By Wu Peng, Senior Process Instrumentation Engineer · Last reviewed August 20, 2026
Cooling tower water level control keeps the cold water basin between two limits: high enough that the circulating pumps never draw air, low enough that water never runs out the overflow. Three methods do the job. A mechanical float valve, a conductivity probe panel driving a solenoid valve, or a continuous level sensor feeding a controller.
An evaporative tower loses water constantly, roughly 2.5 to 3 GPM per 100 tons of cooling at a 10 °F range. The makeup system therefore works every hour the tower runs. This guide compares the three control methods, lays out the setpoint stack, and sizes the makeup flow with the water balance math. It then explains why probes foul, and covers low-level pump protection, freeze protection and sensor selection.
Contents
- Control methods compared
- The setpoint stack
- Sizing the makeup
- Why probes foul
- Low level and pumps
- Freeze protection basics
- Choosing the sensor
- FAQ
Control methods compared
Each method trades cost against precision and remote visibility. The published specifications below come from manufacturer datasheets and manuals for widely installed systems.
| Method | How it works | Limits |
|---|---|---|
| Mechanical float valve | A float on an arm throttles the makeup line directly. No power, no panel. Capacity depends on supply pressure; published tables for a 1/2 in valve run about 11 GPM at 50 psi. | No alarms, no remote signal, level wanders with valve wear and wave action. Setting drifts and needs seasonal checks. |
| Conductivity probes + solenoid | Probes of different lengths sense water with a current-limited low-voltage signal; one published panel uses 17.5 V DC at 400 microamperes. Relays open a 110 V solenoid on the makeup line and drive alarms. Some panels add a time delay, 6 seconds in one published design, so waves do not short-cycle the valve. | Probes plate over and foam defeats them; needs a stilling chamber and periodic cleaning. Very pure makeup water may not conduct enough to sense. |
| Continuous level sensor | An ultrasonic, radar or submersible sensor reads the level as 4-20 mA into a controller or BMS. All setpoints live in software; one commercial system holds level within 1/8 in with wave compensation. | Highest first cost; needs loop power and a controller. Ultrasonic units need a stilling pipe in spray and wind. |
Note the two voltages in a probe system: the sensing signal at the probes is low-voltage DC by design, while the makeup solenoid actuates at line voltage. They are separate circuits.
Small packaged towers usually ship with a float valve. Probe panels dominate mid-size HVAC and light industrial towers. Continuous sensors take over where the basin feeds critical chillers or process loops. There, a trend and an early alarm are worth the cost of the instrument.
The setpoint stack
A full level control scheme stacks five to seven functions at different heights. One widely installed probe panel uses a reference probe plus separate probes for makeup on, makeup off, high alarm, high cutoff, low cutoff and low alarm. Fewer functions just means fewer probes; 3-probe to 6-probe panels are standard catalog items.
The geometry follows three rules. The fill band sits in the middle: the valve opens at the makeup-on level and closes at the makeup-off level. Spacing those two apart keeps the solenoid from chattering.
The low alarm and low cutoff sit below the fill band, above the pump protection limit. The high alarm sits below the overflow, and an independent high-level device is good practice so one stuck relay cannot flood the basin.
Where do the absolute heights come from? One float-valve manufacturer instructs setting the valve to shut off about 1/2 in below the overflow connection, with the pumps running. Nothing then spills on pump shutdown when water drains back to the basin. That drain-back is worth remembering with any control method: the running level and the standing level differ, and setpoints are set at the running level.
The same logic drives pump-start behavior in reverse. When the pumps start, the basin level drops as water fills the distribution piping. A fill band set too tight will call for makeup on every pump start and then overflow when the pumps stop. The deadband thinking is the same as for storage tanks, which we cover with worked numbers in the water storage tank level guide.
Sizing the makeup
Makeup has to replace three losses at once: evaporation, blowdown and drift.
M = E + B + D, with E = 0.00085 x recirculation GPM x Delta T (F) and B = E / (COC − 1)
1,500 GPM tower at 10 °F range: E = 0.00085 x 1,500 x 10 = 12.75 GPM. At 5 cycles of concentration, B = 12.75 / 4 = 3.19 GPM. Drift at 0.01 percent = 0.15 GPM. Makeup M = 16.1 GPM
Two evaporation factors circulate in the water treatment trade and they are both defensible. The simple rule is 0.001 x GPM x Delta T, which equals 1 percent of recirculation per 10 °F of range. The corrected factor 0.00085 credits roughly 15 percent of the heat rejection to direct warming of the air rather than evaporation. Use 0.001 when you want margin and 0.00085 when you want a realistic annual water number.
Cycles of concentration (COC) come from the conductivity ratio between tower water and makeup water. Most systems run best between 4 and 6 cycles; pushing from 3 to 6 cuts total makeup by about 20 percent, while going higher invites scale. Chloride, silica and TDS limits are water chemistry questions that set your achievable COC. They are separate from level control, but they size the blowdown your level system must keep up with.
