How do you inspect a calorifier for legionella?

Inspecting a calorifier for legionella comes down to three questions: does the whole stored volume reach 60°C, is the base free of sediment and cooler water, and is a sample drawn from the bottom drain valve clear? A calorifier is simply a hot water storage vessel, and its weak point is the base, where the incoming cold feed enters and where debris settles.

Because stored water stratifies, the temperature at the top of the vessel tells you very little about the bottom. The layer sitting above the drain valve can be several degrees cooler than the flow temperature at the outlet, and if it drifts into the 20 to 45°C range it becomes the one part of the system where legionella can multiply. That is why a proper calorifier inspection centres on the base, not the outlet.

Why stratification puts the risk at the base

Hot water is less dense than cold, so it rises. Inside a calorifier the hottest water collects at the top, where the flow outlet usually sits, and the coolest water settles at the bottom, where the cold feed comes in. This layering is called stratification, and it is entirely normal. The problem is that a thermostat or outlet probe near the top can read a comfortable 60°C while the base sits well below it.

Two things collect in that cooler base layer. First, sediment: scale, corrosion debris and biofilm drop out of the flow and gather at the bottom, giving legionella and the amoebae that harbour it somewhere to live. Second, cooler water that turns over slowly, because the main draw-off is happening higher up. A base that runs at, say, 35°C over a bed of sediment is close to an ideal incubator, which is exactly why HSG274 Part 2 directs attention there.

What a calorifier inspection covers

A competent inspection works through the vessel from the flow outlet down to the base:

• Flow and return temperatures. Measure the temperature of water leaving the calorifier (the flow) and returning to it (the return). The flow should be around 60°C and the return should not fall below 50°C. A low return suggests the stored volume is not holding temperature.
• Temperature throughout the vessel. Confirm the whole volume reaches 60°C. Where the base runs cooler, one approach is to raise the entire contents to 60°C and hold them, so the cooler layer is pasteurised rather than left as a reservoir.
• The base drain sample. Draw water from the bottom drain valve and look at it. Cloudy water, rust or visible debris points to sediment build-up and a base that is not turning over. This is also the right point to take a sample for legionella testing if the risk assessment calls for it.
• Sediment and scale. Where the vessel has an inspection hatch, open it and check the internal surfaces for scale, corrosion and sludge. Clean out any accumulation at the base.
• The sacrificial anode. Many calorifiers carry a magnesium sacrificial anode that corrodes in place of the steel shell. Check its condition and replace it as it depletes, because a failed anode leads to internal corrosion and more debris.

Where there is no inspection hatch and the vessel cannot be opened, the drain sample and a temperature profile of the vessel become the main evidence. Temperature profiling means measuring at several points, or over the depth, to show whether a cold layer is sitting at the base.

How often a calorifier should be inspected

HSG274 Part 2 sets a two-part rhythm. Flow and return temperatures are checked monthly, and the calorifier is inspected internally once a year where an inspection hatch allows, or at an interval set by the written risk assessment. Monthly figures are recorded on a temperature chart so that a drifting return shows up before it becomes a problem.

What the inspection does and does not tell you

An inspection is a snapshot. A clear drain sample and a vessel holding 60°C on the day tell you the base was in good order at that moment; they do not certify the system as safe indefinitely, and they do not replace the wider control scheme of temperature monitoring, flushing of little-used outlets and outlet checks. If a sample returns a positive result, follow your detection response and review treatment options rather than treating the single clean inspection as reassurance.

The value of the exercise is in the record. Each temperature reading, each drain sample and each internal check should be written down, dated and signed, so the log book shows a continuous picture of how the vessel behaves. A single good inspection with no history behind it demonstrates very little.

Where this sits in the guidance

Calorifier inspection is one strand of the monitoring regime described in HSG274 Part 2 and underpinned by ACOP L8. The written scheme in your risk assessment should name the calorifier, set the inspection frequency and say who carries it out, and the results feed back into your overall monitoring records. Treat the vessel as the storage heart of the hot water system, keep its base clean and hot, and record what you find each time.