A cooling tower logbook records what a hot-and-cold-water logbook never touches: biocide dosing and water-treatment readings, conductivity and bleed, dip-slide counts, Legionella sample results, drift-eliminator and pond inspections, and clean-and-disinfect dates. Most control parameters are logged weekly, samples typically quarterly, and the tower is cleaned at least twice a year — though your risk assessment sets the actual cadence [2][3].

If you already keep a tidy water logbook for taps, calorifiers and tanks, none of it covers the tower on the roof. Evaporative cooling runs warm, runs wet and makes aerosol on purpose, so it is controlled by a chemical treatment programme, not a temperature regime. The records that prove that programme is working are different records, kept on a different rhythm.

This is the cooling-tower side of the logbook: the entries HSG274 Part 1 expects, grouped by how often each one falls due [2].

Why your water logbook doesn’t cover the tower

The guide to What a compliant Legionella logbook must contain (with a template) sets out the asset register, temperature logs and remedial trail that hold a building’s water records together. A cooling tower sits outside almost all of it. There are no sentinel taps to swab, no TMVs to service, and the temperature you care about is the recirculating water staying treated, not a hot outlet reaching a target.

What you are evidencing instead is that an engineered treatment scheme kept a colonisable system under control between cleans. That means dosing readings, microbiological trends and physical-condition checks — and, uniquely, a system that has been notified to the local authority [4]. Notification is a duty in its own right; Cooling tower notification duties and local authority registers covers what to notify and when, and your logbook only needs to hold the reference.

Cooling towers and evaporative condensers: high-risk systems explains why these systems sit at the top of the risk hierarchy. The logbook is how you prove you treated them like it.

The cooling tower logbook checklist, by frequency

Record a finding against each line — a reading, a date, a defect — not a tick. Group your cooling tower log sheet the same way, so a gap shows up as a frequency that lapsed rather than a line lost in prose. Treat every interval below as guidance your risk assessment confirms or tightens [2].

Whenever the system state changes

  • Log commissioning of a new or returned tower, with the pre-service clean and disinfection.
  • Record every shutdown and restart, and any work that disturbed the system, before it goes back into use.
  • Note changes of treatment contractor, dosing chemistry, or the responsible person.

Continuously / daily (where automated)

  • Confirm the automated dosing and bleed system is running and free of alarms.
  • Record any automatic biocide or inhibitor top-up, and any dosing alarm.

Weekly

  • Log the biocide dosing record: type, dose or concentration, and that levels sit inside the scheme’s range.
  • Record conductivity and bleed-off, and inhibitor concentration, against target.
  • Take and log dip-slide aerobic counts, dated and read at the right incubation point.
  • Walk the unit: a brief cooling tower inspection checklist of obvious slime, debris, leaks and noise.

Monthly

  • Review the dip-slide and parameter trend, not only the latest reading.
  • Inspect the drift eliminators for damage, fouling and fit.
  • Check the pond, pack and wetted surfaces for scale, sludge and biofilm; check the float, make-up and that the tank is covered.

Quarterly (or as the assessment sets)

  • Log the Legionella sample result, the laboratory, and the action level it is judged against [3].
  • Review the quarter’s records against the scheme and capture the responsible person’s sign-off.

At least twice a year

  • Record the clean and disinfection: date, contractor, method, and the post-clean checks. This is the cooling tower cleaning frequency people most often under-record [2].

Annually

  • Record the review of the risk assessment and the written scheme of control.
  • Confirm the asset register and the local-authority notification are still accurate.

How to record it so it stands up

Three habits separate a cooling tower maintenance record that reassures an investigator from one that worries them.

Record at the tower, dated and initialled by the person who took the reading — not back-filled at a desk on the last Friday of the month. A run of identical weekly figures entered in one sitting is the first thing a competent auditor distrusts.

Log the response, not just the reading. A dip-slide count or biocide level outside your action level is only half an entry; the corrective action, the recheck and the close-out date are what turn it into evidence of control rather than a record of a problem you watched happen.

Keep the tower’s records traceable to the asset register and the scheme of control, so any reading ties back to a specific unit, a target and a named owner. Retention follows HSE guidance, not the size of your binder [1].

The entries people skip

The bleed and conductivity log goes missing most often, because dosing gets the attention and bleed control quietly drifts. Without it, biocide is diluted or over-concentrated and nobody can see why the counts moved.

The shutdown-and-restart record is the next casualty. A tower brought back after a quiet spell without a pre-use clean is a known amplification route, and the logbook is usually silent on exactly the weeks that matter [2].

The corrective-action trail is the one that decides how a folder reads under scrutiny. HSG274 Part 1 explained: controlling Legionella in cooling towers and evaporative condensers sets out the control scheme itself; the logbook’s job is to prove that when a parameter went out of range, something happened next.

A note on what this checklist is

These intervals and parameters are recognised good practice, not fixed law. The dosing ranges, sampling cadence and cleaning frequency that apply to your tower come from a competent, site-specific risk assessment and the written scheme built on it. This is record-keeping guidance, not legal, medical or water-treatment design advice for your plant.

Your next step

Lay your last quarter of cooling tower records beside your scheme of control and test one thing: can you point to a dated, initialled entry for every weekly check, the last two dip-slide trends, the most recent Legionella sample, and a clean-and-disinfect date still in date? Any frequency you cannot evidence is the entry to fix this week — and the recurring gap a digital logbook surfaces the day it falls due, rather than the day an inspector asks.

FAQ

Can the cooling tower go in the same logbook as the hot and cold water?

You can keep them under one cover, but hold the tower as a clearly separate section with its own asset ID, parameters and frequencies. The two systems are controlled in completely different ways, so mixing weekly temperature rounds with weekly biocide and dip-slide readings on one sheet makes both harder to audit. A digital logbook can hold both under one site while keeping the tower’s treatment log, sampling and cleaning trail distinct.

What’s the difference between the weekly check and the monthly inspection?

The weekly check is mostly the water chemistry and microbiology that keep the treatment proven: biocide, conductivity, inhibitor and dip-slide counts, plus a quick look for anything obviously wrong. The monthly inspection is the physical condition that lets the chemistry work: drift eliminators, the pond and pack, scale and biofilm, and the make-up and level controls. Your risk assessment can move either onto a tighter interval [2].

Do automated dosing and remote sensors replace the written log?

No — they change how the entry is made, not whether you need one. Automated dosing and continuous conductivity or biocide sensors are a strong control, but the record still has to show the values, the alarms, and what was done when a parameter went out of range. The advantage is that a connected system logs each reading with a timestamp you did not write by hand, so the weekly evidence builds itself and missed checks become visible rather than assumed.

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, “Testing and monitoring your water system for legionella”. https://www.hse.gov.uk/legionnaires/testing-monitoring-water-system.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