Hotel Legionella Risk: The Low-Occupancy Compliance Gap

Most hotel Legionella risk assessments treat occupancy as a static input. The risk profile is calculated for the building as it was designed to operate, not the building as it operates on a Tuesday in February when half the rooms are empty.

This is the gap. A hotel water system designed for 85 percent occupancy does not behave the same way at 55 percent. The guest room that has not been let for ten days contains a length of pipework — from the riser to the basin, to the shower, to the WC cistern — that has been holding water in the 20 to 45 degree growth range for the entire period. When that room is next let, the first aerosolising outlet used is the shower. The exposure window is the first thirty seconds.

HSG274 Part 2 and ACoP L8 set out the framework. Hot water stored at or above 60°C. Hot water distributed to reach 50°C at outlets within one minute of running. Cold water maintained below 20°C. Outlets used infrequently flushed at least weekly. HSG274 Part 2 was reissued as a second edition in March 2024, but the substantive guidance on hot and cold water systems has not changed — only Part 1, covering evaporative cooling, was revised. The framework is not the issue. Application in a variable-occupancy building is the issue.

First, flushing protocols that assume the guidance wrote itself. Housekeeping or maintenance teams are told to flush unoccupied rooms weekly. The protocol rarely specifies duration, outlet sequence, or temperature verification. "Running the tap" is not flushing. HSG274 specifies running outlets until representative temperature is reached — meaning hot water to 50°C or cold water below 20°C. Without measurement, the flush is undocumented and unverifiable.

Second, temperature regimes that fail under low demand. Hot water systems sized for peak occupancy can struggle to maintain flow and return temperatures when demand drops. A calorifier storing at 62°C during full occupancy may return to 54°C in distribution-loop readings during low-season weekday periods. The storage temperature remains nominally compliant. The distribution temperature quietly does not.

Third, risk assessments treated as calendar items, not living documents. ACoP L8 removed the fixed two-year review interval in its fourth edition and now requires the risk assessment to be treated as a living document — reviewed whenever there is reason to suspect it is no longer valid. Sustained low occupancy is reason to suspect. An assessment that covered the building's pre-change operating pattern no longer reflects the current risk.

The post-pandemic UK sampling data is instructive. A UKAS-accredited study by Feedwater Ltd of hotels reopening after extended closures found a marked increase in Legionella-positive samples — in some systems, two-thirds of samples tested positive — including in systems previously considered under control. The bacteria do not require a building to close fully to colonise. They require temperature, stagnation, and time. A hotel running at sustained reduced occupancy provides all three in sufficient measure.

For hotel operations directors and estates teams, the audit question is not "do we have a Legionella risk assessment?" It is "does the current assessment reflect how the building is actually operating this quarter?"

A risk assessment dated 2024 that assumes 2019 occupancy patterns is not evidence of compliance. It is evidence of a gap.

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