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Playbook: Fan Coil Unit Faults
Applies toFCU-0001 (cycling), FCU-0002 (SAT low heating), FCU-0003 (SAT high cooling), FCU-0004 (cooling leak), FCU-0005 (heating leak), FCU-0006 (fan proof), FCU-0007 (simultaneous commands)
Fix complexityRemote fix (cycling/leak detection) · On-site (valve replacement)
Typical time15 min remote / 1–2 h on-site per unit
Typical cost$0 remote / $150–$600 per valve replacement
Energy impactEEM-03 (fix leaking valves): 0.5–5% site energy in cold climates. FCU faults are insidious — each unit wastes a small amount, but hotels and apartments may have hundreds of FCUs. Leaking valves (FCU-0004/FCU-0005) are classified as CRITICAL_WASTE because they represent energy being added and removed simultaneously at the zone level.

Adapted from HVAC FDD Reference v1.0, Remediation Playbooks (pp. 171–172).

Step 1 — Verify the fault

  1. Fan proof (FCU-0006): confirm this is an active fan coil, then compare the final fan command with independent current, airflow, speed, rotation, or auxiliary proof. Check occupancy/mode ownership, condensate and freeze interlocks, service state, and local hand control before condemning the fan.
  2. Simultaneous commands (FCU-0007): trend both physical valve commands from the same FCU. Verify the deployed thresholds/duration, then exclude intentional dehumidification/reheat, freeze protection, valve exercise, and commissioning before treating the overlap as a control conflict.
  3. Excessive cycling (FCU-0001): count operating state transitions per hour — more than 7/hr indicates a problem. The most common cause is a narrow deadband between the heating and cooling setpoints.
  4. SAT deviations (FCU-0002/FCU-0003): confirm SAT is below setpoint at full heating (FC-002) or above setpoint at full cooling (FC-003). Rule out plant-side issues first — is the HW/CHW supply temperature adequate?
  5. Leaking valves (FCU-0004/FCU-0005): with the valve commanded to 0%, measure the temperature drop (cooling) or rise (heating) across the coil. Any measurable temperature change when the valve is commanded closed confirms a leak. In multi-story buildings, check gravity circulation: hot water can thermosiphon through vertically oriented coils even with the valve closed.

Step 2 — Remote fix

  1. Cycling: widen the deadband between the heating and cooling setpoints to at least 2 °F (1 °C). Check for sensor noise causing mode oscillation — apply a software filter or averaging if available.
  2. SAT deviations: verify that the central plant is providing adequate supply temperatures. If the CHW supply is too warm or the HW supply too cold, the FCU coils cannot produce the expected output regardless of valve position.
  3. Leaking valves: if the leak is small, a temporary workaround is a seasonal lockout that disables the leaking coil’s valve entirely during the opposite season (e.g. lock out the heating valve in summer). This eliminates the simultaneous heating and cooling effect while the valve awaits replacement.
  4. Simultaneous commands: after confirming no intentional cooling-plus-reheat requirement, correct overlapping loop deadbands, priority-array overrides, mode transitions, or the ordinary heating/cooling interlock. Do not defeat humidity, freeze, condensate, or equipment-protection logic.

Step 3 — On-site service

Only qualified personnel may open panels or approach rotating fans. Apply site lockout/tagout and verify absence of hazardous energy; never bypass condensate, freeze, or electrical protection to clear an FDD finding.

  1. SAT deviations with adequate plant supply: inspect the coil for fouling or air locks. Bleed air from the coil piping. Clean the coil surface.
  2. Leaking valves: the valve seat is worn or the valve body is corroded. Replace the valve ($150–$600 depending on size and type).
  3. Gravity circulation: install a check valve on the coil piping to prevent thermosiphon flow, or reorient the coil piping to eliminate the vertical loop.
  4. Hotels and apartments with hundreds of FCUs: prioritize by measuring the waste at each unit — waste_kW = |temp_change| × airflow × cp_air. Replace the worst offenders first.
  5. Fan proof: verify the proof switch before replacing equipment, then inspect the motor/ECM, wheel, bearings, contactor, and wiring under LOTO.
  6. Simultaneous commands: if command logic is correct, verify each actuator and valve position independently; use FCU-0004/0005 temperature evidence to distinguish command conflict from a valve that leaks when commanded closed.

Step 4 — Confirm resolution

  1. After repair, verify the valve closes to zero leakage: no measurable temperature change across the coil when the valve is at 0%.
  2. For cycling: verify state transitions drop below 7/hr.
  3. For fan proof, use an OEM/site-approved safe stop/start test and confirm final command and independent status agree within both configured windows.
  4. For FCU-0007, verify no above-threshold overlap during ordinary modes and confirm any designed dehumidification/reheat mode still operates correctly.
  5. Monitor for 48 hours before closing the fault.