The 20/50/60 Rule for Legionella Temperature Control

The 20/50/60 rule is the practical shorthand for UK Legionella temperature control: cold below 20°C, hot reaching at least 50°C at the outlet within a minute, and hot stored at 60°C in the cylinder or calorifier. Keep water out of the 20-45°C danger zone and Legionella can't multiply.

The numbers in one table

Cold < 20°C

Cold water can warm up in pipework that runs through warm voids, ceilings or lofts; in poorly insulated cold tanks; and in long horizontal runs in heated buildings. If you measure cold at a sentinel and it's 24°C in July, you've got a Legionella growth condition — and you need to look at lagging, tank position, dead legs and pipe routing.

Hot stored at 60°C

The cylinder or calorifier should be sat at 60°C at all times. On an unvented G3 system this is set on the cylinder thermostat as part of the install and is verified at every annual service. On a vented cylinder it's the immersion thermostat. The temptation to "save money" by setting lower is short-sighted — see our pillar on safe hot water temperatures.

Storage at 60°C is also the reason Pipe Assassin always installs proper thermostatic mixing valves on baths and on any basin where vulnerable users wash — 60°C delivered neat will scald.

Hot at outlet ≥ 50°C within a minute

The distribution temperature is the proof that hot water is actually getting to the user quickly, before it spends long enough in the pipework to drop into the danger zone. HSG274 Pt2 sets the target at 50°C+ within 1 minute at sentinel outlets in general premises; healthcare uses 55°C+ for added safety margin.

A hot outlet that takes 90 seconds to reach 45°C tells you something is wrong: long dead leg, undersized pipework, poor insulation on the flow, a failed pumped circulation loop, or cross-flow at a mixer/TMV. None of those are fixed by turning the cylinder up; they're fixed by sorting the pipework.

Sentinel outlets and monitoring

A "sentinel outlet" is the most representative test point for a section of pipework — typically the nearest and furthest outlet on each loop. HSG274 expects routine checks:

1. 1

   Monthly hot & cold at sentinels

   Two readings per outlet per month — hot within 1 min, cold within 2 min.

2. 2

   Monthly calorifier flow & return temperatures

   Flow ≥ 60°C, return ≥ 50°C if there's a return loop.

3. 3

   Annual checks on representative outlets on subordinate loops

   On a rotating basis so every outlet sees a check at least annually.

4. 4

   Annual cold water tank inspection

   Lid sealed, insect screens, water temperature, condition of tank and water.

5. 5

   Record everything

   Date, outlet ID, hot temp, cold temp, by whom. This is your evidence the controls are actually being done.

How to measure (without getting it wrong)

• Use a calibrated probe or surface thermometer. Calibration check at 0°C and 60°C annually.
• For hot, time from opening the tap; record the temperature at 1 minute. For cold, record at 2 minutes.
• For TMV-protected outlets, measure both the delivered (blended) temperature and, where accessible, the inlet hot — the latter is the system temperature, not the blended one.
• For showers, measure inside the head where possible — not into a cold cup that artificially lowers the reading.

Frequently Asked Questions

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