If your instant gas hot water system has stopped heating, the cause is usually one of five things: no gas or power supply, low water flow, an ignition fault, an active error code, or mineral scale clogging the heat exchanger. Stop immediately and evacuate if you smell gas, then call your emergency gas line from outside. If there’s no gas smell, check your power and gas supply first before touching anything else.
TL;DR:
- Most instant gas hot water system failures are caused by flow restrictions, mineral scale buildup, or flue blockages, which require licensed technicians to fix safely.
- Ensuring the gas and electrical supplies are active, along with clearing inlet filters and verifying flow rates, can prevent many common issues without professional help.
- Repeated lockouts or error codes often point to internal faults like scale on the heat exchanger or a faulty flow sensor, necessitating professional diagnosis and repair.
- Regular maintenance, including descaling and cleaning inlet filters every one to two years, can significantly extend the system’s lifespan and reduce unexpected failures.
- Homeowners should prepare detailed information such as error codes and photos before calling a technician, as this speeds up diagnosis and repairs.
Table of Contents
- How does an instant gas hot water system actually work?
- Why is my instant hot water not hot? Common failures explained
- What should I check before calling a plumber?
- How do I stop instant hot water issues coming back?
- When does an instant hot water fault need a professional?
- What does a licensed technician actually check?
- What I’d tell every homeowner before they call anyone
- How Priority Plus Plumbing gets your hot water back on today
- Where to check further
- Sources
- FAQ
How does an instant gas hot water system actually work?
An instant or continuous flow gas hot water system only fires up when it detects water moving through it. A flow sensor inside the unit picks up water passing through the pipework and signals the burner to ignite. No flow, no heat. That’s why a trickle from a partly blocked tap or a low flow showerhead can sometimes stop the unit from firing at all.
Most units also need mains electrical power to run the electronic ignition and control board, even though the actual heating comes from gas. A tripped circuit breaker can leave you with a perfectly good gas supply and a unit that still won’t light. Once the burner ignites, the heat exchanger transfers heat directly to the water passing through it, rather than storing a tank of hot water. Australian Gas Networks confirms both gas and electricity need to be present for the ignition to spark.
There’s a minimum flow rate every unit needs to activate, typically a few litres a minute depending on the model. Fall below that threshold and the burner simply won’t engage. Cold inlet water in winter also means the unit works harder to reach set temperature, which can feel like a delay even when nothing is faulty.
Why is my instant hot water not hot? Common failures explained
Most instant hot water issues fall into a handful of recognisable patterns. Working through them by symptom is faster than guessing at causes.
No hot water at all. Start with the basics: is the gas supply turned on, and if you’re on LPG, is the bottle empty? Check that the power to the unit hasn’t tripped at the switchboard, and confirm the isolation valve near the unit is fully open. A blocked inlet filter or a failed flow sensor will also stop the system dead, even with gas and power both present.
The unit shuts down mid‑shower. This is one of the more alarming instant hot water problems, and it’s usually a safety mechanism doing its job rather than a random glitch. A dirty burner, a failing ignitor, a faulty flow sensor, or a thermostat reading incorrectly can all trigger a shutdown. Some units go into lockout after detecting an overheat condition, which is the system protecting itself rather than malfunctioning outright.
Temperature spikes then cold blasts. Variable water flow is the usual suspect here, particularly if someone else in the house flushes a toilet or starts a tap while you’re in the shower. Municipal pressure dips do the same thing. If it’s happening with no other explanation, a failing sensor is worth suspecting, and Metropolitan Hot Water notes flow restrictions and scale build-up are frequent culprits behind this exact symptom.
Lukewarm water that never gets properly hot. If this happens whenever two outlets run at once, the unit is probably undersized for your household’s simultaneous demand rather than faulty. It’s a common issue after a renovation adds a second bathroom without upsizing the hot water system to match.
Low pressure or the unit won’t start. Clogged inlet filters and aerators are the most common cause, but a faulty pressure relief valve or a hidden pipe leak can also starve the unit of the flow it needs to activate.
Ignition or pilot faults, and error codes. Every manufacturer uses its own code system on the control panel, and writing the exact code down before you do anything else will save a technician significant diagnostic time.
Blocked flue or scale on the heat exchanger. A restricted exhaust or fan failure can force the unit into a safety lockout, and scale build-up on the heat exchanger reduces heating efficiency over time, sometimes triggering the same error codes as a flue blockage.
Worth knowing: According to Metropolitan Hot Water, flow restrictions, mineral scale, and blocked flues are among the most consistently reported causes of instant gas hot water failures, and most require a licensed technician to clear safely rather than a DIY fix.
What should I check before calling a plumber?
Run through these steps in order. Stop the moment any step points to a genuine gas hazard.
- Smell gas? Leave the property immediately and call the emergency gas line from outside. Do not touch switches, the unit, or try any reset.
- No gas smell? Check other gas appliances in the house are working, then check your gas meter or LPG bottle level.
- Check power. Confirm the circuit breaker for the hot water unit hasn’t tripped and hasn’t been switched off at an isolation switch nearby.
- Open valves and clean filters. Make sure the isolation valve is fully open, then clean the inlet filter and any aerators on the affected tap.
