If your shower mixer is running cold even when turned to full hot, the most likely culprits are a worn or failed mixer cartridge, a problem with your hot water system, or an issue with how the pipes are connected. The good news is that some of these fixes are well within a confident DIYer's reach — but others will need a licensed plumber.
Start With the Basics: Check Your Hot Water Supply
Before pulling apart your shower mixer, confirm that hot water is actually reaching the shower. Turn on a hot tap elsewhere in the house — the kitchen or bathroom basin — and check whether you're getting hot water there.
- No hot water anywhere: The problem is with your hot water cylinder or heat pump water heater, not the shower mixer itself. Check whether the cylinder's thermostat is set correctly (typically 60°C to prevent Legionella bacteria growth), the element hasn't failed, or a circuit breaker has tripped.
- Hot water at other taps but not the shower: The issue is localised to the shower mixer or its supply line.
If your hot water cylinder is the problem, repairs to the cylinder, its element, or any gas components are restricted work and must be carried out by a licensed plumber or gasfitter.
The Most Common Cause: A Worn Mixer Cartridge
The mixer cartridge is the internal valve that blends hot and cold water. Over time — especially in areas with hard or chlorinated water — cartridges wear out, crack, or seize up. A failed cartridge can prevent hot water from flowing through even when the handle is at full hot.
How to Replace a Shower Mixer Cartridge
Replacing a cartridge is a task many homeowners can do themselves, provided you're comfortable with basic plumbing. Here's the general process:
- Turn off the water supply to the shower. This is usually done at the isolation valves under the shower or at the main toby (water meter) if there are no isolators.
- Remove the handle — typically held by a grub screw hidden under a decorative cap.
- Unscrew the trim plate to expose the cartridge body.
- Pull out the old cartridge — note its orientation before removal, or take a photo.
- Take the cartridge to a plumbing merchant (such as PlaceMakers or Mitre 10) to match the replacement. Cartridge designs vary widely between brands.
- Insert the new cartridge in the correct orientation, reassemble, and test.
Important: If you're not confident working with plumbing fittings, or if the mixer is concealed in the wall (a concealed mixer), it's best to call a professional. Incorrect reassembly can cause leaks inside the wall cavity — a costly problem.
Check for a Thermostatic Cartridge Fault
If your shower has a thermostatic mixer (one with a separate temperature-limiting dial), there's a thermostatic cartridge that automatically regulates water temperature. These can fail in a way that locks the output to cold. Thermostatic cartridges are more complex and more expensive to replace — and getting the temperature calibration right matters for safety. This is a job best left to a licensed plumber.
Under the NZ Building Code clause G12, hot water delivered to showers must not exceed 45°C to prevent scalding. A correctly set thermostatic mixer helps meet this requirement.
Could It Be Crossed or Reversed Pipes?
If the shower was recently installed or replumbed, it's possible the hot and cold supplies were connected in reverse. You can check this by feeling the pipes behind the wall (if accessible) — the hot supply pipe should be warm when the hot water system is running. Reversed connections are a plumbing error and must be corrected by a licensed plumber.
Low Hot Water Pressure
In some cases, the hot water side has significantly lower pressure than the cold side. The cold water can then dominate the mix, leaving the output feeling cold even at full hot. This can happen if:
- The hot water isolation valve is partially closed — check and open it fully.
- There's a blockup or scale buildup in the hot supply line.
- Your hot water system is a low-pressure (open-vented) cylinder, which is common in older NZ homes.
If you have a low-pressure cylinder, you may need a mixer specifically rated for unequal pressures, or a pressure-balancing valve. A plumber can advise on the right solution for your setup.
Sediment or Scale Buildup
In some regions of New Zealand, mineral-rich water can cause scale to build up inside the mixer body or cartridge ports, restricting hot water flow. Soaking removable parts in a descaling solution can sometimes help, but a heavily scaled cartridge usually needs replacing.
When to Call a Professional
Call a licensed plumber (find one at tradedirectory.co.nz) if:
- You have no hot water throughout the house (hot water system fault).
- The mixer is a concealed (in-wall) type.
- You suspect reversed or incorrectly connected pipes.
- The mixer is thermostatic and needs recalibration.
- You're not confident working with plumbing fittings.
- There are signs of leaks inside the wall cavity (damp patches, mould, soft wall linings).
Under the Plumbers, Gasfitters, and Drainlayers Act 2006, certain plumbing work is restricted to licensed practitioners. While homeowners can carry out like-for-like minor repairs on their own home (such as replacing a tap washer or cartridge), any work involving new pipework connections or gas systems must be done by a licensed tradesperson.
Safety Note
Always turn off the water supply before opening any mixer or valve. If your hot water system is gas-powered, never attempt repairs to the gas components yourself — this is restricted work under NZ law and must be carried out by a registered gasfitter.
Master Plumbers
