A gas heater that keeps shutting itself off is almost always doing so for a safety reason — the good news is that means the safety systems are working. The frustrating news is that something needs attention before your heater will run reliably again. Below are the most common causes, what you can safely check yourself, and when you must call a licensed gasfitter.
How Gas Heater Safety Shutoffs Work
Modern gas heaters — whether flued, unflued, or ducted — are fitted with several automatic safety devices designed to cut the gas supply if something goes wrong. These include:
- Thermocouple or thermopile – senses whether the pilot flame is lit; if it can't detect a flame, it shuts off the gas valve to prevent unburned gas from escaping.
- Overheat (high-limit) thermostat – cuts the heater off if the unit gets too hot, usually due to restricted airflow.
- Oxygen depletion sensor (ODS) – fitted to many unflued heaters; shuts the unit down if oxygen levels in the room drop too low.
- Flame failure device – similar to the thermocouple; kills the gas if the main burner flame goes out unexpectedly.
Understanding which device is triggering the shutoff helps narrow down the cause.
Common Reasons Your Gas Heater Keeps Cutting Out
1. Dirty or Faulty Thermocouple
The thermocouple is one of the most common culprits. Over time, carbon deposits build up on the tip, preventing it from accurately sensing the pilot flame. The heater lights, runs briefly, then shuts off — often within 30–60 seconds. A thermocouple replacement is a straightforward job for a gasfitter and is relatively inexpensive.
2. Weak or Unstable Pilot Flame
If the pilot flame is small, yellow, or flickering rather than a steady blue, it may not be heating the thermocouple sufficiently. This can be caused by a partially blocked pilot orifice, low gas pressure, or a draught affecting the flame. Do not attempt to adjust gas pressure or internal gas components yourself — this is restricted work under NZ law.
3. Overheating Due to Blocked Airflow
If the heater's air intake or outlet grilles are blocked by dust, furniture, or curtains, heat builds up inside the unit and the high-limit thermostat trips. Check that:
- There is clear space around the heater as specified in the manufacturer's instructions.
- The grilles and filters are free of dust and lint.
- Furniture or drapes haven't been moved too close.
Cleaning the grilles and filters is something most homeowners can do safely — just make sure the heater is switched off and cool first.
4. Low Oxygen / Poor Ventilation (Unflued Heaters)
Unflued gas heaters consume oxygen from the room as they burn. If the room is very well sealed — common in modern NZ homes built to tighter insulation standards — the oxygen depletion sensor may trigger a shutoff. This is a genuine safety warning: open a window slightly to improve ventilation. Energywise has useful guidance on safe use of unflued heaters in NZ homes.
5. Gas Supply Issues
Low gas pressure — caused by a nearly empty LPG cylinder, a partially closed valve, or a regulator fault — can cause the burner to cut out. Check:
- LPG cylinder level (heft it or use a gauge strip).
- That the isolation valve at the cylinder or meter is fully open.
- Whether other gas appliances in the home are also affected.
If you suspect a regulator fault or a problem with your natural gas supply, contact your gas retailer or a licensed gasfitter.
6. Faulty Gas Valve or Control Board
If none of the above apply, the issue may be an internal gas valve or electronic control board fault. These components must only be inspected and replaced by a licensed gasfitter — attempting to disassemble gas valves yourself is both dangerous and illegal under the Gas Act 1992.
7. Flue Blockage (Flued Heaters)
For flued heaters, a blocked or damaged flue can cause combustion gases to back up, triggering safety shutoffs. Bird nests, debris, or a damaged flue terminal are common causes. A gasfitter or chimney professional should inspect the flue.
What You Can Safely Check Yourself
| Check | Safe for homeowner? |
|---|---|
| Clean external grilles and filters | ✅ Yes |
| Check LPG cylinder level | ✅ Yes |
| Ensure adequate ventilation | ✅ Yes |
| Check isolation valve is open | ✅ Yes (turn only, don't modify) |
| Adjust gas pressure or regulators | ❌ No — licensed gasfitter only |
| Replace thermocouple or gas valve | ❌ No — licensed gasfitter only |
| Inspect or modify flue | ❌ No — licensed gasfitter only |
NZ Regulations and Restricted Work
Under the Gas Act 1992 and associated regulations, any work on gas fittings — including internal components, gas valves, regulators, and connections — is restricted work that must be carried out by a gasfitter licensed by the Plumbers, Gasfitters and Drainlayers Board (PGDB). Always ask to see your gasfitter's current practising licence before work begins.
⚠️ Safety note: If you smell gas at any point, do not attempt to operate the heater. Ventilate the area immediately, avoid switches and open flames, and call your gas retailer's emergency line or 111 if necessary.
When to Call a Professional
Call a licensed gasfitter if:
- The heater continues to cut out after you've checked airflow, ventilation, and gas supply.
- You suspect a thermocouple, gas valve, or regulator fault.
- The pilot flame is weak, yellow, or won't stay lit.
- You have a flued heater and suspect a blocked or damaged flue.
- You can smell gas anywhere near the appliance.
A qualified gasfitter can diagnose the fault safely, carry out any restricted repairs, and certify the work. You can find licensed gasfitters in your area at tradedirectory.co.nz.