Quick Answer
Cycles of concentration (CoC) measure how many times minerals in cooling tower water have been concentrated compared to the fresh makeup water — at 4 cycles, tower water holds 4 times the mineral content of the supply. The number matters because it controls how much water you send down the drain as blowdown: at 3 cycles roughly 50% of makeup water becomes blowdown, while at 6 cycles only about 20% does. Raising cycles from 3 to 6 can cut total water use by roughly 20%, lowering both water and sewer bills. Most commercial towers run best between 5 and 8 cycles, with the exact target set by local water chemistry. Run too low and you waste water; run too high and you risk scale and corrosion.
If you manage a building with a cooling tower, you’ll eventually hear someone mention “cycles of concentration.” It sounds technical, but the concept is straightforward — and understanding it can save your building significant money on both water bills and equipment maintenance.
The Plain-English Explanation
Cycles of concentration — often shortened to CoC or just “cycles” — measure how many times the minerals in your cooling tower water have been concentrated compared to the fresh makeup water coming in. Think of it like making soup: if you keep simmering and the water evaporates, the broth gets more and more concentrated. Your cooling tower does the same thing. Water evaporates to cool your building, but the dissolved minerals stay behind, making the remaining water increasingly mineral-rich.
If your tower operates at 4 cycles of concentration, the water circulating through the tower has 4 times the mineral content of your fresh water supply. At 6 cycles, it’s 6 times as concentrated.
Why the Number Matters
The cycles of concentration number directly determines how much water your cooling tower wastes through blowdown — the intentional draining of concentrated water to prevent mineral buildup. Here’s the relationship: at 3 cycles, about 50 percent of your makeup water goes to blowdown (and the sewer). At 6 cycles, only about 20 percent goes to blowdown. At 10 cycles, it drops to about 11 percent.
The math is simple. Going from 3 cycles to 6 cycles can reduce your total water consumption by approximately 20 percent — and since that saved water also means lower sewer charges, the financial impact is substantial. For a 500-ton cooling tower using 100,000 gallons per month, a 20 percent reduction saves 20,000 gallons monthly. At a combined water and sewer rate of $15 per thousand gallons, that’s $300 per month or $3,600 per year from this one optimization alone.
The U.S. Department of Energy recommends operating cooling towers at a minimum of 5 cycles, with many systems capable of safely running at 7 or higher depending on local water quality.
Too Low: Wasting Water
Towers running at fewer than 3 cycles are flushing an excessive amount of water down the drain. This happens when blowdown valves are set too aggressively, when conductivity controllers malfunction, or when chemical treatment programs aren’t properly calibrated. Low cycles mean you’re buying more water, paying more sewer charges, and using more treatment chemicals — all unnecessarily.
Too High: Risking Equipment Damage
Running at too many cycles isn’t free either. As mineral concentration increases, the risk of scale formation, corrosion, and biological growth rises. Scale deposits on heat transfer surfaces act as insulation, forcing your chiller to work harder and consuming more energy. Corrosion can damage expensive fill media, basins, and piping. The optimal range balances water savings against these equipment risks.
Most commercial towers perform best between 5 and 8 cycles, though the right target for your system depends on your local water chemistry. Hard water areas with high calcium and silica may need to stay closer to 5, while facilities with softer water can safely push to 8 or higher. A water treatment professional can test your supply water and recommend the optimal target.
How to Measure Your Own Cycles
You don’t need a laboratory to get a working number. Cycles of concentration is simply the ratio of a dissolved mineral’s concentration in the tower water to its concentration in the makeup water. In practice, most operators use conductivity as the proxy: measure the conductivity of the tower basin water, measure the conductivity of the fresh makeup water, and divide. A basin reading of 2,400 microsiemens against makeup water at 400 microsiemens means you’re running at 6 cycles.
Chloride testing is a useful cross-check because chlorides stay dissolved even when calcium is dropping out as scale. If your conductivity ratio says 6 cycles but your chloride ratio says 8, minerals are precipitating somewhere in the system — a warning sign worth raising with your water treatment vendor. Handheld conductivity meters cost $100 to $300, and taking a weekly reading takes minutes.
The Water Balance Behind the Number
Every gallon entering a cooling tower leaves one of three ways: evaporation (the cooling work), blowdown (the intentional drain to control minerals), or drift (fine mist, usually under 0.1% in modern towers). The relationship between blowdown and cycles follows a simple formula: blowdown equals evaporation divided by (cycles minus 1). A tower evaporating 60,000 gallons per month at 4 cycles needs 20,000 gallons of blowdown; the same tower at 7 cycles needs only 10,000 gallons.
That formula is also the foundation of sewer credit documentation. Utilities want to see that your claimed evaporation is consistent with your metered makeup and blowdown volumes, and cycles of concentration is the number that ties the whole water balance together. Keeping a monthly log of makeup volume, blowdown volume, and conductivity readings gives you exactly the paper trail most credit programs ask for.
How This Connects to Sewer Credits
Here’s why cycles of concentration matter for your sewer bill: higher cycles mean more evaporation and less blowdown. More evaporation means a larger percentage of your water qualifies for sewer credits, since evaporated water never reaches the sewer. So optimizing your cycles doesn’t just save water — it also increases your sewer credit potential. For a detailed look at how cooling tower water loss breaks down between evaporation, blowdown, and drift, see our article on cooling tower water loss.
According to the EPA’s WaterSense program, optimizing cooling tower operations is one of the most impactful water conservation measures a commercial building can take.
Curious what continuous visibility looks like in practice? See how RPM’s 24/7 cooling tower monitoring works.
Ready to Find Out What You Could Save?
RPM Water Equity Solutions helps commercial facilities recover money lost to sewer billing assumptions. If your building has cooling towers, you may be paying sewer charges on water that never reaches the sewer system.
Request your free assessment today and find out how much you could recover.
Check Your Cycles
If you don’t know your cooling tower’s current cycles of concentration, that’s a problem worth fixing today. Your water treatment vendor can test it, or a simple conductivity meter comparison between makeup water and tower water gives you a rough answer. If your cycles are below 5, there’s almost certainly room to optimize — and every cycle you gain means less water wasted and more money saved.
