If you've ever been startled by a sudden blast of scalding water from a tap or shower, you'll understand why water temperature control matters. A tempering valve (sometimes called a hot water tempering valve or thermostatic mixing valve/TMV) is a small but critical plumbing device that blends hot water from your cylinder with cold water to deliver a safe, consistent temperature at your outlets. In New Zealand, these valves are required by law in most residential situations — and for very good reason.
How Does a Tempering Valve Work?
Your hot water cylinder typically stores water at 60°C or higher to kill harmful bacteria like Legionella. However, water at that temperature can cause a serious scald in just a few seconds. A tempering valve is installed on the hot water outlet of your cylinder and automatically blends the hot supply with cold water before it reaches your taps, shower, and bath.
The valve contains a thermostatic element that constantly adjusts the mix to maintain a set output temperature — typically around 55°C for general household use, or 45°C for higher-risk areas like bathrooms used by young children or elderly people. If the cold water supply fails, a well-designed valve will also shut off the hot flow to prevent scalding — an important safety feature.
Why Is a Tempering Valve Required in NZ?
Under the New Zealand Building Code, specifically Clause G12 (Water Supplies), hot water delivered to sanitary fixtures in new and altered residential buildings must be controlled to prevent scalding. MBIE Building Performance provides guidance on these requirements, and they apply to both new builds and significant alterations.
Key points:
- Bathrooms and ensuites used by the general public or vulnerable people (children, elderly, people with disabilities) typically require outlets limited to 45°C.
- Kitchen and laundry outlets may be permitted at up to 55°C where higher temperatures are needed for hygiene.
- The Tenancy Services Healthy Homes standards also reinforce the importance of safe hot water in rental properties.
- BRANZ research has consistently highlighted scalding as a preventable household injury, particularly for young children and older adults.
Tempering Valve vs Thermostatic Mixing Valve — What's the Difference?
| Feature | Tempering Valve | Thermostatic Mixing Valve (TMV) |
|---|---|---|
| Temperature control | Good — maintains set temperature | Excellent — very precise control |
| Fail-safe (cold supply failure) | Basic | Advanced shut-off |
| Typical use | General household hot water outlet | Showers, baths, high-risk areas |
| Cost (as a guide) | Lower | Higher |
| NZ compliance | Meets G12 for most residential use | Meets G12 and higher-risk requirements |
For most standard NZ homes, a tempering valve on the hot water cylinder outlet is sufficient. In rest homes, early childhood centres, or homes with very young children, a full TMV at individual fixtures may be recommended or required.
Signs Your Tempering Valve May Need Attention
Tempering valves don't last forever. Watch out for these warning signs:
- Water temperature fluctuates noticeably between uses
- Water is consistently too hot at the tap (above the set temperature)
- Water is consistently too cool, even when the cylinder is set correctly
- Visible corrosion or leaking around the valve body
- The valve is more than 5–8 years old and has never been serviced
Most manufacturers recommend annual inspection and periodic replacement — check your valve's documentation or ask your plumber for the recommended service interval.
What Can a Homeowner Do?
As a homeowner, you can:
- Check the temperature at your taps using a simple thermometer to confirm the valve is working within the expected range.
- Visually inspect the valve for signs of corrosion or dripping.
- Keep records of when the valve was installed or last serviced.
However, adjusting, replacing, or installing a tempering valve is restricted plumbing work under the Plumbers, Gasfitters, and Drainlayers Act 2006. This must be carried out by a licensed plumber.
When to Call a Professional
You must engage a licensed plumber for any of the following:
- Installing a new tempering valve or TMV
- Replacing a faulty or end-of-life valve
- Adjusting the set temperature
- Commissioning after installation (testing and certifying the output temperature)
Unlicensed plumbing work is illegal in New Zealand and can void your home insurance. The Plumbers, Gasfitters and Drainlayers Board (PGDB) maintains a register of licensed practitioners — always check your plumber is registered before work begins.
If you need a qualified local plumber, you can search for one on tradedirectory.co.nz to find trusted professionals in your area.
Safety note: Never attempt to remove or bypass a tempering valve to get hotter water at the tap. Doing so creates a serious scalding risk, particularly for children and elderly household members, and may breach your building consent conditions.