A calorifier is not a tank you fit and forget. It is a piece of plant with an inside that fouls, a base that runs cooler than its gauge suggests, and a drain valve that is the single cheapest tool you have for Legionella control. Maintain those three things on a schedule and you remove most of the risk a stored-hot-water vessel carries.

The core principle is simple. Heat alone does not keep a calorifier safe, because the vessel is not one temperature. Sludge, scale and corrosion debris settle at the bottom, the cold feed enters there, and that lower layer can sit warm rather than hot — the exact band where Legionella multiplies. Inspection finds the fouling, drain-off removes it, and a temperature check on the drain water tells you whether the base is actually reaching control [1][2].

Why the base, not the body, is the problem

The flow thermometer on the crown reads the hottest water in the vessel. Useful, but it is the least representative point you could measure. Incoming cold water sinks; particulate matter drops out of suspension and forms a sediment bed; and that bed both insulates the base from the heating coil and gives bacteria a nutrient-rich surface to colonise. You can have 60°C at the top and a soft, tepid layer underneath that no flushing of outlets will ever clear.

That is why HSE guidance treats the calorifier as its own monitored asset, with internal inspection and drain-off checks set by the site risk assessment rather than left to chance [2]. Store at 60°C, distribute so outlets reach 50°C within about a minute, and the system works — but only if the bottom of the vessel is part of “the system” in your records [3].

The three jobs, applied on a real site

The annual internal inspection. At a frequency your risk assessment justifies — commonly annual — the calorifier is isolated, drained and opened so the internal surfaces and any sacrificial anode can be examined. You are looking for scale, corrosion, biofilm and the depth of sediment at the base. On a hard-water site you may find a centimetre of scale a year; on a soft-water site, almost none, and the anode tells the corrosion story instead. Where a vessel has no inspection hatch, the drain-off and its sample become your only window inside, which raises the importance of doing them properly.

The drain-valve blowdown. Between full inspections, opening the base drain valve and running off several litres clears loose sediment before it consolidates. The water that comes out is evidence: catch it in a clean container and read its clarity, debris and temperature. Cloudy, gritty or cool drain water is a flag — it says the base is fouled or not reaching the stored temperature the gauge claims [2][4]. The pragmatic call is to log the drain temperature every time, because a slow downward drift over several months is the early warning a single annual inspection would miss.

Base sludge removal. When inspection or drain samples show real accumulation, the vessel needs proper cleaning — drain fully, remove the sediment, and check the anode and internal coating. In my view this is the job most often deferred and most worth doing, because sludge is simultaneously a Legionella habitat and an insulator that pushes your energy bill up while hiding a cool zone from the thermostat.

Field checklist: calorifier inspection and drain-off

Group the work into before, during and after, and make every line recordable — a tick, a reading, an initial.

Before

  • Confirm the asset ID, location and last inspection date against the logbook.
  • Check permit-to-work, isolation points and that a competent person is doing the task.
  • Note the flow thermometer reading and the return temperature before you start.
  • Have a clean clear container ready for the drain sample.

During

  • Isolate and, for a full inspection, drain the vessel safely (mind scalding — stored water is hot).
  • Open the base drain valve; run off until the water clears; record clarity and any debris.
  • Measure and record the drain-water temperature, not just the crown gauge.
  • For internal inspection: examine surfaces, measure sediment depth, assess the anode and coating.
  • Photograph the base and any corrosion for the asset record.

After

  • Flush through, refill, vent air and confirm the vessel reheats to the stored set-point.
  • Verify the nearest and furthest outlets reach the distribution target on recovery.
  • Record findings, defects raised and the next due date; close the work order.
  • Schedule any remedial clean, anode replacement or repeat sample.

Trade-offs worth naming

More frequent drain-off costs a little labour and a little hot water down the drain, and over-frequent full inspections mean more downtime and more disturbance of a coating that is doing its job. Against that, a fouled base is an open invitation to Legionella and a quiet drain on efficiency. The balance your risk assessment should strike is frequent, low-effort drain checks with temperature logging, punctuated by a thorough internal inspection at the interval the assessment sets [2]. Let the drain-sample trend, not the calendar alone, tell you when to escalate.

This is general guidance. Calorifier design, water chemistry and vessel condition vary enormously from site to site, so the inspection interval, drain-off frequency and what counts as an actionable sediment depth must be fixed by a competent person through your own site-specific Legionella risk assessment, not lifted from an article.

The next step you can take today: pull your calorifier’s last inspection and drain-off records and check two things — when the base was last drained, and whether anyone has ever logged the drain-water temperature rather than the crown gauge. If those entries live on a paper sheet in the plant room and no one can find the trend, that is the gap a digital logbook closes, by tying each drain reading to the asset and flagging the next due date before it slips.

FAQ

How often should a calorifier be drained and inspected?

There is no universal number. HSE guidance points to internal inspection — commonly annual — and periodic drain-off, with the actual frequency set by your site risk assessment based on water quality, vessel condition and inspection history [2]. Treat that as the starting expectation and verify it for your premises.

What does cloudy or gritty drain water tell me?

It indicates sediment accumulation at the base, where particulate matter, scale and corrosion debris settle. That bed can insulate the base from the heating coil and create a warm-but-not-hot zone favourable to Legionella, so it is a prompt to sample, clean and recheck the base temperature rather than just close the valve [2][4].

Is the flow thermometer on top enough to prove control?

No. The crown gauge reads the hottest, least representative water. Reaching 60°C storage and 50°C at outlets within about a minute matters, but you also need evidence the base is reaching temperature — which the drain-off reading provides [3][2].

Sources

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