Now close the loop on the valve itself: the makeup valve must deliver M at your actual supply pressure. Published float-valve capacity tables put a 1/2 in valve at about 11 GPM at 50 psi, below the 16.1 GPM demand in the example above. A 3/4 in valve delivers about 27 GPM at 30 psi. Sizing the valve from the water balance instead of from the pipe stub is the single most skipped step we see in tower makeup design.
Why probes foul
Conductivity probes have no moving parts, which is why they last, and a single sensing path, which is why they still need attention. The relay senses water by passing a tiny current from each probe through the water to the reference probe. Anything that blocks that path, or bridges it, fools the panel.
Three mechanisms cover almost every probe service call. Mineral plating and biofilm coat the probe tip until the panel never sees “wet,” so the fill valve stays open and the basin overflows. Foam in the stilling chamber conducts enough to fake a high level and defeat a low alarm at the same time. And very pure makeup water, RO or demineralized, may fall below the panel sensitivity; one published design senses down to a fixed 50,000 ohm-cm and no further.
The fixes are physical, not electronic. Install the probes in a stilling chamber so waves and foam stay out. Clean the tips on a schedule tied to your COC, since higher cycles mean faster plating, and specify a probe sheath where the catalog offers one. The current-limited low-voltage sensing signal already minimizes electrolytic plating; no probe panel avoids mineral scale.
Low level and pumps
The low cutoff probe is pump protection, not housekeeping. A condenser water pump below a tower basin often runs with only a few feet of static head at its suction. Lose part of that head and the suction goes sub-atmospheric. Air is pulled in through vents and fittings instead of being pushed out, bubbles expand inside the impeller, and the pump sounds like it is cavitating even when NPSH margin looks fine on paper.
Drop the level further and a vortex forms over the suction outlet, pulling air straight down from the surface. The classic symptoms are a rumbling pump, swinging discharge pressure and air in the condenser tubes. The cure is simple: keep the basin at its design operating level, and wire the low cutoff to stop the pumps before the vortex forms.
Basin heaters carry the same submergence logic. An electric basin heater burns out fast in air, so heater panels interlock the element with a dedicated low-water cutoff that proves the element is covered before it energizes. If your tower has a heater, one probe in the stack belongs to it.
Freeze protection basics
Level control and freeze protection meet in three places on a winterized tower. First, the makeup piping. The reliable pattern keeps all pressurized water lines in a heated space and feeds the tower through non-pressurized, self-draining lines, so there is nothing outside to trace or burst. Second, the basin heater interlock described above, driven by the cold water temperature and its own low-water cutoff.
Third, the level control itself should live where ice cannot reach it. One manufacturer mounts the electronic water level control inside the tower precisely so no heat tracing is required. If the site shuts down for winter, the alternative is simpler: drain the basin and exposed piping completely and let the level system rest.
Choosing the sensor
For the sensing element itself, match the technology to the job rather than the habit.
A cable float switch or a multi-point float level sensor replaces a probe stack where the makeup water is too pure for conductivity sensing. A submersible level transmitter sits on the basin floor and reads the level as 4-20 mA; it ignores foam and spray completely. An ultrasonic level transmitter measures without touching the water, which matters in aggressive treatment chemistry, but needs a stilling pipe; our stilling well guide gives the dimensions.
Whichever sensor you pick, mount it in calmed water away from the makeup discharge and the pump suction, and give alarms their own device where the consequence justifies it. The full range is on our float level sensors and level instruments pages.
Application example
HVAC, building chilled-water system (Singapore). The customer needed a continuous level reading on a chilled-water system at 35 °C, specified as 4-20 mA on 24 V DC with 0.5 percent FS accuracy. We quoted a submersible level transmitter with a 2 m range and 2 m cable, first in a no-display build. The customer then asked for a display version and delivery terms to be quoted alongside it for an item-by-item comparison.
FAQ
How are cooling towers controlled?
Four control loops run in parallel. Fan speed or staging holds the cold water temperature, makeup control holds the basin water level, blowdown control holds conductivity (cycles of concentration), and chemical dosing protects against scale and biology. This guide covers the level loop; the others have their own sensors and controllers.
How to check water level in cooling tower?
Look into the basin at a calm spot, ideally the stilling chamber, with the circulating pumps running. The level should sit in the normal band below the overflow; one manufacturer sets float shutoff about 1/2 in below the overflow connection. Check again with pumps off, since drain-back raises the standing level.
How does a water level controller work?
Conductivity types pass a current-limited low-voltage signal from probes of different lengths through the water to a reference probe. One published panel senses with 17.5 V DC at 400 microamperes. Water on a probe completes the circuit and a relay opens or closes the makeup solenoid. Fill starts at the longer probe and stops at the shorter one.
What is a VFD for a cooling tower?
A variable frequency drive on the tower fan motor. It varies fan speed to hold the cold water temperature setpoint, saving energy compared with cycling the fan on and off. A VFD is part of the temperature loop, not the level loop; basin level still needs its own makeup control and low-water protection.
Request a quote
Tell us the basin depth, the makeup water quality and the outputs your panel expects, and we will propose a float, submersible or ultrasonic solution for the tower. Reach our application engineers or use the form below.
Written and technically reviewed by Wu Peng and the Instranova engineering team.