Daikin Fan Coil Error Code A4

Full Diagnosis

Freeze (anti-frost) protection has tripped on the fan coil's water coil. Unlike Daikin's refrigerant-circuit indoor units, a fan coil has no compressor or expansion valve — cooling and heating are supplied entirely by chilled or hot water circulated through the coil, and the fan simply moves indoor air across it. When the coil's water temperature drops low enough to risk ice forming on the fins, the controller closes the modulating water valve (or breaks the circulating-pump interlock) and stops the fan to protect the coil from splitting. Unlike A3 (drain level fault, a condensate-handling issue), A4 is a water-side temperature protection fault tied directly to the hydronic loop, not the drain pan.

All Recorded Causes

Insufficient chilled/hot water flow through the coil (partially closed isolation or balancing valve, air-locked coil, undersized circulator, clogged strainer) Water temperature setpoint at the plant/chiller set too low for the coil design, or a sequencing fault delivering water colder than the coil can tolerate Defective or drifted water temperature thermistor giving a false low reading Loose or corroded thermistor connector Control valve stuck partially closed, starving the coil of flow

Verify Before Replacing Parts

1. Confirm the exact Daikin model family and fan coil context before acting on a generic code description.
2. Verify airflow first, then confirm operating pressures and temperature relationship before adjusting charge, since insufficient chilled/hot water flow through the coil (partially closed isolation or balanc….
3. Look for restrictions, iced coils, or dirty heat-transfer surfaces that can mimic a charge problem, and separately confirm water temperature setpoint at the plant/chiller set too low for the coil design, or a sequ….
4. After the correction, run the system long enough to confirm the fan coil fault does not return under normal load.