The chiller trips on high condenser pressure every afternoon in peak summer. The engineer checks the chiller — pressures, compressor, refrigerant. Everything looks fine on paper. Then someone walks up to the roof and looks at the cooling tower. The fill media is partially blocked with scale, one fan motor has seized, and the water treatment dosing pump stopped working three weeks ago. The chiller was never the problem.
Cooling towers are the most maintenance-neglected piece of plant in most commercial HVAC systems. They sit on the roof, out of sight, and tend to get the minimum attention until something fails downstream — usually the chiller — and the investigation leads back to a tower that hasn’t been properly maintained in months.
A structured cooling tower maintenance checklist prevents that pattern. This guide covers the complete PPM schedule — from weekly rounds through to seasonal shutdown and recommissioning — with the water treatment requirements, Legionella control obligations, and fault diagnosis that keeps cooling towers operating efficiently and safely all year round.
Why Cooling Tower Maintenance Matters More Than Most FM Teams Realise
A poorly maintained cooling tower does three things: it degrades chiller performance by raising condenser water temperature, it creates a Legionella risk by providing warm, nutrient-rich water for bacterial growth, and it shortens equipment life through accelerated corrosion, scale, and biological fouling. All three consequences are expensive. All three are preventable.
The connection between cooling tower condition and chiller performance is direct. If your condenser water return temperature rises from 29°C to 35°C because the tower isn’t cooling effectively, your chiller’s coefficient of performance drops significantly — it works harder, consumes more energy, and runs at higher head pressure. Every degree of condenser water temperature above design adds approximately 2–3% to chiller energy consumption. Over a summer season, a poorly maintained tower costs real money in electricity bills before it causes a single fault alarm.
The Legionella obligation is non-negotiable. Cooling towers are the highest-risk water system for Legionella in most commercial buildings — they generate aerosols, operate at warm temperatures, and provide ideal conditions for bacterial growth. In most jurisdictions, cooling towers must have a documented risk assessment, a written control scheme, regular microbiological sampling, and continuous water treatment. This is statutory compliance, not optional good practice.
| 📌 For the Legionella risk assessment process that underpins your cooling tower water treatment programme, see the Legionella Risk Assessment guide. |
Cooling Tower Maintenance Checklist by Frequency
The most effective cooling tower maintenance programmes are frequency-banded. Here is the complete schedule your team needs:
Weekly Cooling Tower Checks
Weekly checks are visual and operational — they take 20–30 minutes per tower and require no specialist equipment. The purpose is to catch developing problems before they become faults.
| ✓ | Task | What to Check / Accept Standard |
| ☐ | Inspect basin water level | Should be at design operating level. Low level = make-up valve fault or excessive drift loss. High level = overflow valve fault. Log actual level. |
| ☐ | Check all fans are running | Walk to tower and confirm all fan motors are running and rotating in the correct direction. A stopped fan reduces cooling capacity immediately. |
| ☐ | Check make-up water valve operation | Confirm float valve or solenoid make-up valve is functioning. A stuck-open make-up valve wastes water and dilutes treatment chemicals. A stuck-closed valve drops basin level. |
| ☐ | Check water distribution — spray heads or nozzles | Water should be evenly distributed over the fill. Blocked spray heads cause dry spots in the fill media — accelerating biological growth in those areas. |
| ☐ | Check overflow and drain — clear and unobstructed | Blocked overflow causes basin flooding. Blocked drain prevents maintenance drainage. Clear any debris weekly. |
| ☐ | Check water treatment dosing pumps are operating | Confirm biocide and scale inhibitor dosing pumps are running and chemical containers have sufficient product. No dosing = Legionella and scale risk within days. |
| ☐ | Record bleed-off / blowdown operation | Bleed-off should be operating on a conductivity controller. Check conductivity reading — target typically 1500–2500 µS/cm depending on make-up water quality and treatment programme. |
Monthly Cooling Tower Maintenance Tasks
Monthly tasks go deeper into the tower condition and water quality. Allow 1–2 hours per tower.
