Legionella water temperatures: what landlords should record

Legionella bacteria multiply in water roughly between 20°C and 45°C, with the middle of that range most favourable. Keeping hot water hot and cold water cold denies them those conditions. That is why temperature, rather than sampling, is the practical everyday control in domestic systems.

Stored hot water

Around 60°C, where hot water is stored in a cylinder or calorifier. Not applicable to a combi or other instantaneous system.

Distributed hot water

50°C or above at the outlet, reached within about a minute of running, on unblended hot outlets.

Cold water

Below 20°C, reached within about two minutes of running. Cold water sitting above 20°C is the more commonly missed problem.

Blended outlets

An outlet fed through a thermostatic mixing valve delivers a lower, deliberately safe temperature. The reading tells you about the blend, not about the hot water distribution temperature.

Hot water at 50°C or above will scald. Where occupants are vulnerable to scalding — young children, older people, anyone with reduced mobility or sensation — thermostatic mixing valves are used at baths and basins to deliver a safe temperature at the point of use. This is a genuine trade-off between two real risks, and it is why blended outlets exist.

Do not turn a boiler or cylinder thermostat up in pursuit of a number without considering scalding, the system design and the manufacturer's instructions. If the system will not reach expected temperatures, that is a finding for a competent heating engineer, not something to force.

Equally, do not transplant the monitoring routines used in hospitals or large commercial buildings into a two-bedroom flat. Those regimes exist for systems with stored water, complex distribution and susceptible occupants.

1. 1

   Identify the outlet

   Record the room and the outlet — "first-floor bathroom basin, hot" — so the reading can be matched to a place, not just a property.

2. 2

   Note whether it is blended

   If the outlet is fed through a mixing valve, record that alongside the reading so it is interpreted correctly later.

3. 3

   Run the outlet

   Run hot for about a minute and cold for about two minutes, or until the temperature stabilises, keeping spray to a minimum.

4. 4

   Measure consistently

   Use the same method each time — a surface or immersion thermometer used the same way — so readings across visits and properties are comparable.

5. 5

   Record the number, date and time

   A reading with no date and no named outlet is of very little value at the next review.

6. 6

   Photograph the thermometer at the outlet

   A photo showing the display at the point of measurement is simple, strong supporting evidence.

• Cold water above 20°C after running: look for pipework passing through warm spaces, a poorly sited or uninsulated tank, or a very short run from a warm area
• Hot water well below 50°C at an unblended outlet: check the appliance or cylinder thermostat setting and get a heating engineer to look at the system
• A reading that swings between visits: check the method before concluding the system changed
• Anything you cannot explain: record it as a finding and get competent advice rather than averaging it away

A temperature reading tells you whether a control is working at that outlet at that moment. It does not tell you whether Legionella bacteria are present, and it is not a substitute for laboratory sampling — nor is sampling generally expected in a simple domestic system.

Describe them accurately in your record as temperature monitoring or assessment observations, never as Legionella tests.

Measurements are taken outlet by outlet with a guided flushing timer, so the run time is consistent. Each reading is stored against the named outlet with its date, whether the outlet is blended, and the thermometer photo attached to that reading.

The report presents them as monitoring evidence with the caveats intact, so nobody reading it later mistakes a temperature for a laboratory result.