A thermometer held under one tap tells you about one tap, for one moment. That is the quiet trap in temperature monitoring: a folder full of in-range readings can sit happily next to a system that is slowly losing its grip. Hot water that left the calorifier at the right figure but arrives tepid at the end of a long run. A cold supply that creeps up every afternoon as the plant room warms. The number on the page was real. The control behind it was not.

So the question worth asking on a round is not “did this outlet pass?” but “does the pattern across the building show a system that can hold temperature?” Legionella multiplies in roughly the 20–45°C band [1], and temperature is the control most UK sites lean on hardest. Reading it well is what separates a monitoring programme from a clipboard habit.

This is a checklist piece, because temperature work genuinely is a set of points to hit in order. The value, though, is in how you read what the list hands back.

Why a structured round beats a wander with a probe

An unstructured check tends to drift to the easy outlets — the ones near the cupboard, at a sensible height, that run warm without complaint. Those are exactly the outlets least likely to tell you anything. The points that expose a failing system are the awkward ones: the far end of a wing, the basement store, the shower nobody booked last week.

A structured round forces you past the convenient taps and gives every reading a context: which sentinel it is, what the scheme expects, what last month said. Without that, a temperature check is just a number with no opinion attached.

What a proper temperature round covers

Work it in groups. Each item is something to do and record, not just to glance at.

Before you start

  • Pull the written scheme of control and the last round’s results, so flagged outlets and open actions travel with you rather than getting picked up afterwards.
  • Confirm this month’s sentinel outlets — typically the nearest and furthest from each calorifier or storage tank — plus whatever is due on rotation [2].
  • Check the thermometer or surface probe is in calibration and note its ID against the readings.

Hot water

  • Read the calorifier flow and the return, not just the flow. A healthy store feeding a cold return points to poor circulation long before any outlet complains.
  • Confirm stored hot water sits at the figure your scheme sets — commonly 60°C or above [3].
  • At sentinel taps, run the hot and check it reaches the distribution target, often around 50°C within about a minute (higher in healthcare premises) [3]. Note how long it took, not only the final reading.

Cold water

  • Run the cold, let the temperature stabilise, then read it; the usual target is below 20°C [3].
  • Compare against the incoming mains temperature where you can. A large rise between the supply and the outlets flags warming somewhere in storage or distribution.

Low-use and blended outlets

  • Flush, then read, infrequently used outlets, and record how long the temperature took to recover. Slow recovery is a finding in its own right.
  • At outlets fed through a thermostatic mixing valve, expect the blended water to read tepid by design. Check upstream of the valve where the scheme requires it, rather than judging the system on a spout that is meant to deliver warm water.

Recording and close-out

  • Log the reading, the time, the outlet ID and — for any exception — the action taken, before you leave site.
  • Raise out-of-range results to the responsible person the same day, by the route the scheme names. A reading nobody acted on is not monitoring; it is paperwork.

Turning readings into a record that holds up

The habit that lifts a programme is writing down the decision, not just the figure. A bare “48°C” tells a future reader nothing. “48°C at the far shower, recovered to 51°C after 90 seconds, flagged to RP, flushing frequency under review” tells them the system was understood and handled.

That matters because temperature records are the evidence your control regime works. HSE is clear that the type and frequency of monitoring follow the system and the risk assessment, not a fixed national timetable [4]. Your records have to show that the chosen frequency is actually being met and that exceptions get closed — which is far easier when the trend is visible. Two consecutive months of a sentinel drifting toward the danger zone is a story; a single isolated number is not. If the cold side is the one drifting, the cold water storage tanks are usually where the heat is getting in.

The checks that quietly get dropped

These are the ones that turn an honest-looking logbook into false comfort.

  • Not running the outlet long enough. Reading the cold before it has stabilised, or the hot before it has reached temperature, produces a number that flatters the system. Note the stabilisation time and you stop fooling yourself.
  • Reading flow but never the return. A correct stored temperature with a poor return is a circulation fault hiding in plain sight.
  • Letting sentinel rotation lapse. When the same accessible taps get read every month, the difficult outlets — the ones most likely to fail — never get a look in.
  • Recording the number but not the action. An out-of-range result with no follow-up is the gap an investigation finds first.
  • Comparing only against the limit, never against last month. The limit tells you pass or fail today. The trend tells you whether you are about to fail next time. Low-use periods make that drift worse, which is why temperature monitoring during building shutdowns needs its own thought rather than the standard round.

Where this guidance stops

The figures above are the temperatures commonly cited in HSE guidance, but the limits, monitoring points and frequencies that apply to your building come from a competent, site-specific risk assessment and your written scheme — not from a general article. Healthcare, sites with vulnerable users, and systems with stored or recirculated hot water can carry tighter or different requirements. Confirm your own numbers against your assessment before you change anything, and treat a temperature reading as one strand of evidence, never as a clearance certificate for the whole system.

Common questions

What hot and cold temperatures should a routine check be looking for?

As general guidance, hot water is commonly stored at 60°C or above and expected to reach around 50°C within roughly a minute at the outlet, while cold water is kept below 20°C [3]. Treat these as the typical targets to confirm; your risk assessment and scheme set the figures that actually apply to your system.

How long should I run an outlet before I take the reading?

Long enough for the temperature to stabilise rather than reflect water that has been sitting in the pipe. For hot outlets, note the time taken to reach the distribution target; for cold, let it settle before reading. Recording the recovery time, especially at low-use outlets, often reveals more than the final number.

If a sentinel outlet reads in the danger zone, what do I do before the next round?

Record the reading, the time and the outlet, then escalate it to the responsible person the same day through the route your scheme sets — don’t wait for the next visit. The reading flags a possible loss of control; the cause (circulation, stagnation, a failing valve, or warming storage) needs identifying and the corrective action logged and verified.

Sources

[1] HSE, “Legionnaires’ disease. The control of legionella bacteria in water systems - Approved Code of Practice and guidance (L8)”. https://www.hse.gov.uk/pubns/books/l8.htm [2] HSE, “Legionnaires’ disease: Technical guidance (HSG274)”. https://www.hse.gov.uk/pubns/books/hsg274.htm [3] HSE, “Hot and cold water systems”. https://www.hse.gov.uk/legionnaires/hot-and-cold.htm [4] HSE, “Testing and monitoring your water system for legionella”. https://www.hse.gov.uk/legionnaires/testing-monitoring-water-system.htm