A cooling tower can give Legionnaires’ disease to someone who never set foot on your site. It does it by throwing a fine mist of water into the air that the wind can carry hundreds of metres. That single property is why HSG274 Part 1 is the most demanding of the three technical parts the Health and Safety Executive publishes [2].

Part 1 is the evaporative cooling section: cooling towers, evaporative condensers and the closed-circuit (fluid) coolers that work the same way. It sits beneath the L8 Approved Code of Practice alongside Part 2, for hot and cold water, and Part 3, for other risk systems [1][2]. Far fewer buildings have an evaporative system than have taps and a calorifier — but the ones that do carry a consequence that reaches past the boundary fence.

So Part 1 reads differently from the rest of HSG274. It assumes a system that runs warm, runs wet, and deliberately makes aerosol. The control scheme it describes is an engineered water-treatment programme, not a temperature regime.

Why evaporative cooling tops the risk hierarchy

An evaporative system sheds heat by evaporating water. To do that efficiently it breaks the water into droplets and pulls air through them, which means a working tower is, by design, an aerosol generator. If the recirculating water is colonised, every fan-assisted plume becomes a delivery mechanism for Legionella that can travel well beyond the plant it serves [3].

That is the difference from a shower or a tap, where the aerosol stays in one room and reaches one person. A contaminated cooling tower exposes passers-by, neighbouring buildings and people downwind who have no connection to your premises at all. It is the classic source behind community outbreaks, which is why these systems are treated as a category of their own. For the wider picture of why they rank where they do, see on cooling towers and evaporative condensers as high-risk systems.

Where Part 1 sits in the HSG274 set

HSG274 comes in three technical parts under one Approved Code of Practice [1][2]:

  • Part 1 — evaporative cooling systems: cooling towers, evaporative condensers and some closed-circuit coolers.
  • Part 2 — hot and cold water systems, the part nearly every occupied building lives in. See on HSG274 Part 2.
  • Part 3 — other risk systems, such as spa pools, humidifiers, air washers and similar plant.

Read the part that matches your plant, not all three from cover to cover. A site with a cooling tower and conventional plumbing needs Part 1 and Part 2 together: the temperature controls in Part 2 do almost nothing for a tower, and the chemical treatment programme in Part 1 has no bearing on your hot water calorifier.

The notification duty other systems don’t carry

Evaporative cooling brings a legal duty that hot and cold water does not. Cooling towers and evaporative condensers must be notified to the local authority for the area in which they sit, and you must tell the authority again when a device is taken out of use [4]. The local authority keeps a register of these devices, so that if a cluster of cases appears in an area, investigators already know where every tower is.

Notification is quick, cheap and frequently forgotten when plant changes hands or a unit is quietly decommissioned. Getting it wrong is an easy enforcement finding. The detail of what to notify and how the registers work is covered in on cooling tower notification duties.

What a Part 1 control scheme has to deliver

Part 1’s control scheme rests on a handful of measures that work together. Drop one and the others rarely compensate.

Water treatment and biocide regime. A recirculating tower concentrates dissolved solids and nutrients as it evaporates, so it needs a continuous chemical programme: biocides to suppress microbial growth (commonly alternating oxidising and non-oxidising types), plus scale and corrosion inhibitors to keep surfaces clean enough for the biocide to reach the water it is dosing. Scale, sludge and biofilm all shelter bacteria from the very chemicals meant to kill them [2].

Drift eliminators. These baffles strip water droplets out of the exhaust air before it leaves the tower, cutting the volume of aerosol released. Damaged, fouled or missing eliminators are a direct and visible increase in how much potentially contaminated water the tower puts into the air, and they are a standard inspection point.

Cleaning and disinfection. Towers need periodic cleaning and disinfection to remove the deposits that treatment alone cannot — generally at least twice a year, and additionally before commissioning, after a shutdown, and following any work that disturbs the system [2]. A tower that is chemically dosed but never physically cleaned will still grow biofilm in its pack and pond.

