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 at 60 °C↗ or above 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. But water that hot burns quickly. At 68 °C a full-thickness scald can take as little as one second↗. At 50 °C it takes up to five minutes↗, which is enough time to pull away.

How a tempering valve works Water leaves the storage tank at 60 °C or hotter. 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 are not 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 early childhood centres, schools and nursing homes it drops to 45 °C↗. Kitchen sinks and laundry taps are not 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 fitting a new hot water system counts. Your plumber should fit or check a tempering valve as part of the job. If a quote does not mention one, ask.

Looking after the valve

Tempering valves wear. If your shower suddenly runs much hotter or colder than it used to, have a licensed plumber check the outlet temperature rather than adjusting the valve yourself.