What is a fan coil unit?

A fan coil unit (FCU) sits in or near the zone and conditions the air that zone already has. A small fan pulls room air across a coil; the coil is usually chilled water for cooling and hot water or electric for heating. The unit does not replace the central plant or an air handling unit. In FCU HVAC layouts the plant makes the water (or the primary air), and the FCU finishes comfort at the room. Hotels, offices, meeting rooms and perimeter zones use them where a full VAV branch is overkill or the fabric already has fan-coil risers.

Main parts

Typical pieces: the fan (often multi-speed or ECM), one or two coils, a control valve and actuator on the water side, a filter, and a local thermostat or zone controller. Many commercial FCUs also expose discharge or leaving-air temperature and a condensate drain float on the BMS. Two-pipe vs four-pipe is the quick piping split. Two-pipe systems share one supply/return pair and change over between heating and cooling seasons (or modes), so simultaneous heat and cool in adjacent zones is limited. Four-pipe systems give each coil its own chilled and hot circuits, so zones can heat and cool independently. Poor changeover, wrong valve fail positions, and low valve authority show up in both layouts.

FCU vs AHU vs VAV

An air handling unit is central plant: filters, coils, fans and dampers that serve many zones through ductwork. A VAV box is a zone terminal that meters primary air from that AHU. An FCU is also a zone terminal, but it moves and conditions room air at the terminal with its own fan and coil water (or electric heat). Some buildings mix them: AHU primary air for ventilation, FCUs for zone temperature. The FCU is not a substitute for the AHU, and it is not a VAV box with a different label.

Faults that show up in meter and BMS data

Stuck-open, stuck-closed and leaking control valves are the classic FCU signatures. A valve that never closes leaves the coil live while the zone overshoots; a valve that never opens leaves the zone hunting setpoints with the fan still running. Poor valve authority (valve ΔP too small vs the circuit) makes the loop look busy while flow barely changes. Fan speed that does not track the command, loaded filters and fouled coils cut capacity and raise fan energy. Terminal faults also feed low delta-T on the chilled-water loop: bypass or leakage at many FCUs returns water that is too warm for the plant design. Zone temperature that chronically misses setpoint is often the first operator complaint. Industry FDD work (including LBNL fan-coil fault datasets) trains detectors on stuck valves, fan faults and similar degradations. The patterns are real; what matters operationally is ranking them from the points you already trend.

What to check before trusting a fault alert

A "stuck valve" finding can be a stuck sensor, a bad feedback potentiometer, a calibration offset, or a data gap on the BMS poll. Before you send a technician, co-trend valve command vs position (or inferred flow), zone temperature vs setpoint, fan command vs speed or status, and coil leaving temperature when available. Confirm the point is updating (not frozen at last-good). Filter a short outage or a setpoint override before you treat the signature as hardware. Setpoint tracking that misses for hours across occupied periods is a stronger signal than a single spike history-based detectors often ignore.

What analytics catches

FrostLogic Explore is Sensor Intelligence. It reads FCU points already on the BMS and returns a prioritised queue. Evidence draws on Explore's anomaly detectors, including setpoint tracking: chronic miss of zone setpoint that threshold history alone often misses. The same stack spots equipment degradation and sensor faults early (valve/fan mismatch, frozen points, coil-side drift). It does not invent a parallel sensor set, and it is not a CMMS. Writes start off; predictive maintenance in Explore is the commercial path for FCU degradation ranked by cost and urgency.