A data centre is engineered to keep water as far from the IT as physically possible. Leak-detection rope under the access floor, pre-action sprinklers held dry until two events agree, cabinets sealed against humidity swings. The whole culture is water-phobic. Which is precisely why Legionella gets overlooked here.

The cooling that protects those servers is, increasingly, wet. The push for lower PUE and lower water-energy cost has driven designs straight back towards evaporation: cooling towers, adiabatic spray on dry coolers, evaporative pre-cooling, air washers and humidification. Every one of those produces, or can produce, a respirable aerosol [1]. The bacteria do not care that they are in a Tier III hall.

If you run the mechanical side of a critical facility, this is a register problem before it is a treatment problem. You cannot control a source you have not written down.

The wet plant hiding in a “dry” building

Stop walking the white space and walk the heat-rejection topology instead. The aerosol-generating systems that belong on a Legionella asset register tend to be these.

Cooling towers and evaporative or closed-circuit coolers rejecting heat from the chiller plant. These are the textbook high-risk systems and, for open towers and evaporative condensers, usually notifiable ones. They are well understood in their own right — Cooling towers and evaporative condensers: high-risk systems covers the mechanism — but in a data centre they are frequently the only heat rejection for a live load, which changes how you maintain them.

Adiabatic or evaporative pre-cooling retrofitted to dry coolers and air handling units: spray nozzles or wetted pads sitting upstream of the coil pack, wetting the incoming air so the “dry” cooler can reject more heat on a hot day. They wet the coil, they make droplets, and they run hardest in exactly the warm conditions Legionella prefers.

Direct and indirect evaporative cooling units and air washers conditioning supply air, where water and the airstream meet by design.

Humidifiers holding server-hall relative humidity inside the operating envelope. Spray, atomising and evaporative types all create fine droplets — that is the point of them.

The mundane domestic water in offices, kitchens, showers and welfare blocks. A data centre is also a workplace, and its hot and cold water carries the same risk as any industrial facility: little-used outlets, oversized incoming mains for fire flow, and showers nobody runs for weeks.

What nobody tells you about evaporative cooling in data centres

The generic guidance stops at “cooling towers are high risk”. Here is the part that actually trips up critical-environment teams.

The efficiency case quietly imports the risk. The line item that makes a facility look good on the sustainability report — water-side economisation, adiabatic assist, a lower PUE — is the same line item that puts water into the airstream. Nobody decides to add Legionella risk; they decide to cut energy and water cost, and the risk arrives bundled with the saving. Treat any WUE or PUE initiative that involves evaporation as a trigger to revisit the risk assessment.

Adiabatic systems slip through the regulatory net because they are filed under “air handling”. The M&E schedule calls it a dry cooler or an AHU. The Legionella risk assessment, scoped years earlier around “cooling towers”, never names it. So a system that sprays water into air sits outside both the control scheme and, potentially, the notification list. This is the single most common gap I see flagged in these buildings.

Intermittent operation is the trap, not the running hours. Adiabatic pre-cooling and spray humidifiers fire only when conditions demand it. Between calls for cooling, water stands in spray headers, sumps and pad reservoirs — warm, still, in the 20–45°C band where Legionella thrives [1]. Seasonal start-up after a long dry spell, when the first hot day brings the sprays back online, is a classic colonisation-then-aerosolisation window.

Uptime culture fights the controls. You cannot drain, clean and disinfect a tower that is the sole heat rejection for a populated hall unless the system was designed to be concurrently maintainable, with redundancy that lets you take one cell offline. Where it was not, “we will do it at the next window” can quietly mean never, and restricted access makes even routine dosing checks a coordinated event rather than a walk-round.

Alternative make-up water brings its own load. Harvested rainwater, recovered condensate, grey water or a borehole feeding evaporative systems all help the WUE number and all carry their own microbiology and nutrients. The water you save money on is not necessarily the water you want in a spray nozzle.

Notifiable — and to whom

Open wet cooling towers and evaporative condensers must be notified to the local authority, and a notifiable system carries duties beyond the risk assessment itself [2]. Closed-circuit evaporative coolers that spray onto a coil generally fall in the same category, because they still generate the aerosol. Verify the position for your specific plant.

