Instant Gas Hot Water Problems: What You Can Fix Yourself (And What You Can't)

Instantaneous gas hot water systems have become a firm favourite in many Wanaka homes. They're energy-efficient, compact, and deliver a continuous supply of hot water on demand. But when something goes wrong, it can be difficult to know whether you're dealing with a simple fix or something that needs a licensed professional.

This guide covers the most common instant gas hot water problems, which ones are safe and legal to sort out yourself, and which ones should always be handled by a qualified hot water plumber.

How Does an Instant Gas Hot Water System Work?

Rather than storing pre-heated water in a cylinder, an instantaneous system heats water as you need it. Cold water enters the unit and passes through a heat exchanger — a coiled copper pipe — where a gas burner heats it on the way through. The burner only fires when a hot tap is turned on, which means no energy is wasted keeping a tank warm. The result is instant hot water and lower running costs.

Because the setup is different from a traditional electric storage unit, the problems that can occur are different too.

Five Problems You May Be Able to Fix Yourself

These are issues that many homeowners can check and resolve without any tools, trade experience, or a plumbing licence.

Check the power supply first. Even though it's a gas system, most instant hot water units also rely on electricity for ignition and controls. If the power point has been accidentally switched off or a safety switch has tripped at the switchboard, the unit won't start at all. Check the power point and reset the switchboard if anything has tripped.

Try resetting the unit. After a power outage or surge, the system may simply need a restart. Turn the unit off at the power point, wait about 30 seconds, then switch it back on and test the taps. This can clear minor electronic faults without any further action needed.

Check the tap flow rate. Instant gas systems need a minimum flow rate to trigger the burner — usually around 3 litres per minute. If you're using a very low-flow shower head or only opening the tap slightly, the unit may not fire, or the water may go cold mid-shower. Try turning the tap on fully to see if the unit engages properly.

Confirm the gas supply is on. If you've recently moved in, had gas work done, or had another appliance serviced, the gas isolation valve near the unit may have been turned off. Check that the valve is in the open position (parallel to the pipe), and if you're on LPG, make sure the bottle isn't empty.

Adjust the temperature settings. If the water isn't hot enough, the fix might simply be adjusting the controller to a higher setting. For safety, the recommended maximum temperature for bathroom outlets is 55°C. If the temperature isn't responding to adjustments, the issue is likely internal and will need a professional.

Problems That Need a Licensed Professional

The following faults require specialist knowledge, specific tools, or are legally required to be carried out by a licensed plumber or gasfitter in New Zealand. Attempting to fix these yourself can be dangerous and may void your warranty.

Burner or ignition failure is one of the most common reasons for a professional callout. If the unit won't ignite, or you hear clicking with no flame, the cause could be a blocked burner, faulty ignition module, or a sensor issue. A licensed gasfitter can safely access the internal components, test and replace sensors, and check flame performance.

Brand-specific faults are also worth mentioning. Different manufacturers use different components, sensors, and control boards, so the same symptom can have a different cause depending on the brand. Rinnai units, for example, often display error codes related to water pressure or flame failure, while Bosch units may need specific reset procedures or controller reprogramming. Diagnosing these correctly requires brand-specific knowledge and tools — not a DIY approach.

Fluctuating water temperature — where the hot water goes hot, then cold, then hot again — can point to a faulty tempering valve, flow rate fluctuations, a dirty inlet filter, or a partially blocked heat exchanger. These components need to be tested and serviced by a qualified plumber. Never attempt to open sealed valves or internal parts yourself.

Visible leaks or error codes on the display often indicate something more serious, such as a blocked flue, a failing heat exchanger, internal corrosion, or a gas valve fault. These issues are potentially dangerous if mishandled. In New Zealand, repairs to gas appliances must be carried out by a licensed gasfitter — it's not just good advice, it's the law.

Low water pressure or inconsistent flow when using multiple taps at once can mean the unit is undersized for your household, or that the inlet strainer is blocked with sediment. Mineral build-up can also restrict flow over time. A plumber can flush the system, service the unit, assess your usage needs, and recommend upgrades if necessary.

Still Having Trouble?

If you've worked through the basic checks and the problem persists, don't risk damaging your unit or breaching gas regulations by going further yourself. Wanaka Plumbing & Gas provides fully licensed support for all instant gas hot water issues — from temperature problems and error codes through to major repairs and full system replacements.

We work with all major brands and service homes throughout Wanaka and the surrounding area.

Get in touch with Wanaka Plumbing & Gas today for expert help with your instant gas hot water system.