years of diagnosing hot water failures across Sydney, I have found that a structured inspection resolves the issue every time. Your plumber will check the power supply, gas supply, and all safety switches first. They will test the thermostat, heating elements (electric), or burner and pilot assembly (gas) with diagnostic equipment.
They will inspect the tempering valve, check for leaks at all connections, assess the anode rod condition (storage systems), and test the pressure relief valve. For continuous flow systems, they will check error codes, clean the inlet filter, and test ignition. You get a diagnosis and a fixed-price quote before any work begins.
Typical Repair and Replacement Costs in Sydney
| Repair | Typical Cost Range |
| Thermostat replacement | $150 to $300 |
| Heating element replacement (electric) | $200 to $400 |
| Tempering valve replacement | $250 to $450 |
| Thermocouple replacement (gas) | $150 to $300 |
| Continuous flow inlet filter clean | $100 to $200 |
| Full hot water system replacement (supply and install) | $1,200 to $3,500+ |
These are indicative ranges. Your plumber should provide a fixed-price quote specific to your system before starting any work.
Why Hot Water Problems Are Common in Older Sydney Homes
Sydney’s housing stock spans more than a century, and each era tends to have specific hot water system types that are now reaching or past their expected lifespan.
- 1970s to 1990s electric storage in Northern Beaches homes: Properties in Dee Why, Manly, and Collaroy commonly have electric storage systems that are 15 to 25 years old. Heating elements corrode, anodes dissolve, and thermostats fail with age. If your electric hot water is not working, but cold is, and your system is over 10 years old, component failure is the most likely cause.
- Off-peak tariff systems across Sydney: Many older electric storage systems run on controlled load tariffs (sometimes called off-peak or Tariff 31). They heat overnight only. If the off-peak relay, timer, or meter fails, the system stops heating, and you wake up to cold water. This is a Sydney-specific cause that many homeowners don’t realise.
- Gas storage in the 1990s to 2000s in North Shore and Lower North Shore homes: Properties in Mosman, Cremorne, and Neutral Bay with gas storage systems from this era are now seeing pilot light failures, thermocouple wear, and sediment buildup after 15 to 20 years of use.
- Undersized systems in renovated homes: Properties across Frenchs Forest and Forestville that have added bathrooms, en-suites, or granny flats during renovations may still have the original hot water system sized for the original household. The system runs out of hot water faster than expected, especially in winter when the inlet water is colder.
- Ageing tempering valves: Tempering valves have a recommended service life of around 5 years, but are rarely replaced proactively. A failed tempering valve is one of the most common causes of lukewarm water in Sydney homes and is frequently misdiagnosed as a hot water system failure.
How to Prevent Hot Water Breakdowns
Most hot water failures are preventable with basic, system-specific maintenance. At Hello Plumbing, we recommend the following.
Service Your System Annually
A licensed plumber checks all safety components, tests the thermostat and heating source, inspects for leaks, and assesses overall system health. Annual servicing catches failing components before they leave you without hot water. Under the Plumbing Code of Australia, all hot water work must be carried out by a licensed plumber.
Replace the Anode Rod Every 5 Years (Storage Systems)
The sacrificial anode rod protects the inside of your tank from corrosion. Once it dissolves completely, the tank itself starts corroding. Replacing the anode every 5 years extends tank life significantly and is one of the cheapest ways to avoid a full system replacement.
Test the Tempering Valve Every 5 Years
A plumber can test the tempering valve output temperature and replace it if it is not delivering within the safe range. This prevents both lukewarm water and scalding risks.
Know Your System Type and Age
Check the compliance plate on your hot water system. It shows the manufacture date, model number, and system type. If your system is over 10 years old, plan for replacement rather than waiting for an emergency failure. Hello Plumbing offers scheduled maintenance plans across Sydney and the Northern Beaches, so nothing gets missed.
One recent Hello Plumbing client, Ann Burgess, shared her experience after a hot water system issue: “I called Hello Plumbing after my hot water system started beeping, to indicate a problem. Andy arrived promptly that afternoon, inspected the problem, stopped the annoying beeping, and scheduled a repair for the next day. His team installed new valves and pipework and got the hot water heater going again. Since this repair a month ago, the hot water system has been working very well, with no further problems. Many thanks to Andy, Damon, and Ryder.”
That kind of prompt, professional response is what you should expect when your hot water stops working.
Areas We Service
Hello Plumbing diagnoses and repairs hot water system problems across Sydney, including the Northern Beaches, North Shore, Lower North Shore, Upper North Shore, Inner West, Eastern Suburbs, Western Sydney, and the Hills District. Whether your electric storage system has failed in Balgowlah or your continuous flow unit won’t ignite in Narrabeen, our licensed plumbers diagnose and fix it the same day.
Get Your Hot Water Fixed Today
If your hot water is not working but cold is, and basic checks haven’t resolved it, don’t suffer through cold showers. Call Hello Plumbing on 1300 303 055 or book online today. Every job comes with:
- Licensed plumbers with current NSW qualifications (Licence No. 480727C)
- Same-day service and 24/7 availability for hot water emergencies
- $0 call-out fee when booked online
- On-time guarantee — if we’re late, we pay you $100
- Fixed-price quotes with no hidden fees
- 15% pensioner discount on all services
- In-house payment plans with no interest and no finance companies
- Lifetime labour warranty on all workmanship
Get your hot water back today.
Frequently Asked Questions
Why is my hot water not working, but cold water is?
The most common causes are a tripped circuit breaker, a failed heating element or thermostat (electric systems), a pilot light that has gone out (gas systems), or a failed tempering valve. Cold water bypasses the hot water system entirely, so if cold works but hot doesn’t, the fault is inside the hot water unit itself.
Why is my hot water tap not working, but the cold is?
If only one tap has no hot water while others work fine, the hot water system is not the problem. The most likely cause is a failed mixer cartridge inside the tap or shower valve, a seized isolation valve under the sink, or a localised blockage in the hot water pipe feeding that fixture.
Why is my electric hot water not working, but the cold is?
Check the circuit breaker first. Electric systems often have a dedicated breaker, sometimes on a separate controlled load meter for off-peak tariffs. If the breaker has not tripped, the heating element, thermostat, or off-peak relay may have failed. A licensed plumber can test all components.
How much does it cost to fix a hot water system in Sydney?
Costs depend on the fault. A thermostat replacement costs $150 to $300. A heating element replacement runs $200 to $400. A tempering valve replacement costs $250 to $450. A full hot water system replacement with supply and installation starts from $1,200 and goes up depending on the system type and size.
Should I repair or replace my hot water system?
If your system is under 8 years old and the repair is a single component (element, thermostat, thermocouple), repair is usually the better option. If the system is over 10 years old and has multiple failing components or a leaking tank, replacement is the smarter long-term investment.
How do I prevent my hot water from breaking down?
Have your system serviced annually by a licensed plumber. Replace the anode rod every 5 years in storage systems. Test the tempering valve every 5 years. Know your system’s age and plan for replacement before an emergency failure forces a reactive, more expensive decision.
