---
title: "Cooling towers and evaporative condensers: high-risk systems"
source_url: https://legionella.io/articles/cooling-towers-and-evaporative-condensers-high-risk-systems/
canonical_url: https://legionella.io/articles/cooling-towers-and-evaporative-condensers-high-risk-systems/
pillar: "Building Types & Use Cases"
summary: "Cooling towers are the only Legionella system that can reach the public. Where the money goes, the duty to notify the council, and when to retire the tower."
primary_keyword: "cooling tower Legionella"
date_published: 2025-08-28
date_reviewed: 2026-06-26
author: "Legionella.io editorial team (REMOTE TECH LTD)"
reviewed_against: "HSE L8 and HSG274 guidance"
region: "United Kingdom"
license: "(c) REMOTE TECH LTD. Quote freely with attribution and a link to source_url."
---

# Cooling towers and evaporative condensers: high-risk systems

A cooling tower is the only water system on most sites that can make a stranger ill. It throws a fine mist into open air, and that mist — drift — can carry Legionella well past your fence line to people who never set foot in the building. That one fact reorders the whole cost conversation. With a shower, the worst case is a single person standing in front of it. With an evaporative cooling system, the worst case is a community cluster traced back to your roof.

So the honest budgeting question is not "what does the annual service cost?" It is "what does it cost to run this tower to a standard that holds up — and would the site be safer and cheaper without one?" Cooling tower Legionella control is the most demanding water-safety regime most buildings run, and the most consequential to get wrong. Most tower budgets only answer the first half.

## Why these systems sit at the top of the risk list

HSE ranks water systems by the foreseeable risk they create, and cooling towers and evaporative condensers sit right at the top — above hot and cold water systems — because of the aerosol they produce by design [3]. Their whole purpose is to shed heat by evaporating water, which means warm, nutrient-rich water in constant contact with air, plus a plume that leaves the plant. Legionella reaches people when contaminated water is broken into breathable droplets [5], and a tower does exactly that, continuously, at volume, pointed at the sky. (The mechanism is worth understanding in full — see [How Legionella spreads through water systems](https://legionella.io/articles/how-legionella-spreads-through-water-systems/).)

Because that profile is so different from a tap or a calorifier, HSG274 deals with evaporative cooling systems in their own dedicated part, separate from hot and cold water, with their own water treatment and monitoring expectations [2]. The management duties under the ACoP L8 apply to a tower like any other system [1], but you cannot run a tower on a domestic hot-and-cold routine and call it controlled. [HSG274 guidance explained: practical Legionella control](https://legionella.io/articles/hsg274-guidance-explained-practical-legionella-control/) walks through how that guidance is structured.

There is also a duty that attaches to almost nothing else on site. If you operate a cooling tower or evaporative condenser, you must notify your local authority of the device — and tell them again when it is taken out of use [4]. It is a short form. Missing it is a visible, easily-checked compliance gap, and it signals to a regulator that the rest of the regime may be just as loose.

## Where the money actually goes

The cost of a wet tower is dominated by work that recurs whether or not anyone is watching. Break it into four running-cost drivers and the budget stops being a black box.

| Cost driver | What it covers | What pushes it up |
| --- | --- | --- |
| Water treatment | Continuous biocide programme (typically an oxidising/non-oxidising pairing), scale and corrosion inhibitors, automatic dosing and controlled bleed-off | Tower size, hard water, heat load, poor control of dosing |
| Cleaning and disinfection | Periodic strip-down clean and disinfection of the tower, pack and sump | Fouling, dead areas, worn drift eliminators, deferred cleans |
| Monitoring and sampling | Routine checks on dosing and condition, dip-slide counts for general bacteria, Legionella sampling at the assessed frequency | Number of towers, manual record-chasing, acting on out-of-spec results |
| Physical condition | Drift eliminators that genuinely catch drift, sound packing, accessible sump and isolation valves | Age, corrosion, poor original access design |

Two of those drivers are the ones people quietly stretch, and both punish it. Cleaning is commonly carried out at least six-monthly as a general expectation, more often where the system fouls quickly [2] — defer it and you trade a planned clean for an emergency one. Skimp on water treatment and you are not saving money; you are pre-paying for the next disinfection.

