If your gas hot water unit sparks and lights but then cuts out within a few seconds, you're not alone — this is one of the most common gas hot water complaints in NZ homes. The good news is there are a handful of well-known causes, and a licensed gasfitter can usually diagnose and fix the problem in a single visit.
How a Gas Hot Water Unit Stays Lit
Before diving into causes, it helps to understand the basics. When you call for hot water, the unit opens a gas valve, sparks an igniter, and lights the burner. A flame-sensing device — usually a thermocouple or flame rod — then confirms the flame is present and signals the gas valve to stay open. If that signal isn't received or is too weak, the valve closes as a safety measure and the flame dies. This is a deliberate safety feature, not a fault in itself.
Common Reasons the Flame Won't Stay Lit
1. Faulty or Worn Thermocouple
The thermocouple is the most frequent culprit in older continuous-flow and storage gas hot water systems. It's a small sensor that sits in the flame and generates a tiny electrical signal to hold the gas valve open. Over time, thermocouples degrade, corrode, or simply wear out. A weak or failed thermocouple means the valve closes the moment the igniter stops sparking.
2. Dirty or Misaligned Flame Sensor
In newer electronic-ignition units, a flame rod or ionisation sensor does the same job as a thermocouple. If the sensor tip is coated in carbon or mineral deposits, or has shifted out of the flame path, it can't detect the flame reliably. The unit then shuts off as a precaution.
3. Low or Interrupted Gas Supply
- Check that your LPG bottle isn't empty (if applicable) or that your natural gas account is active.
- A partially closed isolation valve on the gas line to the unit can restrict flow enough to cause nuisance shutdowns.
- If other gas appliances in the home are also behaving oddly, the issue may be upstream — contact your gas retailer or network operator.
4. Blocked or Dirty Burner
Dust, insects, and mineral scale can partially block burner ports. A weak, uneven flame may not reach the sensor reliably, causing the unit to cut out. This is more common in units that haven't been serviced for several years.
5. Venting or Flue Problems
A blocked or poorly performing flue can cause the unit to overheat or starve of combustion air, triggering a thermal cutout. Check that the flue outlet is clear of debris, bird nests, or vegetation.
6. Faulty Gas Valve or Control Board
Less commonly, the gas valve solenoid or the electronic control board itself may be failing. These components are more expensive to replace and require a licensed gasfitter to diagnose and fit.
7. Water Flow or Pressure Issues
Many modern continuous-flow units require a minimum water flow rate to activate and stay on. A partially closed tap, a blocked inlet filter, or low mains pressure can cause the unit to cycle off. Check the cold-water inlet filter screen for sediment build-up — this is one of the few maintenance tasks a homeowner can safely do (refer to your unit's manual first).
What You Can Safely Check Yourself
As a homeowner, you can safely:
- Check the LPG level or confirm your natural gas supply is active.
- Inspect the flue outlet externally for obvious blockages (do not modify the flue).
- Check the cold-water inlet filter for sediment if your manual describes how.
- Reset the unit using the manufacturer's reset procedure (usually a button on the front panel).
- Check isolation valves are fully open.
Do not attempt to adjust, repair, or replace any gas components yourself. In New Zealand, all work on gas appliances and fittings is restricted work under the Gas Act 1992 and must be carried out by a gasfitter licensed by the Plumbers, Gasfitters and Drainlayers Board (PGDB). Attempting gas repairs without a licence is illegal and extremely dangerous.
Safety Note
If you smell gas at any point — do not attempt to relight the unit. Turn off the gas supply at the meter or bottle, ventilate the area, avoid switches and open flames, and call your gas retailer's emergency line or 111 if you suspect a serious leak. WorkSafe NZ provides guidance on gas safety in the home.
When to Call a Professional
You should call a licensed gasfitter if:
- The unit continues to cut out after basic checks.
- You suspect a faulty thermocouple, flame sensor, gas valve, or control board.
- The unit is more than 8–10 years old and hasn't been serviced recently.
- You notice soot, unusual smells, or yellow/orange flames (should be blue).
- Any gas pipework, fittings, or components need to be replaced.
A qualified gasfitter will carry the right test equipment to measure thermocouple output, check gas pressure, and inspect the burner and flue safely. Regular servicing — typically every 2–3 years — can prevent many of these issues and extend the life of your unit. You can find licensed gasfitters in your area on tradedirectory.co.nz.
For rental properties, landlords should also be aware of obligations under the Healthy Homes Standards regarding heating and hot water.
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