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Hot Water Cost

Tempering valves and the 50 °C rule

Hot water has to be stored hot enough to be safe from bacteria, then cooled enough to be safe on skin.

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Stored hot water must be kept above 60 °C to control Legionella. Water reaching a bathroom tap in a home must be no hotter than 50 °C. A tempering valve mixes cold water into the hot supply to bridge the two.

Why there are two temperatures

Legionella bacteria can grow in lukewarm water, so the tank itself stays hot. Water that hot burns fast, though. At 68 °C a full-thickness scald can take as little as one second. At 50 °C it takes five minutes, which is plenty of time to pull away.

How a tempering valve works Water leaves the storage tank above 60 °C. One pipe runs straight to the kitchen and laundry taps, which can run hotter. The other pipe reaches a tempering valve, where cold water is mixed in, and leaves at no more than 50 °C for the shower, bath and basin. Storage tank 60 °C+ Kitchen tap runs hotter Tempering valve Cold water in Bathroom 50 °C max
Fig. 2 — The tank stays above 60 °C to control Legionella. A tempering valve mixes cold water in so bathroom fixtures deliver no more than 50 °C. Kitchen and laundry taps aren't personal hygiene fixtures, so they can run hotter.

Where the lower limit applies

The limit covers fixtures used for personal hygiene: showers, baths and basins. In aged care, early childhood centres, the parts of schools children use and facilities for people with a disability it drops to 45 °C. Kitchen sinks and laundry taps aren't personal hygiene fixtures, so they can run hotter.

What it means when you replace a system

The limit applies when heated water plumbing is installed, and the National Construction Code's explanatory notes include replacing the water heater, like-for-like or not. Your plumber should fit or check a tempering valve as part of the job. If a quote doesn't mention one, ask. What else a quote should show.

Looking after the valve

Tempering valves wear. If your shower suddenly runs much hotter or colder than it used to, get a licensed plumber to check the outlet temperature rather than adjusting the valve yourself. If the whole system is due for replacement, the replacement cost guide has current prices.