Layered on top is the **friction cost** finance never models. Towers live on roofs and in awkward plant rooms, so access can need permits, scaffolding or a dry-weather window. Cleaning means taking the tower offline and coordinating with whatever depends on the cooling. Visits cluster, samples have to be chased to the lab and back, and someone competent has to act on a high count rather than file it.

Then there is the **failure cost**, which is in a different league from any other water system. A linked case or cluster from a tower triggers an outbreak investigation, immediate shutdown and disinfection, environmental sampling across the area, enforcement attention, and the kind of coverage that names the building. Because the plume reaches the public, the people harmed may have no connection to you at all. There is no honest single figure to quote here, and inventing one would be worse than useless. The point is the asymmetry: a predictable annual programme on one side, an unbounded public-facing event on the other.

## The decision the numbers actually force

Lay those drivers out and the real lever becomes obvious: the cheapest tower to run safely is often the one you no longer have.

Where a process genuinely needs evaporative cooling, the answer is to fund the programme properly and stop treating monitoring as optional. But where the heat load could be met another way — a closed-circuit cooler, a dry air-cooled system, an adiabatic alternative — retiring the wet tower removes the biocide regime, the six-monthly cleans, the sampling burden and the notification duty in a single move. It also removes the only system on site that can reach the public.

That is a capital-versus-running-cost decision, and it deserves to be made on purpose rather than inherited. Plenty of towers still spin only because nobody re-asked the question at the last refurbishment — so a major service, a re-treatment or a drift-eliminator replacement is the moment to put keep-or-replace on the table.

For the towers you do keep, a simple rule sorts the spend: put the next pound where a weakness either increases what people breathe in or undermines your ability to prove the tower is controlled. A failing drift eliminator or a skipped clean outranks a fourth confirmatory sample on a sump that is already well dosed and well documented.

## Making the case to whoever signs it off

When the number goes upstairs, frame it as three things at once: a known, modest running cost; an unbounded, public-facing failure cost; and a live question about whether the asset should exist at all. Two points tend to land. First, the regulator already knows the tower is there — the notification duty made sure of that — so weak records are not a private oversight. Second, monitoring and treatment frequencies are set by the assessment and HSG274, not by what finance would prefer to spend [2]; cutting them is not a saving, it is accepting risk you have chosen not to measure.

## Before you rely on any of this

This is general guidance, not a control scheme for your tower. The treatment chemistry, the cleaning interval, the sampling frequency and the action levels all depend on your water, your plant and the judgement a competent person makes about them — and an evaporative cooling system carries duties (notification, water treatment, drift control) that a hot-and-cold system simply does not. Use a competent provider who works to a recognised standard, and let your written scheme set the figures, not a web page.

## FAQ

### Do I really have to tell the council about a cooling tower?
Yes. Operators of cooling towers and evaporative condensers must notify the local authority of each device, and notify again when it is taken out of use [4]. It is a brief form, but it is one of the first things an inspector checks, and an unnotified tower undermines confidence in everything else you claim to control.

### Is an evaporative condenser as risky as a cooling tower?
Treat it the same way. Both shed heat by evaporating water into an air stream, and both can throw drift, so HSE lists them together at the top of the risk hierarchy [3] and HSG274 covers them under the same evaporative-cooling regime [2]. The label on the unit does not change the duties.

### We dose the water well — do we still need to clean and sample?
Yes. Treatment, cleaning and monitoring are separate controls, not substitutes for each other. Good dosing slows fouling but does not lift the biofilm and debris that a physical clean removes, and sampling is how you find out whether the whole programme is actually working rather than just running [2].

## Related reading

- [HSG274 guidance explained: practical Legionella control](https://legionella.io/articles/hsg274-guidance-explained-practical-legionella-control/)
- [How Legionella spreads through water systems](https://legionella.io/articles/how-legionella-spreads-through-water-systems/)
- [Neglected water systems: the danger of stagnation](https://legionella.io/articles/neglected-water-systems-the-danger-of-stagnation/)
- [Communication gaps: unclear duties leading to failures](https://legionella.io/articles/communication-gaps-unclear-duties-leading-to-failures/)

## Sources

[1] 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
[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] CDC, "How Legionella Spreads". https://www.cdc.gov/legionella/causes/index.html