Cooling towers are the most dangerous piece of equipment in most commercial buildings, and they are also the most neglected by building controls. A cooling tower that is poorly controlled wastes energy. A cooling tower with poor water treatment grows Legionella. A cooling tower with no monitoring does both, and nobody knows until the energy bill arrives or the environmental health officer does. The 2012 Legionella outbreak in Edinburgh — traced to a cooling tower on a council building — infected 92 people and killed 4. The tower had inadequate water treatment and insufficient monitoring. It was not an unusual installation. It was a typical one.
This article covers how BMS integration addresses both the energy performance and the Legionella compliance requirements of cooling tower systems, what the control sequences look like in practice, and why the monitoring requirements are not optional extras but legal obligations under L8 and HSG274.
A cooling tower rejects heat from a building's cooling system by evaporating water. Warm condenser water from the chiller enters the tower at the top, flows over fill media (structured packing that increases the surface area for evaporation), and is cooled by ambient air drawn through the tower by fans. The cooled water collects in the basin at the bottom and is pumped back to the chiller condenser. The process works because evaporation absorbs latent heat — roughly 2,260 kJ per kilogram of water evaporated. This is enormously efficient: a cooling tower can reject megawatts of heat using only the fan motor power and the latent heat of evaporation.
The controls challenge is threefold. First, controlling the fans to deliver the right amount of cooling — too much and you overcool the condenser water, wasting fan energy and potentially reducing chiller efficiency; too little and the condenser water temperature rises, increasing chiller energy consumption and potentially triggering a high-pressure trip. Second, managing the water treatment chemistry to prevent scale, corrosion, and biological growth. Third, monitoring the system continuously to demonstrate Legionella compliance.
On older cooling towers with fixed-speed fans, the BMS controls fan staging based on the condenser water return temperature (the temperature of water leaving the tower basin and returning to the chiller). A typical two-cell tower has four fans (two per cell). The staging sequence is: below 25°C condenser return, all fans off (natural draught provides sufficient cooling); at 25°C, Stage 1 fans start (one fan per cell); at 27°C, Stage 2 fans start (two fans per cell); at 30°C, all fans running. The sequence reverses as the condenser water temperature drops. Hysteresis of 1-2°C on each stage prevents rapid cycling.
The problem with fixed-speed staging is that each stage change is a step change in cooling capacity. The condenser water temperature oscillates between stages — too cold after a stage starts, too warm before the next stage starts. This oscillation translates directly into varying chiller efficiency. The chiller operates most efficiently at a stable, optimal condenser water temperature — typically as low as the tower can deliver without excessive fan energy.
Variable speed drives on the cooling tower fans eliminate the step changes. The BMS modulates the fan speed continuously using a PID loop to maintain a target condenser water return temperature. The control signal is a 0-10V or 4-20mA output from the BMS controller to the VSD. As the outside wet-bulb temperature drops (meaning the air can absorb more moisture and provide more cooling), the fan speed reduces. On a mild spring day with a wet-bulb temperature of 10°C, the fans might run at 30-40% speed to maintain 22°C condenser water. On a hot summer afternoon with a wet-bulb of 20°C, the fans run at 80-100%.
The energy savings from VSD fan control follow the same cube law as pump VSDs — reducing fan speed by 50% reduces fan power by approximately 87%. On a cooling tower with two 15kW fan motors running 2,500 hours per year, VSD control typically saves £4,000 to £6,000 annually compared with fixed-speed staging. The VSD cost (£2,500 to £4,000 per motor) pays back within a year. For more on how VSD control delivers energy savings across building services, see our article on reducing commercial building energy costs with BMS.
The approach temperature is the difference between the condenser water leaving the tower and the ambient wet-bulb temperature. A tower with a 5°C approach at design conditions delivers condenser water at wet-bulb + 5°C. The closer the approach, the more fan energy is needed — diminishing returns set in below about 3°C approach, where the fan energy required to force the last degree of cooling out of the tower exceeds the chiller energy saved by having slightly cooler condenser water.
The BMS can optimise this trade-off dynamically. Rather than maintaining a fixed condenser water setpoint (which may require maximum fan speed on a hot day but wastes fan energy on a cool day by cooling the condenser water far below what the chiller needs), the BMS calculates the optimal condenser water temperature that minimises the combined energy of the chiller compressor and the tower fans. This is a more sophisticated control strategy that requires the BMS to know both the chiller's part-load efficiency curve and the tower's performance characteristics — but on a large installation, the savings from this whole-system optimisation are 10-15% of total cooling plant energy beyond what simple condenser water reset delivers.
The condenser water circuit connects the chiller condenser to the cooling tower. The BMS controls the condenser water pumps (start/stop, VSD speed control on variable-flow systems), the tower isolation valves (motorised butterfly valves that isolate individual tower cells when they are not needed), and any bypass or three-way valves that allow condenser water to bypass the tower during cold weather (to prevent the condenser water from becoming too cold and causing chiller low-pressure faults).
