Sentinel outlets and TMVs explained

By the Germ3 Water team. Updated 20 October 2026.

What a sentinel outlet is

A sentinel outlet is a tap or shower chosen to represent part of your water system. They are usually the first and last outlets on each hot and cold water circuit. That means the nearest and the furthest from the calorifier or tank.

If the furthest outlet reaches the right temperature in time the rest of the circuit almost certainly does too. Your risk assessment should list your sentinel outlets by location.

The targets

For most hot and cold water systems HSG274 Part 2 sets these monthly targets:

• Hot water should reach at least 50°C within one minute. In healthcare it should reach 55°C.
• Cold water should fall below 20°C within two minutes.

Calorifier flow and return temperatures are checked monthly too. Stored hot water should be at least 60°C.

How to take a reading

1. Use a calibrated digital thermometer with a probe.
2. Turn the tap on fully and start a timer.
3. Hold the probe in the flow of water.
4. Read the temperature at one minute for hot or two minutes for cold.
5. Write the reading and the time taken in your logbook.

Do it the same way every month. Readings taken in different ways are hard to compare.

Where TMVs fit in

A thermostatic mixing valve blends hot and cold water to a safe temperature for washing. That protects people from scalding. It also means the water after the valve sits at a temperature where legionella can grow.

Your checks at a TMV outlet tell you the valve is working. They do not tell you the hot supply is hot enough. Where you can check the hot water going into the valve as well. Keep the pipe run after the valve as short as possible.