Adiabatic and spray-pre-cooling systems are the genuine grey area. Whether a given unit is notifiable depends on its design and how it wets the air, and reasonable engineers disagree — so confirm the status of each unit with your local authority and a competent advisor rather than assuming “it is only an AHU” [2]. Either way, notifiable or not, an evaporative source still needs assessing and controlling under the L8 framework [3].

Keep RIDDOR in view too: a confirmed case of Legionnaires’ disease connected to a work water system can be reportable, which is exactly the conversation you do not want to have without records to hand [2].

Where the BMS helps, and where it does not

The building management system already trends the data you need: approach temperatures, fan and pump speeds, sump levels, conductivity, fill-and-bleed cycles and run hours. The opportunity is to make the Legionella-relevant parameters first-class points rather than buried trends — biocide dosing status, conductivity against the target cycles of concentration, sump temperature, spray on/off state, and run hours that can flag a unit that has sat stagnant through a cool spell.

The limit is just as important. A BMS optimises for availability and efficiency; it does not protect water hygiene unless someone configures it to. Integrating Legionella control with Building Management Systems goes into how to wire those parameters in. In my view a single hard alarm on a failed dosing pump earns more than another temperature graph nobody opens — the BMS is most valuable when it tells a human that a control has stopped, not when it merely records that things are normal.

Controls that survive 24/7 operation

The treatment fundamentals are not exotic: a written cleaning and disinfection schedule, an oxidising and non-oxidising biocide regime with dosing verified at the plant, and monitoring through dip slides and periodic Legionella sampling. Two points deserve specific attention here.

Drift eliminators decide how much viable aerosol leaves the system, so they should be specified properly and physically inspected, not assumed intact after years of vibration. And the discharge geometry matters more than people expect: a tower exhaust or an adiabatic plume sited next to an AHU fresh-air intake, a loading bay or a spot where people walk is a direct exposure path. Map where the aerosol goes, not just where it is made.

Then design for the drain-down you will actually need. End-of-season shut-down with a full clean, and start-up disinfection before the sprays return, are where intermittent systems are won or lost — and only possible if access and redundancy let you take a unit offline without dropping the hall.

This is general guidance, not a control scheme for your site. Which systems are in scope, whether a given adiabatic unit is notifiable, the biocide and monitoring regime, and the sampling frequency are all decisions for a competent person working from a current, site-specific risk assessment under ACoP L8 [3]. We do not give legal, medical or M&E design advice, and a data centre’s redundancy topology will shape what is practicable in ways no generic article can anticipate.

What to do next

Put the cooling schematic and the Legionella asset register side by side and mark every point where water meets air: each tower, every adiabatic cooler, every humidifier and air washer, and the welfare water. For each one, answer three questions today — is it on the current risk assessment, is it notified if it should be, and who can clean it without dropping the load? The adiabatic coolers are where the gap usually hides.

That cross-check is the kind of recurring, evidence-backed task that a paper file loses between refits and a digital logbook keeps visible — so the spray system added during last summer’s PUE project does not survive three maintenance cycles unassessed.

FAQ

Are adiabatic dry coolers notifiable like cooling towers?

It depends on the design, and it is a genuine grey area. Open cooling towers and evaporative condensers are clearly notifiable to the local authority [2]. Adiabatic and spray-pre-cooling units sit less clearly, so confirm each unit’s status with your local authority and a competent advisor. Notifiable or not, an evaporative source still needs assessing and controlling under L8 [3].

Does a closed-circuit (dry) cooler avoid Legionella risk?

Only if it is genuinely running dry. Many closed-circuit and hybrid coolers spray water over the coil in evaporative mode on hot days, which generates an aerosol and brings them back into scope. Check whether your unit ever wets the coil before assuming it is risk-free.

Can we skip Legionella controls because the data hall is sealed and dry?

No. The risk is not in the sealed white space; it is in the cooling and humidification plant that conditions it, and in the building’s domestic and welfare water. A “dry” hall can sit directly downstream of a wet cooling tower and an evaporative humidifier [1].

Sources

[1] HSE, “Legionnaires’ disease: Technical guidance (HSG274)”. https://www.hse.gov.uk/pubns/books/hsg274.htm [2] HSE, “Other duties: RIDDOR and notification of cooling towers or evaporative condensers”. https://www.hse.gov.uk/legionnaires/what-you-must-do/duties.htm [3] 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