What the Building Code says
New Zealand's Building Code is Schedule 1 of the Building Regulations 1992, and clause G12 covers water supplies. Among its aims is protecting people from injury caused by a hot water system exploding, or from contact with excessively hot water.
On scalding, G12 says that where hot water is supplied to fixtures used for personal hygiene, it "must be delivered at a temperature that avoids the likelihood of scalding".
On Legionella, the clause is just as short: a hot water system "must be capable of being controlled to prevent the growth of legionella bacteria".
It also requires any vessel that makes or stores hot water to carry safety features. They must relieve excessive pressure in normal and abnormal conditions, and limit temperatures so a rupture is unlikely to produce flash steam.
Why we don't print temperatures
Clause G12 itself gives no figures. The numbers sit in the Acceptable Solution G12/AS1, which is one way of complying with the clause. The current edition is the Third Edition, Amendment 14, valid from 1 November 2024, and Amendment 13 is no longer current.
We couldn't read the official G12/AS1 text when we wrote this, and the figures quoted elsewhere don't all agree, so we publish none. Guides that quote older editions may be out of date. Your plumber works from the current documents and can tell you the settings for your system.
Tempering valves: how both jobs get done
A tempering valve is a three-port mixing valve that blends hot and cold before the water reaches the tap. The standard for them, NZS 4617, says they're intended to prevent scalding in places such as homes, schools and motels, and G12/AS1 Amendment 14 cites it.
That's how a cylinder can store water hot enough to control Legionella while the shower runs at a safe temperature. A tempering valve can also be the reason water seems too cool: Rheem says a faulty one can cool hot water excessively, and an approved plumber can test it and replace it. See tempering valves.
Why a plumber sets it, not you
Turning a cylinder down can look like an easy saving, but it can undermine the Legionella control the Code asks for. Turning it up raises the risk of scalding wherever nothing tempers the water.
Rheem's guide for its mains pressure electric cylinders says any temperature adjustment needs an authorised person. Leave the setting, and any tempering valve, to an authorised plumber, who can set the two to work together.
Signs the temperature needs a look
- Water at a basin or shower hot enough to make you snatch your hand away.
- Hot water that has turned lukewarm all over the house.
- Big swings at the shower when someone else turns on a tap.
- Steam or boiling sounds from the cylinder. Switch it off at the switchboard and ring a plumber.
- A relief valve that discharges all day.
What you can and can't do
You can
- Run each hot tap and note which ones are too hot or not hot enough.
- Check the hot water switch at the switchboard if every tap is cold.
- Photograph the rating label so the plumber knows the model.
- Switch the cylinder off at the switchboard if you see steam or hear boiling.
Leave to an authorised tradie
- Changing the cylinder's thermostat setting.
- Fitting, adjusting or replacing a tempering valve.
- Any repair that means removing covers or casing with tools.
- Gas work on a gas water heater, which is for an authorised gasfitter.
Questions to ask the plumber
- What will the cylinder store water at, and how does that control Legionella?
- What temperature will reach the shower and basins, and how is it tempered?
- Which outlets, if any, get untempered hot water?
- Which edition of G12/AS1 are you working to?
- Will you show me the settings before you leave?
What we would do
Leave the dial alone and ask the question instead. If water at a basin or shower feels dangerously hot, or the hot water has gone tepid, book an authorised plumber who specialises in hot water to check the cylinder setting and the tempering valve together, and to tell you the figures they've used. The two settings work as a pair, and getting one wrong undoes the other.