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Playbook: Economizer Failure
Applies toAHU-0006, AHU-0009, AHU-0011, AHU-0017, AHU-0021, AHU-0030, AHU-0034, RTU-0004, RTU-0005, RTU-0006, CLU-03
Fix complexityRemote fix (40%) · Remote + on-site (40%) · On-site only (20%)
Typical time30 min remote / 1–3 h on-site
Typical cost$0 remote / $100–$400 linkage repair / $500–$1,200 actuator
Energy impactEEM-06: 0–7% site energy. EEM-23 (RTU advanced controls): 3–11% electricity. Cowan (2004): 54% of RTU economizers have at least one fault; disconnected linkages are the single most common failure mode.

Adapted from HVAC FDD Reference v1.0, Remediation Playbooks (pp. 156–157).

Step 1 — Verify the fault

  1. Wait for favorable conditions (outdoor air cooler than return air and above the low-limit cutoff). Is the OA damper stuck at minimum? Is mechanical cooling active (cooling valve open or compressor running)? Both true = confirmed waste.
  2. Compare the OAT sensor to a nearby weather station — a sensor reading high can lock out the economizer. PNNL-27338 AIRCx computes outdoor air fraction OAF = (MAT − RAT)/(OAT − RAT), reliable only when |OAT − RAT| > 5 °F.
  3. Check both failure modes: (1) economizer not activating when favorable, and (2) economizer staying open when OAT is above the lockout — the second brings in excess hot outdoor air.

Step 2 — Remote fix

  1. Check the economizer enable/disable flag in the BAS; enable if off.
  2. Check the high-limit setpoint. Fixed dry-bulb: ASHRAE 90.1 high-limit by climate — 75 °F zones 1A–3A, 70 °F zones 4A–5A, 65 °F zones 5B–8. Differential: free cooling should enable whenever OA is cooler than RA. Raise a too-low setpoint.
  3. Remove seasonal lockouts blocking free cooling during mild weather.
  4. If the OAT sensor has drifted, apply a calibration offset as a temporary fix.
  5. RTUs with integrated economizers: verify DX staging allows the economizer to run with and without mechanical cooling — non-integrated operation loses significant free-cooling opportunity.

Step 3 — On-site service

  1. Manually command the OA damper to 100% and watch it physically move.
  2. If it doesn’t move: check actuator power/air supply; check the linkage — disconnected rod ends are the most common RTU economizer failure (rod-end pop-off, bent crank arm, stripped set screw, broken plastic clip). Replace linkage components ($50–$150).
  3. If the actuator has failed, replace it ($300–$800).
  4. RTUs: verify barometric relief / power exhaust works — inadequate relief creates positive pressure that prevents the damper opening fully.
  5. Inspect damper blade seals — worn seals cause excess outdoor air when commanded closed (RTU-0005).

Step 4 — Confirm resolution

  1. Wait for the next favorable free-cooling period.
  2. Verify the OA damper modulates between minimum and 100% as conditions change, and mechanical cooling drops during favorable periods.
  3. The fault should clear within 24 hours.
  4. Multi-RTU sites: survey all units — if one RTU has an economizer problem, 30–50% of others on the same roof likely do too.