G12 hot water: the tempering valve and what other trades need from you

NZBC clause G12 “Water Supplies” is about getting safe, adequate hot and cold water to the fixtures and away again. The headline tension it solves is the scald-versus-Legionella trade-off, plus pressure relief, backflow protection and approved materials. All sanitary plumbing must be done and certified by a Certifying Plumber registered with the PGDB, so as a builder your job is to know what the plumber needs and where these things land on site.

Why the tempering valve exists — the core G12 idea

Stored water has to run hot enough to kill Legionella, but water that hot scalds in seconds. The tempering valve (TMV) is what resolves that: it blends stored hot water down to a safe delivery temperature at the fixture. Think of it as “hot, store; safe, deliver.”

• Cylinder stores at ≥60°C to kill Legionella.
• A tempering valve / TMV is mandatory where the outlet is reachable by hand, blending down to 55°C max at point of use and 50°C at basins and showers.
• Delivery is capped at ≤55°C for normal homes, and ≤45°C for early-childhood centres, schools, rest homes and hospitals (vulnerable users).

Hot water – cylinder, valves and relief

The cylinder, the relief gear and the backflow protection all sit together. Here is what belongs on the job.

• Hot water cylinder minimum 180 L typical for residential; increase 30–50 L per additional bathroom.
• Pressure-temperature relief valve (TPR) plumbed to outside in a clearly visible location — minimum 600mm from any opening.
• Relief drain: 15mm copper or PEX, falling continuously to outside daylight discharge.
• Mains-pressure hot water requires a double-check valve (DCV) on the cold inlet to prevent backflow.
• An electric heat pump hot water cylinder runs 30–70% cheaper than a resistance element and qualifies for some council rebates.

Approved materials

Only potable-rated materials go inside the building — never garden hose or black poly. The common choices:

• Copper pipe Type B for residential hot and cold; soldered or compression, 15mm a common diameter.
• PEX (cross-linked polyethylene) — faster to install, flexible, and expands rather than bursting if it freezes; joined with brass or copper press fittings.
• Brass DR (dezincification-resistant) fittings required — non-DR brass corrodes in chloraminated water.
• Potable-rated only — never garden hose or black poly inside the building.

Drains to outside (daylight discharge)

Relief and overflow lines have to run to a visible, safe endpoint — not be capped off. Where they go:

• Hot water cylinder relief drain — daylight discharge outside, max 1500mm above ground.
• Air-conditioning condensate drain — daylight discharge, or plumbed to wastewater.
• Roof scupper / overflow — daylight discharge.

Check it at final

Verify the delivered temperature with a thermometer at the tap outlet at final. Factory-set TMVs sometimes ship at 60°C, which is a bath-fill scald risk. And make sure the relief / expansion drain runs to a visible safe tray or daylight — not capped.