Slow a cooling tower fan to half speed, and it draws roughly one-eighth the power.
Not half. One-eighth. Fan power scales with the cube of speed, which is the single most useful piece of physics in the mechanical room and the one most cooling tower system controls are set up to ignore. Most towers in older commercial buildings still run one speed: on. The fan either roars or sits there.
That’s mistake number one, and there are nine more behind it. None of them will trip an alarm. All of them show up in the July electric bill.
1. Running The Fan Wide Open Or Off, With Nothing In Between
A single-speed fan cycling on and off gives you exactly two operating states, and one of them costs full power to deliver more cooling than you asked for. Variable speed changes the math completely, because of that cube relationship. Two-thirds speed is about 30 percent of the power.
The fix is a VFD and a control strategy that modulates against condenser water temperature instead of bang-bang cycling. It’s one of the cheaper interventions in a chiller plant and one of the most commonly skipped.
2. Holding 85 Degrees In October
Chillers get sized around 85-degree entering condenser water because that’s what design day demands. Then the setpoint never moves again.
For most of the cooling season, your tower could deliver water considerably colder than 85 with the same fan energy, because wet-bulb temperature dropped. Colder condenser water means less lift for the compressor and less energy per ton. A fixed setpoint throws that away for eight months to protect against about twelve hours of design conditions.
The real strategy is condenser water reset tracking wet bulb, with a floor set by the chiller manufacturer’s minimum. There’s a balance point where added fan energy cancels the chiller savings, and finding it requires trending both, which brings us to a theme you’ll see repeat here.
3. Running One Cell Hard Instead Of Two Cells Easy
This is the counterintuitive one, and it’s where I see the most money left on the table.
Two cells running at half fan speed move roughly the same air as one cell at full speed. But each half-speed fan draws about an eighth of full power, so the pair pulls around a quarter of what the single cell was drawing. You also get double the wetted surface area, which improves heat transfer on top of the fan savings.
Operators split cells the other way constantly, because “one tower is enough for this load” feels efficient. It isn’t, and the rule worth remembering is to run every cell you have at the lowest speed that still holds the setpoint.
4. Never Looking At Approach Temperature
Approach is the gap between your cold water temperature and the outdoor wet bulb. It’s the tower’s report card, and almost nobody trends it.
A tower that ran a 7-degree approach in May and runs 11 degrees in August at the same wet bulb is telling you something specific: fouled fill, scaled distribution nozzles, clogged strainers, or a drift eliminator problem. That widening approach forces the chiller to work harder for every ton, and it happens gradually enough that nobody notices until the electric bill does something rude.
Trend approach weekly against wet bulb. It’s free, and it’s the earliest warning you’ll get.
5. Running Three Cycles Of Concentration When You Could Run Six
Cycles of concentration is the ratio of dissolved solids in your tower water to what’s in the makeup water. Run it low, and you’re dumping treated, conditioned water down the drain and replacing it with cold city water you then have to heat-reject and treat again.
The Department of Energy’s cooling tower management guidance is direct about the gain: moving from three cycles to six cuts makeup water by 20 percent and blowdown by 50 percent. EPA’s own facility guidance targets six or more cycles for water efficiency, and reports that its Fort Meade science center saved 530,000 gallons just by reducing blowdown.
How high you can push depends on your makeup water chemistry, so this is a conversation with your water treatment vendor, not a dial you turn yourself. Ask them what cycles you’re actually running, because a surprising number of operators have never been told, and the answer usually explains a real chunk of the water bill.
6. Bleeding On A Timer
Related, and worse. If your blowdown is on a timed solenoid rather than conductivity control, you’re bleeding on a schedule that has nothing to do with what the water is doing.
Timers dump on hot days when the tower is cycling hard and on cool days when it barely ran. Conductivity control bleeds when the water says so. This is a small part with a fast payback, and it’s frequently the reason a building’s cycles are stuck at three no matter what the treatment program says on paper.
