A gas instantaneous (continuous flow) hot water unit losing temperature mid-shower is one of the most frustrating household problems — but it's also one of the most common. The good news is there are several well-known causes, some of which you can investigate yourself, while others will need a licensed gasfitter to sort out safely.
How Instantaneous Hot Water Units Work
Unlike a storage cylinder, an instantaneous or continuous flow unit heats water on demand as it passes through a heat exchanger. A flow sensor detects when a tap is opened, ignites the burner, and modulates the flame to reach your set temperature. Because the system is reactive, anything that disrupts flow, gas supply, or the unit's sensors can cause temperature swings.
Common Causes of Temperature Loss Mid-Shower
1. The "Cold Water Sandwich" Effect
This is the most frequently reported issue with instantaneous units. When you briefly turn off the shower (e.g., to lather up) and turn it back on, a slug of cold water sitting in the pipes arrives before the newly heated water catches up. This is a normal characteristic of the technology, not a fault — but it can feel alarming.
2. Fluctuating Water Flow or Pressure
Your unit needs a minimum flow rate (typically around 2–3 litres per minute, but check your model's manual) to stay ignited. If someone flushes a toilet, runs a tap, or a washing machine kicks in, the drop in pressure can cause the burner to cut out momentarily. Low mains pressure in your area can also be a contributing factor.
3. Incorrect Temperature Setting
If the unit is set too low, adding even a small amount of cold water at the mixer tap can drop the delivered temperature noticeably. Check your unit's control panel — most modern units allow you to adjust the set point. MBIE Building Performance notes that hot water delivered to bathrooms should generally not exceed 55°C to reduce scalding risk, so balance safety with comfort.
4. Dirty or Blocked Inlet Filter
Instantaneous units have a small inlet filter screen that can become clogged with sediment, especially in areas with harder water. A blocked filter restricts flow, causing the unit to cycle on and off. Cleaning this filter is generally a straightforward homeowner task — turn off the cold water supply to the unit, unscrew the filter housing, rinse the screen, and reinstall. Always check your owner's manual first.
5. Failing Flow Sensor or Modulating Valve
If the flow sensor is worn or dirty, it may not accurately detect water movement, causing erratic ignition. A faulty modulating gas valve can also prevent the burner from maintaining a steady flame. These are internal components that must be diagnosed and replaced by a licensed gasfitter.
6. Gas Supply Issues
Insufficient gas pressure — whether from a partially closed isolating valve, a regulator that needs adjustment, or high demand on a shared LPG cylinder — can starve the burner of fuel mid-operation. If you're on LPG, check that your cylinder isn't running low. Gas pressure testing and regulator work is restricted work under the Gas Act and must only be carried out by a licensed gasfitter registered with the PGDB.
7. Scaled-Up Heat Exchanger
In areas with hard water, mineral scale can build up inside the heat exchanger over time, reducing its efficiency and causing uneven heating. Descaling is a job for a qualified technician.
8. Unit Undersized for Demand
If your unit's flow rate capacity (measured in litres per minute) is too small for your household's simultaneous hot water demand, it will struggle to maintain temperature when multiple outlets are running. This is worth discussing with a gasfitter if the problem is ongoing.
What You Can Safely Check Yourself
- Clean the inlet filter (with water supply isolated)
- Check the temperature set point on the unit's control panel
- Check LPG cylinder levels if on bottled gas
- Confirm all isolating valves on the cold water supply to the unit are fully open
- Review your owner's manual for model-specific troubleshooting steps
When to Call a Professional
Gas work in New Zealand is legally restricted. Under the Gas Act 1992, only a gasfitter licensed by the Plumbers, Gasfitters and Drainlayers Board (PGDB) may carry out work on gas appliances, pipework, regulators, and connections. This includes:
- Testing or adjusting gas pressure
- Replacing internal components such as flow sensors, modulating valves, or heat exchangers
- Servicing or repairing the burner assembly
- Any work on gas pipework or connections
Attempting restricted gas work yourself is illegal and dangerous. If you smell gas at any point, turn off the gas supply at the meter or cylinder, ventilate the area, and call a licensed gasfitter immediately. For emergencies, contact your gas retailer or WorkSafe NZ.
A qualified gasfitter can also carry out a full service of your unit — most manufacturers recommend annual servicing to keep the appliance running safely and efficiently. You can find licensed local gasfitters on tradedirectory.co.nz.
A Note on Healthy Homes and Rental Properties
If you're a landlord, be aware that Tenancy Services Healthy Homes standards require rental properties to have an adequate hot water supply. A malfunctioning hot water unit that affects habitability should be addressed promptly.