That annoying pattern — lovely hot water one moment, then a blast of cold the next — is one of the most common complaints NZ homeowners have about gas hot water systems. The good news is there are several well-known causes, and some are straightforward to identify. The bad news is that most of the fixes involve restricted gasfitting work, meaning you'll need a licensed professional to carry out repairs.
The "Cold Water Sandwich" Effect
If you're getting a brief burst of cold water sandwiched between hot water — especially at the start of a shower — this is a very common phenomenon with instantaneous (continuous flow) gas hot water units. Here's what happens:
- Hot water sitting in the pipes from the last use arrives first.
- Then cold water that filled the pipes after the unit shut off comes through.
- Finally, the unit fires up and hot water arrives again.
This isn't a fault — it's simply how tankless systems work. Insulating your hot water pipes can reduce the effect, and some newer units have a recirculation feature to minimise it.
The Unit Isn't Igniting Properly
If the water goes cold mid-shower and doesn't recover, the burner may be cutting out or failing to ignite consistently. Common reasons include:
- Low gas pressure — If other gas appliances (like your hob or heater) are also underperforming, low pressure from the supply could be the issue. Contact your gas retailer or a licensed gasfitter.
- Dirty or faulty igniter — The ignition electrode can become sooty or worn over time, causing intermittent ignition failure.
- Blocked burner jets — Dust, insects, or debris can partially block burners, causing the flame to drop out.
- Flame sensor (thermocouple) fault — This safety device detects whether the burner is lit. If it's faulty, it may incorrectly signal the gas valve to shut off.
Flow Rate and Minimum Activation
Instantaneous gas units only fire when water flow reaches a minimum threshold (typically around 2–3 litres per minute, though this varies by model). If you turn the tap down to get a more comfortable temperature, you may accidentally drop below this threshold and the unit shuts off. Try:
- Turning the hot tap up higher and mixing in more cold water at the tap or mixer.
- Checking whether your showerhead or tap aerator is partially blocked, reducing flow.
Temperature Fluctuations from Pressure Issues
Fluctuating mains water pressure — common in older NZ homes or during peak demand times — can cause the unit to cycle on and off. A pressure-limiting valve (PLV) or tempering valve that's worn or incorrectly set can also contribute. These components are part of your plumbing system and must be checked and replaced by a licensed tradesperson.
Venting and Combustion Air Problems
Gas units need adequate combustion air and flue ventilation to operate safely and consistently. If the unit is in a confined space, cupboard, or has a partially blocked flue, it may overheat and shut down on its thermal cut-out as a safety measure. Never block vents near your gas appliance, and ensure the flue is clear of debris — but leave any inspection or repair of the flue to a licensed gasfitter.
Age and Servicing
Most gas hot water units have a service life of around 10–15 years. If yours is ageing and showing intermittent faults, it may simply be due for a service or replacement. Regular servicing by a licensed gasfitter is recommended — check your unit's manual for the manufacturer's suggested interval. BRANZ provides useful guidance on maintaining home systems.
What You Can Safely Check Yourself
As a homeowner, there are a few non-restricted checks you can do:
- Check the gas meter — Make sure the meter isolation valve is fully open and your account is current with your gas retailer.
- Check for error codes — Many modern units display a fault code on a small screen. Note the code and refer to your manual or share it with your gasfitter.
- Inspect the inlet filter screen — Some units have a small filter on the cold water inlet that can become blocked. Check your manual to see if this is user-serviceable.
- Test other gas appliances — If your hob or heater is also underperforming, the issue may be with gas supply rather than the unit itself.
- Check the showerhead — A partially blocked showerhead can reduce flow below the unit's activation threshold.
When to Call a Professional
Under New Zealand law, gasfitting work is restricted work and must be carried out by a tradesperson licensed by the Plumbers, Gasfitters and Drainlayers Board (PGDB). This includes:
- Inspecting, adjusting, or replacing gas valves, burners, igniters, or thermocouples.
- Checking or adjusting gas pressure.
- Servicing or replacing the hot water unit.
- Inspecting or clearing the flue.
Attempting this work yourself is illegal and dangerous — gas leaks and carbon monoxide poisoning are serious risks. Always use a licensed gasfitter. For your safety, if you ever smell gas, turn off the gas at the meter, ventilate the area, and call your gas retailer's emergency line immediately. WorkSafe NZ has further guidance on gas safety.
You can find qualified, licensed gasfitters in your area right here on tradedirectory.co.nz.
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