7. Never Metering Makeup Water
Makeup equals evaporation plus blowdown plus drift. If you don’t meter makeup and blowdown separately, you cannot calculate cycles, spot a stuck float valve, or notice the basin quietly overflowing into the roof drain at 2am.
A basin overflow is one of those failures that can run for weeks. The tower keeps working. The chiller keeps making cold water. Nothing complains except the water bill, and that arrives six weeks later.
8. Single-Pass Cooling Still Hiding Somewhere In The Building
Somewhere in a lot of older buildings there’s a piece of equipment cooled by city water running straight to the drain. An old ice machine, a vacuum pump, a legacy air compressor, occasionally a CT scanner.
EPA puts the penalty in stark terms: single-pass cooling can use roughly 40 times more water to reject the same heat as a tower running at five cycles. Walk your mechanical spaces and look for a supply line going into equipment and a drain line coming out with nothing in between.
9. Cleaning On The Calendar Instead Of On The Condition
Twice-a-year cleaning is a floor, not a strategy. Fill fouls at whatever rate your water chemistry, air quality, and runtime dictate, and a tower next to a construction site fouls faster than one on a quiet roof.
The regulatory side of this got stricter, and New York owners should know where it landed. Under Local Law 159 of 2025, Legionella culture testing moved from every 90 days to at least every 31 days while a tower is in operation, effective in May 2026, on top of the registration, maintenance plan, and annual certification that Local Law 77 already required. The city’s cooling tower requirements page is the reference to work from, and your water treatment provider should be driving the program.
The energy angle and the compliance angle point the same direction here. Clean fill transfers heat better and costs less to run.
10. Keeping Tower Data In A Binder
Every mistake above shares one root cause. The data that would reveal it is either not collected or collected onto paper nobody trends.
Approach temperature, fan speed, cycles, makeup gallons, cell staging, runtime. Each of those is a number somebody wrote down once a week and filed. Trended continuously and alerted on, they’re an early warning system. That’s what modern cooling tower system controls are for: continuous monitoring across the tower, chiller, and pumps, with alerts when something fails rather than when someone notices. Runwise flags failed cooling towers, chiller shutdowns, and other critical faults within seconds, and puts the cooling cost reduction around 17 percent.
The carbon math matters too. Under Local Law 97, buildings over 25,000 square feet face penalties of $268 per metric ton over their cap, and the city is now actively enforcing against properties out of compliance. Summer cooling load is a large slice of that number for most commercial buildings.
The Ten At A Glance
# | Mistake | What you’d see | Where it costs you |
|---|---|---|---|
1 | Single-speed fan | Fan cycling on and off hard | Fan kW, up to 8x on part load |
2 | Fixed 85F setpoint | Same setpoint in May and August | Chiller kW/ton all season |
3 | One cell at full speed | Second cell idle in mild weather | Roughly 4x the fan power needed |
4 | Untracked approach | Nobody trends it | Slow chiller efficiency decay |
5 | Low cycles | Cycles at 2 to 3 | Makeup water and chemicals |
6 | Timed bleed | Solenoid on a clock | Water, sewer, treatment cost |
7 | No makeup meter | Cycles can’t be calculated | Undetected overflow and leaks |
8 | Single-pass cooling | Supply in, drain out | ~40x water per unit of heat |
9 | Calendar cleaning | Fill fouled between services | Heat transfer and compliance |
10 | Paper logs | Weekly readings, no trending | Everything above, undetected |
Where To Start
Pick the free ones first. Ask your water treatment vendor what cycles of concentration you’re running right now, and write the number down. Then ask your operator whether the second cell runs in mild weather or sits idle. Those two questions cost nothing and often surface a five-figure answer.
After that, check whether anything in your building is still cooled by single-pass water. It takes one walk-through, and when you find it, the fix usually pays back inside a season.
The rest of the list needs controls and trending, which is a bigger conversation. But you’d be starting it with actual numbers instead of a hunch, and that changes what your engineer can do with it.
What’s your tower’s approach temperature this week? If you can’t answer that in under a minute, you’ve found your first project.