| ✓ | Task | What to Check / Accept Standard |
| ☐ | Inspect fill media condition | Fill should be clean and structurally intact. Scale build-up reduces heat transfer. Biological growth (slime, algae) indicates inadequate biocide dosing. Physically damaged fill reduces airflow and contact time. |
| ☐ | Inspect and clean basin | Remove debris, leaves, and sediment from basin. A dirty basin provides nutrients for Legionella and biological growth even with correct chemical dosing. |
| ☐ | Check drift eliminators are fitted and undamaged | Drift eliminators prevent water droplets leaving the tower. Missing or damaged eliminators increase water loss and — critically — increase Legionella aerosol generation risk. |
| ☐ | Measure and record conductivity, pH, and chemical residuals | pH target: 7.0–8.5. Conductivity: per water treatment programme. Biocide residual: per chemical supplier specification. Log all readings — trends matter more than single readings. |
| ☐ | Take microbiological water sample | Submit to UKAS-accredited laboratory for Legionella culture and HPC (Heterotrophic Plate Count). Results <100 cfu/ml for Legionella is satisfactory. Above 1000 cfu/ml requires immediate corrective action. |
| ☐ | Check fan motor current draw | Clamp meter on all 3 phases against nameplate FLA. More than 10% above FLA: investigate. Check bearing condition by listening and by hand (for heat) with motor running. |
| ☐ | Inspect basin and structure for corrosion | Check steel components for surface corrosion. Check concrete or GRP basin for cracks. Any structural corrosion: log and arrange repair. Untreated corrosion leads to failure within 2–3 seasons. |
Quarterly Cooling Tower Service
Quarterly service typically coincides with the deeper water treatment review and mechanical inspection that monthly checks cannot cover fully.
| ✓ | Task | What to Check / Accept Standard |
| ☐ | Clean fill media — chemical or high-pressure wash | Annual chemical clean minimum. If microbiological results are poor: quarterly. Use approved biocide solution, contact time per supplier spec, rinse thoroughly before return to service. |
| ☐ | Inspect and clean spray nozzles / distribution headers | Remove and descale blocked nozzles. Confirm even distribution pattern after refitting. Uneven distribution = hot spots in fill = Legionella risk. |
| ☐ | Check gearbox oil level (gear-drive towers) | Gearbox oil level should be at the operating mark on the sight glass. Oil change per manufacturer interval — typically annually or 8,000 hours. Contaminated or low oil = gearbox failure. |
| ☐ | Inspect fan blades and blade pitch | Check fan blades for erosion, cracking, or imbalance. Variable-pitch blades: confirm pitch setting is consistent across all blades. Blade imbalance causes vibration and bearing failure. |
| ☐ | Check all isolation valves in cooling tower pipework | Operate and return to open. Seized valves prevent controlled maintenance isolation and emergency shutdown. Log any valves that are stiff or show signs of internal damage. |
| ☐ | Review water treatment programme with chemical supplier | Quarterly review of conductivity trends, pH control, chemical consumption, and microbiological results. Adjust dosing rates if results show drift. Don’t wait for a positive Legionella result to review. |
Seasonal Shutdown and Recommissioning
In climates where the cooling tower operates seasonally, shutdown and recommissioning are the most critical maintenance events of the year. A tower that is shut down incorrectly retains stagnant water, develops biological growth over the off-season, and starts the next cooling season with contaminated water already in the system.
