RTU Economizer Stuck 100 Percent on a Cold Day: Decision Tree

Why this matters

A light-commercial rooftop unit with the outside-air damper locked at 100 percent on a 25 F morning will freeze its coil, trip safeties, and surprise the customer with a Monday-morning callout. The economizer is supposed to modulate outside air for free cooling - not flood the unit at design heating conditions. Four root causes drive a stuck-open economizer: failed minimum-position pot or controller, actuator drive-train failure or mechanical bind, sensor wiring fault that defeats lockout, and a controller logic mismatch from a recent thermostat swap or BAS reconfigure. Each calls for a different fix. Use this tree to identify which before the coil freezes.

Symptom presentation

Customer or building engineer reports an RTU "running cold" or supply temperature low even when the heat is calling. Inspection shows the outside-air damper actuator driven to roughly 100 percent open while the outdoor temperature is well below the high-limit lockout setpoint (typical ASHRAE 90.1 dry-bulb high-limit 60 to 75 F depending on climate zone). Heating may energize and run continuously fighting the cold air; cooling may not be called at all. Mixed-air temperature reads in the low 30s F, supply temperature drops below the freeze-stat setpoint, freeze-stat trips, unit lockouts.

Variants:

• Damper stuck open and won't move when commanded closed: actuator or linkage failure.
• Damper drives closed on command but returns to 100 percent over time: controller minimum-position drift, or a failed return-air sensor reading hot.
• Damper open only when heating calls (paradoxical): controller wired so heat call enables free cooling - control logic error.
• Damper open and minimum position dial reads 100: someone field-adjusted the pot and forgot.
• Damper open on a BAS-controlled unit only when BAS comm drops: failsafe state in the controller is "open" instead of "minimum."

Quick checks before pulling the economizer hood

Capture outdoor dry bulb, outdoor enthalpy (if equipped), return-air dry bulb, mixed-air dry bulb, supply-air dry bulb, controller setpoints (minimum position, high-limit, changeover), and any active BAS commands. Read the economizer controller LED status (Honeywell W7220, Carrier ProtoNode, Daikin Open Choice, etc. all indicate state on the panel).

Walk the actuator linkage. A floating actuator that drives full open and stays there indicates the controller is commanding open OR the actuator has lost feedback. An actuator that physically resists moving - mechanical bind - feels stiff when manually de-energized.

Confirm the high-limit configuration matches the design intent. A dry-bulb-only controller set to 75 F changeover in a humid climate is wrong by design and may pass economizer logic on a damp morning where enthalpy would have rejected it. ASHRAE 90.1 prescribes both setpoint and changeover type by climate zone.

Isolation tree

Branch 1: actuator and linkage.

• Actuator drives open with a 24 VAC command but does not drive closed when the command is removed - actuator failed. Replace.
• Actuator linkage physically bound at the damper shaft (corrosion, debris, mechanical interference from a service panel) - free the linkage, lubricate, verify smooth travel from 0 to 100 percent.
• Spring-return actuator with a broken spring - the damper will not return to minimum on power loss. Replace.

Branch 2: controller and minimum position.

• Honeywell W7220 (or equivalent) showing "FCC" (free cooling commanded) when outdoor is below high-limit - controller is working as designed but the high-limit setpoint is wrong. Check and correct.
• Minimum position pot reading 100 percent or "OCC" output stuck high - someone field-adjusted or the pot is mechanically broken. Reset minimum position to design (typically 10 to 30 percent occupied per ASHRAE 62.1 ventilation calc).
• Controller LED shows fault code (Honeywell shows numeric fault, Carrier shows alarm bit) - cross-reference the OEM manual for the specific fault. Sensor failures (outdoor air, return air, mixed air) commonly default the economizer to a failsafe state which on some controllers is "minimum" and on others is "100 percent."

Branch 3: sensors.

• Outdoor air sensor reads ambient (the controller thinks it is hot outside even when it is cold) - shorted sensor or wiring fault. Test with ohm meter, replace per OEM.
• Return air sensor reads very high (controller thinks return is hot and welcomes free cooling) - failed sensor or sensor exposed to a heat source (sun on the rooftop curb). Replace or relocate.
• Mixed-air sensor failed open - controller cannot close the loop on supply temperature. Replace.

Branch 4: controls and BAS.

• BAS commanding 100 percent outside air via override - check BAS overrides at the device and at the BAS supervisor.
• BAS comm dropped, controller defaulting to "open" - reconfigure failsafe to "minimum" per OEM bulletin.
• Recent thermostat swap to a model that does not support economizer staging - controller stuck in cooling mode. Reconfigure or replace thermostat with economizer-capable model per ASHRAE 90.1 Section 6.5.1.

Branch 5: freeze protection.

• Mixed air temperature falling below 38 F - immediate freeze risk to the cooling coil. Override damper to minimum manually, lock out the cooling coil, and address root cause before re-enabling.
• Freeze-stat tripped and locked out unit - clear the lockout only after the root cause is identified and corrected.

Confirming the diagnosis

After remediation, run the unit through the morning warm-up cycle and the daytime cooling cycle. Damper should drive to minimum position during heating call, drive open progressively when outdoor air is suitable for free cooling (below the high-limit, above the low-limit for freeze protection), and return to minimum when outdoor air exceeds high-limit. Mixed-air temperature should stay above 45 F at all heating-call operating points. Document baseline and post-fix damper position, mixed-air temperature, and supply temperature across a 4-hour observation.

Remediation summary

References

• ASHRAE Standard 90.1-2022 Section 6.5.1 - air economizer requirements including high-limit shutoff setpoints by climate zone.
• ASHRAE Standard 62.1-2022 - ventilation for acceptable indoor air quality including minimum outdoor air calculation methodology.
• 2021 IECC Section C403.5 - economizer requirements and exceptions.
• Honeywell W7220 Economizer Module installation guide and OEM equivalents (Carrier EconoMi$er, Daikin Open Choice) - sensor fault response and setpoint configuration.
• ACCA Manual CS Commercial - economizer commissioning and damper calibration.