A real-world example of why calculated load matters before blaming the machine
Written by Travis Riley, chiller mechanic and co-founder of Chiller Trend.
It is Saturday night.
The customer calls and says there is something wrong with the chiller.
“It is not making setpoint.”
No obvious alarms. Nothing clearly failed. The machine is running, but it is not pulling the loop down.
That kind of call can turn into a long night fast.
Because once a chiller is not making setpoint, everyone naturally starts looking for what is wrong with the machine.
Refrigerant issue. Sensor issue. Control issue. Flow issue. Something out of adjustment.
Maybe.
But before changing anything, there is a better question:
What is the machine being asked to do compared to what it was designed to do?
The Machine Was Being Pushed Past Design
Once the field readings were reviewed, the picture changed immediately:
Design tonnage: 1,250 tons
Calculated evaporator tonnage: 1,437 tons
Load vs design: 115%
The chiller was not failing.
It was being overloaded.
The way the plant was running had pushed the machine outside the design condition. Evaporator flow was high. Entering evaporator water temperature was high. That combination matters.
High return water temperature increased the temperature difference across the evaporator. Higher-than-design evaporator flow moved more water through the barrel. Together, they increased the BTUs being delivered to the chiller.
The machine was not magically short on capacity.
The plant was feeding it more load than the design number allowed for.
At some point, the math stops being a mystery.
The chiller was not refusing to make setpoint. It was being handed more load than it could physically remove.
Why It Was Not Making Setpoint
At 115% of design capacity, not making setpoint is not surprising.
The machine can be running correctly and still fail to pull the loop down because the load being placed on it is larger than what the chiller was selected to handle.
That is a completely different problem than a failed chiller.
If the machine is already above design load, the issue is not automatically refrigerant charge, controls, sensors, or adjustment.
The first issue is load.
The plant was pushing more load through the evaporator than the chiller could reasonably remove under that condition.
Why This Matters
Without calculated load, this kind of call can turn into a guessing game.
Refrigerant adjustments.
Flow assumptions.
Control changes.
Sensor chasing.
Trying to fix a chiller that may not be the real problem.
But once the load was clear, the direction changed.
The right next steps were not to start adjusting the machine.
The right next steps were to evaluate system load, staging, plant operation, evaporator flow, return water temperature, and whether the current operating condition matched the original design assumptions.
The Result Was Clarity
This was not a long analysis.
It took less than five minutes to go from raw field readings to a clear answer.
That matters in the field.
Less time guessing.
Faster decisions.
More confidence in what you are telling the customer.
The value was not just the number.
The value was having an answer the mechanic could explain and stand behind.
The Question This Should Raise
When a chiller is not making setpoint, the first question should not be:
What do we adjust?
The first question should be:
What is the actual load?
If the machine is only at 70% of design and not making setpoint, that points one direction.
If the machine is at 115% of design and not making setpoint, that points somewhere completely different.
Same complaint.
Different answer.
That is why calculated load matters.
Bottom Line
The chiller was not failing.
It was running at approximately 115% of design capacity.
That changed the whole conversation.
Instead of chasing symptoms, the focus moved to the real issue: the load being placed on the machine.
That is what a good chiller finding should do.
It should make the next decision clearer.
Until then, you are guessing.
And guessing gets expensive.