Case study

Air-Side Economizer Stuck Damper Energy Penalty Case Study

Energy engineering case study on diagnosing a stuck outdoor-air economizer damper using mixed-air temperature, outdoor-air fraction, cooling coil load, energy penalty, actuator checks, and commissioning evidence.

Branch: Energy Engineering
Content: Case study
Updated: Jun 22, 2026
Revision: v1.0.0 · reviewed

An air-side economizer uses cool outdoor air to reduce mechanical cooling. When outdoor conditions are favourable, outdoor-air and return-air dampers modulate so the mixed-air temperature can satisfy the supply-air setpoint with little or no chiller or direct-expansion cooling.

This case study follows a rooftop air-handling unit serving a university laboratory support wing. The building automation system reports that the economizer is enabled during mild weather, but the chilled-water valve remains open and the cooling plant runs every afternoon. The engineering question is whether the cooling load is real or whether the air-side economizer is failing to deliver the commanded outdoor-air fraction.

The purpose is to show how a simple mixed-air balance can expose a stuck damper, estimate the avoidable cooling load, and define commissioning evidence that proves the fault was corrected.

Case Context

The air-handling unit supplies variable-air-volume terminal boxes. The zone loads are moderate during spring shoulder-season operation. Outdoor air is cool and dry enough for economizer operation, and the humidity or enthalpy lockout is not active.

Item	Value or observation
Supply airflow during occupied mode	$12.0\ \text{m}^3/\text{s}$
Return-air temperature	$24.0^\circ\text{C}$
Outdoor-air temperature	$10.0^\circ\text{C}$
Supply-air temperature setpoint	$14.0^\circ\text{C}$
Measured mixed-air temperature	$20.2^\circ\text{C}$
Minimum outdoor-air setting	$25\%$ of supply airflow
Economizer enable signal	active
Outdoor-air damper command	$72\%$ open
Chilled-water valve	modulating open
Chiller system COP for screening	$3.1$
Shoulder-season affected operation	$720\ \text{h/year}$
Electricity price for screening	$\$ 0.16/\text{kWh}$

The operator display initially appears normal because the economizer enable signal and damper command are both active. The suspicious evidence is the mixed-air temperature: it is much closer to return air than to the temperature expected under economizer operation.

Expected Economizer Fraction

For a dry sensible screening calculation, mixed-air temperature can be approximated by:

T_{mix}=f_{OA}T_{OA}+(1-f_{OA})T_{RA}

where $f_{OA}$ is the outdoor-air fraction, $T_{OA}$ is outdoor-air temperature, and $T_{RA}$ is return-air temperature.

The outdoor-air fraction required to reach the supply-air setpoint before mechanical cooling is:

\displaystyle f_{OA,req}=\frac{T_{RA}-T_{SA,set}}{T_{RA}-T_{OA}}

Substitute the operating values:

\displaystyle f_{OA,req}=\frac{24.0-14.0}{24.0-10.0}=\frac{10.0}{14.0}=0.714

The economizer should therefore use about $71\%$ outdoor air under these conditions if humidity, freeze protection, smoke mode, or other overrides are not active. The $72\%$ command is physically plausible.

Outdoor-Air Fraction from Measured Mixed Air

The same mixed-air equation can be rearranged to estimate the actual outdoor-air fraction:

\displaystyle f_{OA,actual}=\frac{T_{RA}-T_{mix}}{T_{RA}-T_{OA}}

Using the measured mixed-air temperature:

\displaystyle f_{OA,actual}=\frac{24.0-20.2}{24.0-10.0}=\frac{3.8}{14.0}=0.271

The unit is actually taking about $27\%$ outdoor air, close to the minimum setting and far below the $71\%$ required for free cooling. That result is consistent with an outdoor-air damper stuck near minimum position, a slipping linkage, a failed actuator, or a control output that is not reaching the damper.

This calculation is a diagnostic screen, not final proof by itself. Mixed-air sensors can be biased by stratification, poor sensor placement, or leakage. However, the result is large enough to justify physical inspection and a direct damper stroke test.

Cooling Load Penalty

With the damper stuck near minimum outdoor air, the cooling coil must cool mixed air from about $20.2^\circ\text{C}$ to $14.0^\circ\text{C}$ . For sensible cooling:

\dot{Q}=\rho C_p \dot{V}(T_{mix}-T_{SA,set})

Take:

\rho=1.2\ \text{kg/m}^3,\quad C_p=1.01\ \text{kJ/(kg K)},\quad \dot{V}=12.0\ \text{m}^3/\text{s}

Then:

\dot{Q}_{fault}=1.2(1.01)(12.0)(20.2-14.0)=90.2\ \text{kW}

Under proper economizer operation, the mixed air should be close to the supply-air setpoint and the sensible coil load for this condition should be approximately zero. The avoidable sensible cooling load is therefore about:

\dot{Q}_{avoidable}\approx 90\ \text{kW}

This is not the whole building cooling load. It is the avoidable coil load for one air-handling unit during this outdoor condition. Latent load, reheat, fan heat, duct heat gain, and control deadband may change the exact value, but the order of magnitude is large enough for action.

Electrical Energy and Cost Impact

Convert the avoidable cooling load to electrical power using the screened chiller-system coefficient of performance:

\displaystyle P_{elec,avoidable}=\frac{\dot{Q}_{avoidable}}{COP}=\frac{90.2}{3.1}=29.1\ \text{kW}

For $720\ \text{h/year}$ of similar shoulder-season operation:

E_{avoidable}=29.1(720)=20{,}952\ \text{kWh/year}

At $\$ 0.16/\text{kWh}$:

C_{avoidable}=20{,}952(0.16)=\$3{,}352\ \text{per year}

The cost looks modest for one air-handling unit, but the reliability signal is more important than the utility line item. A failed economizer can increase cooling plant runtime, reduce spare cooling capacity, hide in a normal-looking control display, and repeat across multiple units if the same actuator or linkage design is used.

