Two rules, one valve
Clause G12 of the Building Code sets two requirements that pull in opposite directions. The installation must be capable of being controlled to prevent the growth of Legionella bacteria. And where hot water serves fixtures used for personal hygiene, it must be delivered at a temperature that avoids the likelihood of scalding.
The clause itself gives no temperature for either. The detail sits in the Acceptable Solution, G12/AS1. A tempering valve is one way to satisfy both at once: hot water stored, tempered water delivered.
What the standard says
Tempering valves are 3-port mixing valves made to NZS 4617. MBIE's Building CodeHub describes them as intended to prevent scalding in homes, schools, motels and residential premises, where ideally every hot water outlet is sourced from a tempering valve. The current G12/AS1, Amendment 14, cites that standard.
The valve often sits on the hot pipe near the cylinder. It's part of the plumbing, so it's covered on our hot water cylinders pages rather than as a tap.
Signs the valve needs attention
- Bathroom hot water that is hotter than it used to be.
- Showers that won't get properly warm while the kitchen tap runs hot.
- Temperature that surges or swings during a shower.
- A drip or weep from the valve body or its fittings.
- A new cylinder went in and nobody mentioned the valve.
What a plumber will do
A plumber will measure the water temperature at the bathroom outlets, then clean, service, adjust or replace the valve, and recheck. Rheem notes that water not being hot enough can come down to a faulty tempering valve, which a plumber can test and replace.
When a cylinder is replaced, ask whether the existing valve stays, gets serviced or is swapped for a new one. Valve work is sanitary plumbing, so it's for someone with a current PGDB licence. Repair and like-for-like replacement of plumbing are among the Building Act's consent exemptions when an authorised person does the work, and the result still has to meet the Building Code.
Your side of it
You can
- Notice when the temperature changed and tell the plumber.
- Find the valve near the cylinder so you can point it out.
- Leave the cylinder thermostat alone.
- Use the shower mixer, not the cylinder, to get comfortable.
Leave to an authorised tradie
- Adjusting, servicing or replacing the valve.
- Any change to the cylinder's temperature setting.
- Fitting a valve to a new or replacement cylinder.
- Checking which outlets the valve feeds.
Questions to ask the plumber
- Is there a tempering valve on our bathroom supply now?
- Does the kitchen get tempered water, or straight hot?
- How did you check the bathroom temperature, and what did it show?
- How often should this valve be serviced?
- Will the valve appear in the job's paperwork?
What we would do
If a shower scalds or runs cool, leave the cylinder setting alone: turning it down swaps a burn risk for a Legionella one. Get a plumber who knows tempering valves to test the outlet temperature and service or replace the valve. If small children or older people live with you, mention it when you book.