Gas Hot Water System Not Working? Diagnose It Yourself

If your gas hot water system is not working, start with a few quick checks before you call a tradie. Most no-hot-water faults come down to gas supply, power to the unit, a pilot light gone out, or low water flow preventing ignition. Work through the steps below in order, and stop if you smell gas or see signs of burning or soot.

Quick checklist: fix the most common issues first

1. Check gas supply is on. See if your stove or other gas appliance works. At the meter, make sure the gas isolation valve is parallel with the pipe. On LPG, confirm the selected cylinder is not empty and the changeover lever is pointing to a full bottle. If you can smell gas, turn the supply off, ventilate, and call the emergency number.
2. Check power to the unit. Many gas systems need electricity for ignition, display and fan. Ensure the power point switch is on, the plug is fully in, and the circuit breaker or safety switch (RCD) has not tripped. Try another appliance in the same outlet. If power was off, restore it and see if hot water returns.
3. Reset the system. Turn power off to the unit for 30 seconds, then back on. Some continuous flow models also have a reset button under a flap. After resetting, run a hot tap for a minute to see if it fires.
4. Relight a pilot (storage units with standing pilot). Access the instructions on the inner panel. Follow the lighting sequence carefully. Hold the pilot button in long enough for the thermocouple to heat. If the pilot will not stay lit, stop and call a licensed gasfitter.
5. Open a hot tap fully for continuous flow units. Low-flow mixers or shower heads can drop below the unit’s minimum flow, so it never ignites. Open the hot tap fully to test. If it now works, clean aerators or consider a shower head with a higher flow rating.
6. Look for an error code. Many continuous flow units display a code. Note it and check the manual on the inside cover. Codes that return after a reset usually need a pro to diagnose.
7. Check the water isolation valves. At the heater there may be a small lever valve on the cold inlet and sometimes on the hot outlet. Make sure they are fully open. Partly closed valves lead to weak flow or temperature swings.
8. Confirm the temperature setting. For storage, the cylinder should be at 60 °C to control legionella; a tempering valve mixes it down to around 50 °C at taps. For continuous flow, a setpoint of 50–55 °C suits most homes. If the dial is turned down, turn it up and test.
9. Inspect for obvious faults. Sooting, scorch marks, melted plastic, water leaking from the tank, or the relief valve running constantly are red flags. Turn the unit off and call a pro.
10. On LPG, check the regulator and hoses. Frosting in very cold weather can briefly reduce pressure. Ensure hoses are in good condition and upright bottles are secured and not tilted.

How gas hot water works (and why it stops)

Storage vs continuous flow

Gas storage systems keep a cylinder of water hot with a burner. Older models use a standing pilot; newer ones have electronic ignition. If the pilot goes out or the gas valve or thermocouple fails, you will get no hot water.

Continuous flow (instantaneous) systems heat water only when a hot tap opens and flow passes a sensor. They need sufficient water flow and gas pressure to ignite and modulate. If flow is too low, a filter is blocked, or power is off, they will not fire.

Tempering valves and temperature

Most homes have a tempering valve mixing hot with cold to deliver about 50 °C to bathrooms. A stuck or failing valve causes warm, not hot water, even when the heater is fine. Storage systems must keep the tank at 60 °C to reduce bacteria risk.

Troubleshooting by symptom

No hot water at any tap

• Gas off. Meter valve closed, LPG cylinder empty, or emergency shut-off used. Turn gas back on only if there is no smell of gas.
• No power to the unit. Check power point, switch, fuse or RCD. Many units need electricity even if they have gas.
• Pilot out (storage). Relight as per the panel. If it goes out again soon after, likely thermocouple or gas valve fault; call a gasfitter.
• Error code (continuous flow). Note the code, reset once. If it returns, service is needed.
• Blocked inlet filter (continuous flow). Many units have a small strainer on the cold inlet. Turn off water and gas, relieve pressure, remove and clean the strainer. If unsure, leave to a pro.

Hot water cuts in and out at taps

• Flow too low for ignition. Ultra-efficient shower heads, clogged aerators or partly closed isolation valves can make the heater cycle. Open the tap fully to test. Clean or replace restrictors that are too aggressive.
• Single-lever mixer letting cold cross over. A worn cartridge can push cold across to the hot line and trick the heater. Shut the cold isolation valve to that mixer and test another hot tap; if hot returns, replace the cartridge.
• Long pipe runs and small draws. Short bursts at a basin may never trigger or deliver heat before the tap is closed. That is normal behaviour on continuous flow.

Water is warm, not hot

• Tempering valve stuck or mis-set. Bathrooms are tempered to about 50 °C. If kitchen hot is also cool, the issue may be at the heater. If only bathrooms are cool, suspect the tempering valve.
• Thermostat set low. For storage, set the cylinder to 60 °C; for continuous flow, try 50–55 °C. Avoid scald risks and comply with local plumbing rules.
• Winter inlet temperatures. In colder months, a small continuous flow unit may struggle to lift temperature at high flow. Try slightly lower flow or a higher setpoint.
• Scaling or sediment (storage). Mineral buildup insulates the burner from the water, reducing output. A service can flush sediment and check the anode in glass-lined tanks.
• Gas pressure or regulator issue. Especially on LPG, low bottle pressure or a failing regulator can limit burner output. Swap to a full cylinder and test.

Pilot will not stay lit (storage)

• Thermocouple fault. If the pilot lights but goes out when you release the button, the thermocouple may be worn or loose. This is a job for a licensed gasfitter.
• Draught or flue issue. Wind blowing down the flue can blow out the pilot. Check for debris around the flue terminal and ensure the area is ventilated.
• Gas control valve fault. Internal safety mechanisms can fail with age. Replacement or a new heater may be more economical on older units.

When to stop and call a licensed gasfitter

• You smell gas, hear hissing, or see bubbles on a soapy water test.
• The burner soots, the flame is yellow, or there are scorch marks.
• The relief valve runs constantly, the tank is leaking, or the unit is rusted.
• Error codes return after a reset.
• The pilot will not stay lit after a careful attempt to relight.
• Vent terminals are damaged, blocked, or too close to openings.
• Any work involving gas pipework, flues, internal components, or sealed panels. That requires a licensed gasfitter and a certificate of compliance.

Maintenance that prevents breakdowns

• Annual service. Have a licensed gasfitter check burner condition, gas pressure, flue integrity, ignition, and safety devices.
• Clean inlet strainers. On continuous flow units, a quick clean of the cold inlet filter screen restores proper flow and ignition.
• Test the temperature pressure relief (TPR) valve. Briefly lift the lever to ensure it operates and reseats. If it dribbles constantly or will not test, have it replaced.
• Check the tempering valve every 1–2 years. Age, scale and debris can make it stick. Recalibration or replacement restores consistent hot water delivery.
• Protect storage tanks. For glass-lined cylinders, replacing the sacrificial anode on schedule helps extend life in hard-water areas.
• Keep vents and flues clear. Remove leaves, dust and insect nests. Maintain safe clearances around the outdoor unit for combustion air.

Work through the checklist calmly and safely. If a simple reset, relight or flow fix does not restore hot water, bring in a licensed gasfitter. They can test gas pressure, combustion and controls properly, and advise whether repair or replacement is the smarter long-term move.