On systems with multiple chillers and multiple tower cells, the BMS manages the mapping between chillers and towers. A common arrangement is dedicated towers — each chiller has its assigned tower cell(s) — but a more flexible arrangement uses a common condenser water header with isolation valves, allowing any tower cell to serve any chiller. The BMS sequences tower cells to match the number of running chillers, optimising the tower capacity to the cooling load.
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Cooling tower water treatment is not optional. The tower operates by evaporating pure water, concentrating the dissolved minerals in the remaining water. Without treatment, scale deposits on the fill media and heat exchange surfaces, reducing the tower's cooling efficiency. Corrosion attacks the tower basin, pipework, and chiller condenser tubes. And biological growth — algae, biofilm, and Legionella bacteria — flourishes in the warm, aerated, nutrient-rich environment of the tower.
The water treatment system typically includes: a conductivity controller that monitors dissolved solids concentration and bleeds (drains) water from the system when conductivity exceeds a setpoint, replacing it with fresh makeup water; a pH controller that maintains the water in the correct pH range for the chosen treatment programme (typically pH 7.0 to 8.5); a biocide dosing system that periodically injects oxidising biocide (sodium hypochlorite or bromine) and non-oxidising biocide to control biological growth; and an inhibitor dosing system that injects corrosion and scale inhibitors.
The BMS integrates with these systems at multiple levels:
The conductivity sensor (typically an inductive or contacting conductivity probe installed in the tower basin or return pipework) provides a 4-20mA signal to the BMS. The BMS trends the conductivity continuously. When conductivity exceeds the treatment programme's setpoint — typically 1,500 to 3,000 microsiemens/cm depending on the makeup water quality and treatment regime — the BMS opens the bleed valve (a motorised solenoid valve on the tower drain) to discharge concentrated water and triggers the makeup water supply to replace it. The BMS logs the volume of water bled and made up, providing data for water consumption monitoring and treatment chemical usage calculation.
Conductivity that consistently runs above setpoint despite bleed indicates either the bleed valve is not opening (solenoid fault, valve blockage), the makeup water supply is interrupted, or the tower is operating at higher load than the water treatment system was designed for. The BMS alarms on sustained high conductivity — this is not just an efficiency issue but a water quality issue with direct Legionella implications.
pH monitoring uses a glass electrode pH sensor, again providing a 4-20mA signal to the BMS. The BMS trends pH and alarms on out-of-range readings. Low pH (acidic conditions) accelerates corrosion. High pH promotes scale formation. The BMS does not typically control pH directly (that function belongs to the water treatment controller), but it provides an independent monitoring point and an alarm path that is separate from the water treatment controller's own alarms — belt and braces.
The BMS monitors the biocide dosing system: dosing pump run status, chemical tank level, and — on systems with an ORP (oxidation-reduction potential) sensor — the oxidant level in the tower water. ORP provides a real-time indication of whether biocide dosing is maintaining effective disinfection. An ORP reading that is consistently low despite the dosing pump running indicates either the chemical is exhausted, the dosing pump is not delivering (diaphragm failure, blocked suction line), or the biological load in the system is overwhelming the dosing rate. The BMS alarms on low ORP — this is directly relevant to Legionella risk.
The Approved Code of Practice L8 (Legionnaires' disease: The control of Legionella bacteria in water systems) and its technical guidance HSG274 Part 1 (The control of Legionella bacteria in evaporative cooling systems) set out the legal requirements for managing Legionella risk in cooling towers. The duty holder — the building owner or their appointed responsible person — must demonstrate that the system is under effective control. The BMS is the primary tool for doing this.
HSG274 requires that cooling tower water temperatures are monitored. Legionella proliferates between 20°C and 45°C, with optimum growth at 35-40°C. Cooling tower water sits squarely in this danger zone during normal operation. The BMS must continuously log the condenser water flow temperature (leaving the chiller — typically 30-35°C), the condenser water return temperature (leaving the tower basin — typically 25-30°C), and the tower basin temperature. These temperatures must be available for review by the water treatment consultant, the responsible person, and — in the event of an investigation — the environmental health officer.
The BMS log of biocide dosing events (pump run times, chemical usage), conductivity readings, pH readings, and bleed valve operation provides a continuous record that complements the monthly water sampling and dip-slide testing carried out by the water treatment contractor. The combination of continuous BMS monitoring and periodic manual sampling provides the evidence base that the system is under control. A gap in the BMS data — a period where the conductivity was not logged because the sensor was offline, or where the dosing pump status was not recorded — is a gap in the compliance record.
L8 requires that any departure from normal operating parameters is identified and acted upon promptly. The BMS alarm system is the mechanism for this. Alarms that should trigger an investigation and corrective action include: high conductivity (exceeding treatment setpoint for more than 4 hours), low ORP (indicating inadequate biocide residual), high basin temperature (exceeding 40°C — indicating the tower is not rejecting heat effectively), biocide chemical tank low level (dosing will cease when the tank empties), and any water treatment equipment fault (dosing pump failure, conductivity sensor fault).
