If your gas hot water system only produces hot water when the tap is turned on full, the most likely cause is that your unit isn't receiving enough water flow to trigger the ignition. This is a very common issue with instantaneous (continuous flow) gas water heaters, and it's usually fixable — though some causes will require a licensed gasfitter to sort out safely.
How Instantaneous Gas Hot Water Systems Work
Unlike a storage hot water cylinder, an instantaneous gas heater has no stored hot water. Instead, it uses a flow sensor (sometimes called a water flow switch or turbine) to detect when water is moving through the unit. Once flow reaches a set minimum activation threshold — typically around 2–3 litres per minute, though this varies by model — the gas valve opens and the burner ignites.
If the flow rate falls below that threshold, the unit simply won't fire. This is a built-in safety feature, not a fault in itself. The problem is when normal everyday use — like running a warm shower at a comfortable mix — doesn't generate enough flow to keep the unit going.
Common Reasons Your Unit Won't Fire at Partial Flow
1. Blocked or Dirty Flow Sensor / Filter
Most units have a small inlet filter screen that can become clogged with sediment, rust particles, or mineral scale over time. A partially blocked filter restricts flow and can prevent the unit from reaching its activation threshold. Cleaning the inlet filter is a straightforward maintenance task — check your unit's manual for instructions, or ask a gasfitter to do it during a service.
2. Scale Build-Up Inside the Unit
In areas with hard water, mineral scale can build up inside the heat exchanger and water pathways, restricting flow. This is more common in parts of NZ with harder water supplies. A gasfitter can descale the unit or advise on a whole-home water filter.
3. Low Mains Water Pressure
If your home's mains water pressure is low — below around 350–500 kPa depending on your unit — the heater may struggle to reach activation flow. Low pressure can be caused by a partially closed toby (main shutoff valve), a failing pressure-reducing valve (PRV), or simply low supply pressure from your water network. Check with your local council or water supplier if you suspect this.
4. Worn or Faulty Flow Sensor
Over time, the flow sensor turbine can wear out, stick, or become coated in scale, causing it to under-read actual flow. This means the unit thinks there isn't enough flow even when there is. Replacing a flow sensor is restricted gasfitting work and must be carried out by a licensed gasfitter.
5. Partially Closed Isolation Valves
Check that the isolation valves on the cold water inlet to your hot water unit are fully open. A valve that's been accidentally left half-closed will restrict flow significantly.
6. Mixer Tap or Thermostatic Mixing Valve (TMV) Restrictions
If you're mixing hot and cold at the tap or through a thermostatic mixing valve, the actual hot water draw may be lower than you think. Some modern water-saving tapware and showerheads also have built-in flow restrictors that can drop flow below the heater's activation point. Try temporarily removing the flow restrictor from the showerhead or tap aerator to test.
7. Unit Sized Too Large for Your Household Demand
Some units have a relatively high minimum flow rate. If your unit was oversized for your needs, it may simply not suit low-flow tapware. A gasfitter can advise whether a different unit or an adjustment to the flow sensor threshold (where the model allows) would help.
What You Can Safely Check Yourself
- Inspect and clean the inlet filter screen (refer to your unit's manual)
- Check isolation valves are fully open
- Remove flow restrictors from tapware temporarily to test
- Check your water pressure using a simple gauge from Bunnings or Mitre 10
- Review your unit's manual for minimum flow rate specifications
NZ Regulations and Your Hot Water Temperature
Under the NZ Building Code clause G12, hot water delivered to sanitary fixtures must be controlled to safe temperatures. If your system has a tempering valve or TMV installed (required in many situations, including all new installs and rental properties under the Healthy Homes Standards), this valve mixes cold water with the hot supply. A faulty or incorrectly set TMV can reduce flow enough to cause ignition problems — another reason to have a gasfitter inspect the full system.
When to Call a Professional
You must use a licensed gasfitter for any work involving the gas components of your hot water system. Under NZ law, gasfitting work is restricted work regulated by the Plumbers, Gasfitters and Drainlayers Board (PGDB). This includes:
- Replacing or adjusting the flow sensor, gas valve, or ignition components
- Servicing the burner or heat exchanger
- Adjusting gas pressure or connections
- Replacing the unit entirely
Plumbing work on the water supply side (such as replacing a PRV or TMV) must also be carried out by a licensed plumber. Always ask to see your tradesperson's licence before work begins.
If you'd like to find a qualified local gasfitter or plumber, you can search tradedirectory.co.nz for professionals in your area.
Safety note: Never attempt to modify, bypass, or repair gas components yourself. Gas leaks and incorrect installations can cause fires, explosions, or carbon monoxide poisoning. If you smell gas, ventilate the area immediately, avoid open flames or switches, and call your gas retailer's emergency line.
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