- Test one outlet. Run a single tap to see if the fault is isolated to one outlet or affects the whole system.
- Note any error code before resetting. Write down exactly what’s shown on the control panel, then check your manual before attempting a single reset, My Plumbing Pros recommends this step specifically so you don’t lose diagnostic information.
- Stop if it locks out again. A unit that keeps tripping or keeps throwing the same code needs a licensed gasfitter, not another reset attempt.
Pro Tip: Photograph the control panel screen showing the error code before you reset anything. If the code disappears after a reset but the fault returns, that photo is often the fastest way for a technician to diagnose the problem over the phone.
How do I stop instant hot water issues coming back?
Regular maintenance is the difference between a unit that lasts and one that keeps failing at the worst possible moment. In hard water areas, scale builds up faster on the heat exchanger, and a professional flush or descale once a year is a sensible baseline rather than an optional extra.
- Clean inlet filters, showerheads and aerators every few months to keep flow rates above the unit’s activation threshold.
- Be careful with low-flow fittings. Some water-saving showerheads restrict flow below what the unit needs to fire, which looks like a fault but isn’t.
- Book a licensed service check every one to two years, more often in hard water regions.
- If your household has grown or you’ve added a bathroom, get the unit’s capacity checked against current demand rather than assuming it’ll cope.
When does an instant hot water fault need a professional?
Any smell of gas, a visible leak, or a suspected carbon monoxide issue is an immediate emergency. Leave the property and call for urgent help straight away.
Repeated lockouts, persistent error codes, or anything involving ignition, the burner, the flue, or internal wiring also warrants an urgent attended visit rather than more DIY troubleshooting. Before you call, gather the unit’s model and serial number, the exact error code shown, a note of what checks you’ve already done, and photos of both the unit and its control panel. This information alone can cut a technician’s diagnostic time significantly, letting them arrive prepared rather than starting from scratch.
What does a licensed technician actually check?
A technician arriving on-site for instant hot water problems runs through a set sequence: supply pressure tests on the gas line, a check of the ignition module and control board, a flue inspection, and an assessment of the heat exchanger for scale or damage. Common on-site repairs include replacing a flow sensor, cleaning or replacing an ignitor, and clearing flue blockages. Replacement gets recommended when the heat exchanger itself is corroded or when repeated repairs on an ageing unit start costing more than a new system would.
Licensed gasfitters offer 24/7 emergency callouts with upfront pricing agreed before work starts and a workmanship guarantee on every job. If your unit’s already thrown a code and you’ve done your safety checks, our hot water repair technicians can take it from there.
What I’d tell every homeowner before they call anyone
The mistakes I see repeated most often are undersizing a unit during a renovation, skipping annual servicing because everything seems fine, and fitting a water-saving showerhead without checking its flow against the unit’s activation threshold. When installing a new system, get the isolation valve placed somewhere accessible, confirm flue clearance meets the manual’s spec, and size the unit for simultaneous outlet use, not just one bathroom running at a time.
— Peter Honey
How Priority Plus Plumbing gets your hot water back on today
There are plenty of guides that’ll walk you through resetting a control panel, but when the fault sits behind the front cover, none of that troubleshooting fixes a failed ignitor or a corroded heat exchanger. Licensed gasfitters run 24/7 emergency callouts, quote upfront before any work starts, and back every repair with a workmanship guarantee.
Before you book, have your unit’s model and serial number ready, note down the exact error code on the display, and take a couple of photos of the control panel and the unit itself. That handful of details lets our technicians diagnose the fault faster once they’re on site, whether it’s a flow sensor, a blocked flue, or scale on the heat exchanger. If your system’s locking out repeatedly or throwing the same code after a reset, get in touch through our emergency plumbing service page and we’ll get someone out to fix it properly, not just patch it for another week.
Where to check further
- Australian Gas Networks for gas emergency numbers and troubleshooting basics.
- Energy Rating for official efficiency benchmarks on gas instantaneous heaters.
- Trade teams tracking preventive servicing schedules often rely on tools like field service software built for plumbers to keep maintenance visits on schedule.
Sources
- Gas Hot Water Not Working? A Troubleshooting Guide: Common Causes — Australia Gas Networks
- Hot Water Not Getting Hot? What’s Wrong and How to Fix It — My Plumbing Pros
FAQ
What are some common problems with instant hot water heaters?
The most frequent faults are ignition failures, low flow preventing activation, blocked flues, mineral scale on the heat exchanger, and units undersized for simultaneous hot water demand.
What is the average lifespan of an instant hot water system?
Most continuous flow gas units last well over a decade with regular servicing, though hard water areas and skipped maintenance can shorten that considerably through scale build-up.
Why do plumbers dislike tankless water heaters?
It’s less about disliking the technology and more that instant systems have tighter flow and ignition tolerances than storage tanks, so a minor issue like a blocked filter or a low-flow showerhead can stop them working entirely, unlike a tank that keeps a buffer of stored hot water.
Why is my instant hot water not hot?
Usually it’s low flow below the unit’s activation threshold, a blocked inlet filter, scale on the heat exchanger, or simultaneous demand exceeding the unit’s capacity. Check power and gas supply first, then flow-related causes, before assuming a major fault.