A real-world example of why field verification matters
Written by Travis Riley, chiller mechanic and co-founder of Chiller Trend.
Nobody knew there was a problem.
The chillers were running. The plant was operating. The BAS was online. Nothing was screaming for attention.
But during a baseline inspection, one extra reading changed the operating story of the plant.
A normal chiller log can tell you what the machine was doing at that moment.
A stronger workflow asks a better question:
Do the readings agree with the rest of the plant?
At one large central plant, the answer was no.
As part of the inspection workflow, independent ambient readings were taken near the cooling towers and cooling tower approach was calculated across the operating chillers.
The tower approach readings were not lining up the same across the plant.
Something did not look right.
That led to verifying the BAS ambient inputs against independent field measurements.
At the time of the field check, the BAS showed an outdoor-air wet-bulb value of 81.7°F. Two independent Testo probes both showed a wet-bulb temperature of 78.2°F.
That is a difference of about 3.5°F.
The BAS outdoor-air temperature and humidity values also did not agree with the independent field readings.
That matters because the BAS was using that environmental input to influence cooling tower control.
The control system was responding the way it was supposed to.
The problem was that it was responding to bad information.
One Extra Reading Changed the Story
This was not a failed chiller.
It was not a major alarm.
It was not an obvious mechanical problem.
It was a believable BAS value that did not match verified field conditions.
That is how these problems hide.
The number is online. The value looks reasonable. The plant keeps running.
But believable is not the same thing as verified.
When a BAS input is wrong, the plant can make bad decisions quietly. The operator may not see a failure. The chiller may not trip. The BAS may not alarm.
In this case, the cost showed up through tower behavior, condenser water temperature, refrigerant lift, and chiller kW.
The Control Chain
The important part was not just that a sensor value was off.
The important part was what that value was connected to.
Bad ambient and humidity data affected the wet-bulb calculation.
Bad wet-bulb information affected tower control.
Tower control affected condenser water temperature.
Condenser water temperature affected chiller lift.
Lift affected chiller kW.
That is the chain.
Sometimes the most valuable chiller finding is not on the chiller.
It is upstream from the machine, hiding inside a control input that everybody assumes is close enough.
Why Normal Logs Miss This
A normal log captures readings.
That still matters.
But the next layer is where the value shows up.
Do the readings agree with each other?
Do they agree with the BAS?
Do they agree with the field?
Do they explain the way the plant is operating?
Without the independent ambient reading, the BAS value may have continued to look believable.
Without calculating tower approach, the inconsistency across the plant may not have stood out.
Without saving and comparing those readings over time, the issue may have stayed buried inside normal-looking plant operation.
The Result
Once the control strategy was corrected, the condenser water setpoint was reduced by 3°F.
After the adjustment, plant efficiency improved from approximately 0.56 kW/ton to approximately 0.50 kW/ton during comparable operating conditions observed during review, while accounting for increased tower fan energy.
That is an improvement of about 0.054 kW/ton.
Using the original value as the baseline, the reduction is approximately:
(0.56 - 0.506) ÷ 0.56 = 9.6%
That supports calling it roughly a 9–10% reduction in kW/ton.
Based on the available operating history and savings model, the estimated savings range was approximately $50,000–$60,000 per year.
That estimate depends on runtime, load profile, weather, electric rate, and how consistently the improved control condition is maintained.
The important point is that this was not a capital project.
It was a field-verified operational correction.
Why This Matters Beyond One Site
One bad BAS input at one large central plant was enough to affect tower behavior and chiller efficiency.
That should raise a larger question for owners and facility leaders:
Is this isolated, or is this happening somewhere else too?
Most large facilities already have plenty of data.
The harder problem is knowing which data can be trusted, which readings need to be verified, and which small changes are actually affecting performance.
Small changes are easier to spot when there is a real baseline to compare against.
The Real Lesson
This was not found because the chiller failed.
It was found because the inspection workflow looked beyond the machine and checked whether the plant inputs made sense.
That is what good chiller work has to become.
Not just:
What are the readings?
But:
What do the readings mean?
Do they agree with the field?
What changed?
What is this costing?
What should be corrected?
A chiller log should not just prove that someone was there.
It should help explain what the plant was doing.
Bottom Line
A bad BAS input can make a good chiller plant run inefficiently.
Because the number can still look believable, it may never create an obvious alarm.
That is why field verification, tower approach, and baseline inspections matter.
And that is why the result is bigger than the tool.
One extra reading exposed a control issue that was quietly costing efficiency.
Sometimes that is all it takes to stop guessing and start seeing the plant clearly.