| ✓ | Task | What to Check / Accept Standard |
| ☐ | Pre-shutdown chemical clean and disinfection | Shock dose with biocide at manufacturer’s specified concentration. Circulate for minimum 4 hours. This kills biological growth before shutdown, not after. |
| ☐ | Drain basin and pipework completely | Never leave standing water in a cooling tower basin during shutdown. Stagnant water at ambient temperature is perfect for biological growth over winter. |
| ☐ | Physical clean of basin, fill, and structure | After draining: manually remove all debris, scale deposits, and biological growth. High-pressure wash basin walls and floor. Inspect fill for damage while accessible. |
| ☐ | Commission water treatment before startup | Before filling for the new season: confirm chemical dosing system is operational, chemical stocks are available, and conductivity/blowdown controller is calibrated. |
| ☐ | Pre-commission microbiological sampling | Before connecting cooling tower to the chiller condenser water system: take a water sample after recommissioning and confirm results are satisfactory. Don’t start operating the system with contaminated water. |
| ☐ | Witness fan and motor operation before connecting to chiller | Run tower with water circulating for minimum 30 minutes. Confirm: all fans running, correct rotation, no abnormal noise, even water distribution, make-up valve functioning, bleed-off operating. |
Water Treatment: The Part Most FM Teams Get Wrong
Cooling tower water treatment is not a case of adding chemicals and forgetting. It’s a continuous control programme that needs to be monitored, adjusted, and reviewed regularly. The three things water treatment must control are: scale (calcium carbonate and other mineral deposits on heat transfer surfaces), corrosion (of steel, copper, and concrete components), and biological growth (including Legionella).
The most common failure is treating water treatment as a contract item rather than a managed programme. An FM team that signs a water treatment contract and then doesn’t check what the contractor is actually doing — chemical consumption, dosing rates, sampling results — is not managing the risk. They’re assuming it’s being managed.
These are the numbers you should know and track monthly:
• Conductivity — target typically 1,500–2,500 µS/cm. Rising conductivity = bleed-off not working. Falling conductivity = excessive bleed-off diluting chemicals
• pH — target 7.0–8.5. Outside this range: scale forms above 8.5, corrosion accelerates below 7.0
• Biocide residual — confirm against your chemical supplier’s target. Zero residual = no protection
• Legionella and HPC counts — monthly minimum, more frequently if results are elevated
• Chemical consumption — track actual consumption vs expected. Sudden drop = dosing pump failure. Sudden increase = system leak
Cooling Tower Fault Diagnosis Quick Reference
When a fault is reported or performance deteriorates, use this table as a starting point before escalating:
| Symptom / Problem | Diagnosis & Action |
| High condenser water return temperature | Check all fans running and rotating correctly. Check fill for blockage or scale. Check basin level and distribution. If tower is operating correctly: condenser water flow rate may be below design — check pump and strainer. |
| Chiller high head pressure alarm | In summer: usually cooling tower issue. Check condenser water temperature vs design (target: 29–32°C supply). If above 35°C: tower is not rejecting heat. Investigate tower before touching chiller. |
| Excessive water consumption / overflow | Check bleed-off (conductivity controller) settings — if set too low, it bleeds off too much. Check make-up valve — if stuck open, it continuously overfills. Check for basin overflow. |
| Fan motor tripped / not running | Check DB for tripped MCB. Check motor overload relay. If tripped: check motor current before resetting. Check fan blade for blockage or damage that increased load. Listen for bearing noise before restart. |
| White scale deposits on fill and basin | Scale indicates inadequate scale inhibitor dosing or pH too high. Review water treatment programme with chemical supplier. Scale on fill must be chemically removed — physical scrubbing damages fill media. |
| Biological growth / slime in basin | Inadequate biocide dosing or wrong biocide type. Shock dose immediately per supplier guidance. Take microbiological sample. Review dosing regime — biological growth means current programme is insufficient. |
| Positive Legionella result from sampling | Results 100–1000 cfu/ml: increase monitoring frequency, review control measures, investigate temperature and treatment. Above 1000 cfu/ml: immediate action — consider restricting use, shock dose, notify duty holder, increase sampling. |
| Excessive vibration from fan | Check fan blade condition and pitch consistency. Check gearbox oil level (gear-drive units). Check motor bearing condition. Fan imbalance causes progressive bearing damage — don’t operate a vibrating tower without investigation. |
Legionella Compliance for Cooling Towers: What You Must Document
Cooling towers have the most demanding Legionella compliance requirements of any building system — because they are the most common source of Legionnaires’ disease outbreaks. Under HSE ACoP L8 (UK), ASHRAE Guideline 12 (North America), and equivalent international standards, cooling tower operators must maintain documented evidence of their control programme.