Diagnostic Evidence

The team checks whether the apparent fault could be caused by measurement error or a legitimate override.

Check	Result	Interpretation
Humidity and enthalpy lockout	not active	economizer should be available
Freeze protection	not active	low-temperature override does not explain minimum outdoor air
Smoke or pressure mode	not active	no life-safety override forcing damper position
Supply fan speed	stable	airflow estimate is not changing rapidly
Return-air sensor	within calibration tolerance	return-air reference is credible
Mixed-air sensor	stratification observed but average remains near $20^\circ\text{C}$	sensor placement adds uncertainty, not enough to explain the full error
Damper command	about $72\%$	controller is requesting economizer operation
Actuator shaft	moves during manual command	actuator motion exists
Damper blade position	remains near minimum	linkage between actuator and blade has slipped

The decisive finding is mechanical: the actuator shaft moves, but the damper blade does not follow it. The building automation system command is not the same as actual blade position.

Measurement Uncertainty

Assume each temperature sensor has an uncertainty of about $\pm0.3\ \text{K}$ after calibration review. A simple uncertainty screen for the outdoor-air fraction uses:

\displaystyle \delta f_{OA}\approx \frac{\sqrt{(\delta T_{RA})^2+(\delta T_{mix})^2}}{T_{RA}-T_{OA}}

So:

\displaystyle \delta f_{OA}\approx \frac{\sqrt{0.3^2+0.3^2}}{14.0}=0.030

Allowing additional uncertainty for outdoor-air temperature and mixing nonuniformity, the practical uncertainty may be closer to $\pm0.05$ . Even then, the inferred outdoor-air fraction is roughly $27\%\pm5\%$ , far from the required $71\%$ . The conclusion does not depend on a small measurement difference.

Fault Mechanism

The fault is a slipped outdoor-air damper linkage. During previous maintenance, the actuator clamp was tightened while the blade was not aligned with the actuator end stop. The control loop can command economizer operation, but the damper blade remains near the minimum outdoor-air position for most of the stroke.

This is different from a controller tuning problem. The control sequence is making a reasonable request. The mechanical system is not delivering the requested air path. Retuning the supply-air temperature loop would hide the symptom while leaving the failed economizer in service.

It is also different from a chiller efficiency problem. The chiller is consuming energy because the air handler is presenting an avoidable coil load. A plant-level optimization would not remove the root cause.

Corrective Action

The corrective action is:

lock out the unit for a controlled damper inspection;
realign the outdoor-air and return-air damper blades with actuator end stops;
tighten and mark the linkage clamp so future slip is visible;
verify full stroke under manual command;
confirm minimum outdoor-air flow separately from economizer operation;
add a fault-detection rule comparing commanded economizer fraction with inferred mixed-air fraction;
trend outdoor-air temperature, return-air temperature, mixed-air temperature, damper command, chilled-water valve position, and supply-air temperature during the next mild-weather period.

The minimum outdoor-air check is kept separate. An economizer fault should not lead operators to reduce ventilation below the required minimum outdoor-air rate while trying to save energy.

RPN Screen

A simple risk-priority-number screen documents why the issue is not only an energy tuning item:

RPN=S\times O\times D

Before corrective action:

RPN_{before}=6(5)(6)=180

The severity ranking reflects energy waste, lost cooling margin, and possible ventilation-risk ambiguity, but not an immediate life-safety condition. Occurrence is moderate because multiple units use similar linkage hardware. Detection is weak because the operator display shows actuator command, not verified blade position.

After repair, seasonal stroke testing, and mixed-air fault detection:

RPN_{after}=6(4)(3)=72

Severity is unchanged because the consequence of recurrence is similar. Occurrence and detection improve because the linkage is corrected, stroke evidence is recorded, and a trend-based diagnostic will flag the fault earlier.

Validation Criteria

The repair is accepted only if measured evidence supports it. Useful release criteria are:

during favourable outdoor conditions, inferred $f_{OA}$ agrees with the economizer sequence within $\pm0.08$ outdoor-air fraction;
supply-air temperature remains within $\pm0.5\ \text{K}$ of setpoint without unnecessary chilled-water valve opening;
minimum outdoor-air flow is verified independently at the required ventilation setting;
damper blades visibly reach full closed, minimum, and economizer positions during a stroke test;
mixed-air sensor placement is reviewed so stratification does not mask future faults;
trend logs show no repeated simultaneous economizer command and mechanical cooling above the accepted deadband;
operators know which alarm indicates a suspected economizer/damper mismatch and which manual override limits apply.

The most important validation point is that the evidence must prove air path, thermal result, and control state together. A command signal alone is not evidence that an air-side economizer is working.

Engineering Lessons

The first lesson is that an economizer is a coupled thermal and mechanical system. The control sequence, actuator, linkage, dampers, sensors, airflow path, and cooling coil all have to agree.

The second lesson is that mixed-air temperature is a powerful diagnostic when used carefully. It converts a vague complaint about “high cooling energy” into a physical estimate of actual outdoor-air fraction.

The third lesson is that energy faults can be reliability faults. A stuck damper may not trip an alarm, but it can consume cooling capacity, increase plant runtime, undermine commissioning claims, and hide behind apparently valid control commands.

Good HVAC commissioning therefore verifies the useful service, not only the control output. The building should receive the required ventilation, the economizer should reduce mechanical cooling when outdoor conditions allow it, and the trend data should make future drift visible before a full season of avoidable energy use accumulates.

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