What should be considered when testing calorifiers for legionella?

When testing a calorifier for legionella, the central consideration is temperature at three points: the stored water should be at least 60°C, the water returning from the distribution loop should be at least 50°C, and the furthest outlets should reach at least 50°C within one minute (55°C in healthcare). Beyond the thermometer, you are considering the cold base of the vessel, the sludge and scale inside it, and whether the system can hold these temperatures continuously, not just on the day of the test.

A calorifier is the engine room of a hot water system: it is where water is stored hot so that legionella is suppressed. But a calorifier that is partly cold, partly fouled, or losing heat around a long loop can be growing legionella at the same time as its top thermometer reads a reassuring figure. Testing is how you find out which of those you have.

Flow, return and the loop

The flow from the calorifier should leave at 60°C or more, because the stored water is the control. The return tells the more interesting story: after travelling around the whole distribution loop, the water coming back to the vessel should still be at 50°C or more. A return below 50°C means water somewhere in the loop is sitting in the 20 to 45°C growth range, usually because of heat loss on long or uninsulated pipe runs, a failed or mis-set pump, or a section of the loop that is not circulating. The monthly check of flow, return and sentinel outlets sits in the monitoring schedule, and the temperature chart sets out the target figures.

The cold base: the part that survives

Cold make-up water enters the calorifier at the bottom, so the base of the vessel is always its coolest zone. In a well-stratified or poorly circulating vessel, the base can sit below 50°C while the top is at 60°C, and sludge, scale and sediment settle into exactly that zone, feeding and sheltering bacteria. This is why testing includes the temperature at the drain valve or base of the vessel: it is the one place that can be quietly wrong while every other reading looks fine. The detail of what an inspection covers is in calorifier inspection.

The annual internal inspection

HSG274 expects the inside of the calorifier to be inspected, normally annually: the lid or inspection hatch comes off and the vessel is checked for sludge, scale, corrosion and any sign of deterioration. Where the inspection finds significant deposits, the vessel is drained and cleaned. The inspection is also the moment to confirm immersion heaters, thermostats and any destratification pump are actually working, because a calorifier can lose its stratification or fail to recover temperature without anyone noticing from the outlet readings alone.

When a test fails

A reading below target is a finding, not just a number to re-measure. A low return points at heat loss or circulation failure in the loop; a cold base points at stratification or a heating fault; a calorifier that cannot hold 60°C may be undersized, scaled up, or have a failed heater or thermostat. Each has a different fix, and the written scheme of control should say what action follows a failed check. Repeated failures with no corrective action recorded are worse than the failure itself, because they show the control is known to be broken. Log every reading and every action in the log book; the temperature checker helps record readings as you take them. The wider context is in legionella temperature control.