At minimum, your cooling tower compliance records must include:
1. Current Legionella risk assessment — reviewed annually or after any significant system change
2. Written control scheme — describing the water treatment programme, monitoring frequencies, and responsible persons
3. Monthly microbiological sampling results — with laboratory name, sample date, outlet location, and result
4. Water treatment records — chemical dosing rates, conductivity, pH, biocide residual — logged every visit
5. Cleaning and disinfection records — dates, method, chemical concentrations, contractor name
6. Corrective action records — any elevated results, what was done, and when results returned to acceptable
Keep all records for a minimum of 5 years. In the event of a Legionella outbreak linked to your building, these records are the evidence that determines whether the organisation defended its duty of care.
| 📌 For the full building PPM programme that incorporates cooling tower maintenance alongside other systems, see the How to Build a PPM Programme guide. |
Frequently Asked Questions
How often should a cooling tower be cleaned and disinfected?
Under HSE ACoP L8 and most equivalent standards, cooling towers should be cleaned and disinfected at least twice per year — once at startup after any extended shutdown, and once during the operational season. High-risk situations (elevated Legionella results, following system repairs, after a period of poor water quality) require additional disinfection events. The pre-season disinfection before reconnecting to the chiller system is the most critical — never start the cooling season with a tower that hasn’t been cleaned since the previous shutdown.
What are the acceptable Legionella levels in cooling tower water?
Under guidance aligned with HSE ACoP L8: below 100 cfu/litre is satisfactory — maintain routine monitoring. 100–1,000 cfu/litre requires investigation: review water treatment, increase monitoring frequency, check temperature and pH, and assess whether control measures are working. Above 1,000 cfu/litre requires immediate action: shock dosing, possible restriction of tower use, increased sampling, and notification of the duty holder. Two consecutive results above 10,000 cfu/litre should trigger tower shutdown and full investigation before return to service.
Why does my cooling tower keep producing scale even with water treatment?
Scale formation in a cooling tower despite water treatment is almost always caused by one of three things: pH drifting above 8.5 (accelerates calcium carbonate precipitation), scale inhibitor dosing rate that is too low for the actual make-up water hardness, or conductivity running too high (concentration of dissolved solids above the threshold where scale precipitates). Ask your water treatment contractor to review the dosing rate against an up-to-date make-up water analysis — if the analysis is more than 12 months old, the make-up water quality may have changed.
Can I operate a cooling tower without a water treatment programme?
No — operating a cooling tower without a water treatment programme is a regulatory violation in most jurisdictions, not just poor practice. In the UK, HSE ACoP L8 requires a written control scheme including water treatment for all cooling towers. Beyond the legal obligation, a cooling tower without water treatment will scale up within one season, corrode within two, and develop biological growth including potential Legionella within weeks. The cost of a water treatment programme is small compared to the cost of tube replacement, basin repairs, or a Legionella outbreak.
How do I know if my cooling tower fill media needs replacing?
Cooling tower fill has a design life of 10–15 years under normal conditions with proper water treatment. Signs that fill needs replacing: physical damage — collapsed, cracked, or severely deformed sections; scale build-up that cannot be removed by chemical cleaning without further damaging the fill structure; significant biological colonisation that persists despite correct biocide dosing; or measured thermal performance below design (rising condenser water temperatures despite correct water flow and fan operation). Replacing sections of severely degraded fill is far cheaper than operating a chiller at reduced efficiency and elevated head pressure for another three seasons.
Conclusion
Cooling tower maintenance is one of the highest-impact items on any commercial HVAC maintenance programme. A well-maintained tower keeps condenser water temperatures at design, keeps chiller energy consumption under control, and keeps your Legionella compliance documented and defensible. A neglected one quietly costs you in energy bills and compliance risk until the day the chiller trips and someone goes up to the roof.
For the chiller troubleshooting that directly connects to cooling tower performance, see the Chiller Troubleshooting Guide . Use the PPM Planner on mepmasterguide.com to schedule your weekly, monthly, and quarterly cooling tower tasks alongside the rest of your HVAC maintenance programme.
