How an instant unit works
Open a hot tap and water starts moving through the unit. A sensor detects the flow, the burner lights, and a heat exchanger warms the water on its way past. Close the tap and the burner shuts off.
Nothing is stored, so the unit's flow rate sets the ceiling. Two showers and a washing machine together can be more than a small unit can keep up with.
Signs an instant unit needs attention
- An error code on the controller or the unit's display.
- Water that goes hot, then cold, then hot again.
- No hot water at a low flow, such as a gently running basin tap.
- The unit clicks but won't fire.
- Water dripping from underneath.
What a plumber will check
The code
A plumber will read the error code against the maker's list, which narrows the fault fast.
Gas and water supply
They confirm the gas pipe and meter can feed the unit at full output, and that water pressure is reaching it.
Ignition and sensing
Igniters, flame sensors and flow sensors are common culprits, and can usually be replaced.
Heat exchanger
Scale or damage here cuts output, and often decides whether a repair is worth doing.
Temperature at the bathroom
An instant unit still has to meet the delivery limits. For bathroom fixtures used for personal hygiene, the National Construction Code caps hot water at 50°C, and lets a tempering valve, a thermostatic mixing valve or a temperature limited heater do the job.
In aged care, health care, childcare, schools and disability facilities the cap is 45°C, and only a thermostatic mixing valve or a thermostatically controlled tap is accepted. Ask the plumber how your unit meets its limit.
Victoria's rule covers instant gas too
The rule talks about gas hot water systems, and its wording doesn't separate tanks from instant units. So from 1 March 2027, an instant gas unit in an existing Victorian home that reaches end of life has to be replaced with an efficient electric option.
Until then, and after, it can be repaired. Check current rules: the rule explained.
Licences for the job
An instant gas unit is a gas job as well as a water one, so ask for a plumber licensed for gasfitting. In NSW, plumbing and gasfitting are both specialist work, and a contractor needs a licence for either at any job value.
In Victoria, only a licensed plumber, not a registered one, can issue the compliance certificate for the work.
What you can try first
You can
- Note the error code and when it appears.
- Check the gas is on and other gas appliances work.
- Run one hot tap at a time to see whether demand is the problem.
- Check nobody has changed the set temperature on the controller.
Leave to a licensed plumber
- Opening the unit's cover.
- Gas pressure tests and gas pipe changes.
- Replacing sensors, igniters and control boards.
- Descaling or replacing the heat exchanger.
Questions to ask
- What does this error code mean on this model?
- Is the gas supply big enough for the unit's full output?
- Is the unit sized for the number of showers we run at once?
- How does it meet the 50°C bathroom limit?
- If it's near end of life in Victoria, which electric options fit?
What we would do
Write the error code down before you call. With the model number, it tells a hot water specialist most of what they need, and makes it likelier they arrive with the right part. If the unit only struggles when several taps run, the answer may be a bigger unit or a better gas supply, not a new sensor.