These alarms must reach a person who can act on them — not sit on a BMS screen in an unattended plant room. Alarm routing to the facilities management team's phones, with escalation if not acknowledged, is essential. For guidance on how BMS alarm management works in practice across commercial buildings, see our article on preventive maintenance BMS strategy.
The cooling tower basin is the reservoir of water at the base of the tower. The BMS monitors the basin level using a float switch, capacitance probe, or ultrasonic level sensor. Low basin level triggers the makeup water valve to open, admitting fresh water to replace evaporation and bleed losses. Very low basin level (below the pump suction) triggers a pump shutdown to prevent dry running — running the condenser water pump with the basin empty will damage the pump and cavitate the piping.
High basin level indicates the makeup valve has stuck open or the overflow is blocked. The BMS alarms on both high and low basin level. Persistent makeup water flow (the BMS trends the makeup valve position or counts the fill cycles) can also indicate a leak in the condenser water circuit — the system is losing water faster than evaporation and bleed alone would account for.
Drift eliminators are the baffles at the top of the cooling tower that capture water droplets entrained in the air leaving the tower. Their function is to prevent these droplets — which contain whatever bacteria and chemicals are in the tower water — from being discharged into the atmosphere. A damaged or missing drift eliminator increases the risk of Legionella-contaminated aerosol being dispersed from the tower. HSG274 requires that drift eliminators are inspected regularly and maintained in good condition.
The BMS cannot directly monitor drift eliminator condition (it is a mechanical component), but it can monitor a proxy: the rate of water loss. A tower that is losing water significantly faster than the expected evaporation and bleed rates — after accounting for bleed volume measured by the BMS — may have excessive drift caused by damaged eliminators. The BMS trending data provides the baseline against which abnormal water consumption can be identified.
In the UK, cooling towers must be managed through winter. A tower that is not needed during the heating season should be drained, cleaned, and isolated — the HSG274 guidance is clear that stagnant water left in an idle tower over winter is a Legionella risk that must be managed. The BMS controls the winterisation sequence: draining the tower basin and condenser water circuit, closing isolation valves, and — on towers that remain in service year-round — managing frost protection.
Frost protection for operational towers includes: basin heaters that prevent the water in the basin from freezing (typically electric immersion heaters controlled by the BMS based on basin temperature, energising at 5°C and de-energising at 10°C), fan control logic that prevents the fans from running when the ambient temperature is below a threshold (typically 2°C, to prevent ice forming on the fill media), and condenser water flow maintenance (keeping the condenser water pumps running at minimum speed to prevent freezing in exposed pipework, even when the chillers are not operating).
The BMS must also manage the spring recommissioning sequence: refilling the basin, flushing the condenser water circuit, dosing the system to the treatment programme's target chemistry, and — critically — carrying out a pre-season clean and disinfection before the tower returns to service. The water treatment contractor performs the physical clean; the BMS confirms that post-clean water quality (conductivity, pH, ORP) is within acceptable parameters before the tower is returned to automatic operation.
Full BMS integration for a cooling tower installation — covering fan VSD control, condenser water circuit control, water treatment monitoring (conductivity, pH, ORP, biocide tank level), basin level monitoring, and temperature monitoring — typically costs £8,000 to £15,000 per tower cell. This includes BMS controller hardware, field sensors, wiring, programming, graphics, and commissioning. On a two-cell tower installation, the total BMS scope is £16,000 to £30,000.
VSD retrofit for tower fans (if not already fitted) adds £2,500 to £5,000 per fan motor. Water treatment instrumentation (conductivity, pH, and ORP sensors with their transmitters) costs £2,000 to £4,000 if not already in place. The total investment for a comprehensive monitoring and control installation on a medium-sized commercial cooling tower system is £20,000 to £40,000.
Against this, the costs of inadequate monitoring are: energy waste from poor condenser water control (£3,000 to £10,000 per year on a medium installation), water treatment failures leading to chiller condenser fouling (condenser tube cleaning: £5,000 to £15,000, or condenser replacement: £30,000+), and — the risk that dwarfs all others — a Legionella incident. Prosecution under the Health and Safety at Work Act for a Legionella failure carries unlimited fines and the possibility of imprisonment for the responsible person. The reputational damage to the building owner or facilities management company is incalculable. The BMS investment is not a nice-to-have. It is the evidence that you are managing the risk.
If your cooling tower has no BMS integration — fans on local control, water treatment on a standalone controller, no continuous data logging — you are operating blind on both energy and compliance. The first step is a survey to establish what monitoring and control is currently in place and what gaps exist. The second step is a specification for the BMS scope, aligned with the water treatment consultant's programme and the L8 risk assessment. The third step is installation and commissioning, ideally during the cooling season when the system is operating and the control sequences can be commissioned under real load conditions.
Alpha Controls integrates cooling tower systems with BMS platforms across London and the South East, including Trend, Distech, Schneider, and Siemens. We work alongside water treatment consultants to ensure that the BMS monitoring meets L8 and HSG274 requirements. Request a survey or call 01474 552200 to discuss your cooling tower controls.
Specialist BMS installation, commissioning, and maintenance across London and the South East. SafeContractor Approved, BCIA Member.
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