Monitoring. The treatment programme has to be proven, not assumed. That means routine checks of control parameters — biocide levels, conductivity, inhibitor concentrations and dip-slide aerobic counts — usually weekly, with Legionella sampling at a frequency set by the risk assessment and conditions, commonly quarterly but more often after any loss of control [2][5]. Readings outside the action levels in your scheme are meant to trigger a defined response, not a note for next time.

Competent management. None of this works without a named responsible person, a written scheme of control, and contractors whose competence you have actually checked. The risk assessment underneath all of it is system-specific; on risk assessing cooling towers covers how to build one.

Field checklist: what a duty holder should be able to evidence

Walk the system against its scheme of control and record a finding against each line — a reading, a date, or a defect — rather than a tick. If you cannot evidence it, you are not controlling it.

Identification and notification

  • Confirm every cooling tower, evaporative condenser and evaporative cooler on site is on the asset register with a unique ID.
  • Confirm each device has been notified to the local authority, and that any decommissioned unit was de-notified.

Treatment and control parameters

  • Check biocide dosing is operating and within the scheme’s range, with recent results recorded.
  • Confirm conductivity and bleed control, and inhibitor levels, against the scheme’s targets.
  • Review dip-slide aerobic counts and the latest Legionella sample result against your action levels.

Physical condition

  • Inspect the drift eliminators for damage, fouling and correct fit.
  • Check the pond, pack and wetted surfaces for scale, sludge, slime and debris.
  • Confirm make-up water and the float and level controls are working, and the tank is covered.

Cleaning, disinfection and records

  • Confirm the last clean-and-disinfection date and the next one due.
  • Confirm any out-of-specification reading triggered a recorded corrective action.
  • Confirm the written scheme of control, the risk assessment and the named responsible person are all current.

What Part 1 does and doesn’t decide for you

HSG274 Part 1 sets out recognised good practice for evaporative cooling. It does not assess your specific tower, and it does not replace a competent, site-specific risk assessment that decides which measures apply, at what set points, and how often you check them. The frequencies and parameters above are working expectations to confirm against your own plant and water conditions, not fixed national rules. This is compliance guidance, not legal, medical or engineering design advice for your premises.

Common questions

Does HSG274 Part 1 apply to dry air coolers and adiabatic systems?

A fully dry air cooler that never wets a surface is not an evaporative system and falls outside Part 1. Adiabatic and hybrid coolers are the grey area: when they spray or wet a surface to boost cooling they can generate aerosol and may need to be controlled as evaporative plant, with some also requiring notification. Your risk assessment, not the sales brochure, should decide how each unit is treated [2][4].

How often must a cooling tower be cleaned and disinfected?

The general expectation is at least twice a year, plus before commissioning, after any prolonged shutdown, and after work that could have contaminated the system — but the right interval is the one your risk assessment justifies for the specific tower and its duty [2]. A heavily loaded or fouling-prone system may need it more often.

Your next step

Pull your asset register and your last quarter of cooling-tower records side by side, and check three things: that every tower is notified to the local authority, that the last clean-and-disinfection date is still in date, and that any reading outside your action levels actually triggered a recorded response. A gap in any one of those is the kind of thing an investigator finds first — and exactly the recurring evidence a digital logbook keeps visible instead of buried in a folder.

Sources

[1] HSE, “Legionnaires’ disease. The control of legionella bacteria in water systems - ACoP and guidance (L8)”. https://www.hse.gov.uk/pubns/books/l8.htm [2] HSE, “Legionnaires’ disease: Technical guidance (HSG274)”. https://www.hse.gov.uk/pubns/books/hsg274.htm [3] HSE, “Systems most likely to create legionella risk”. https://www.hse.gov.uk/legionnaires/risk-systems.htm [4] HSE, “Other duties: RIDDOR and notification of cooling towers or evaporative condensers”. https://www.hse.gov.uk/legionnaires/what-you-must-do/duties.htm [5] HSE, “Testing and monitoring your water system for legionella”. https://www.hse.gov.uk/legionnaires/testing-monitoring-water-system.htm