A Fan Coil Unit (FCU) is a small, local HVAC device that conditions a single room or zone using a fan and a water coil, with no duct network of its own. Chilled or hot water from a central plant flows through the coil; the fan blows room air across it to deliver cooling or heating, typically moving 200 to 2,000 m³/h per unit. FCUs are the standard choice for hotels, apartments, and offices where individual temperature control is needed without the cost of a full air-handling distribution system.
Types of Fan Coil Units
Two-pipe FCUs share one water circuit for both heating and cooling, switched seasonally—simple and cheap but cannot heat and cool simultaneously. Four-pipe FCUs have separate heating and cooling coils, allowing a room to cool while another heats, ideal for mixed-mode buildings. Cassette units mount in the ceiling and throw air in four directions; wall-mounted and floor-standing units suit retrofits; ductable FCUs feed a short local duct run for discreet grilles.
Sizing and Performance Numbers
Capacity scales with airflow and water temperature. A typical 4-row coil at 0.8–1.2 m/s face velocity delivers 1.5 to 15 kW of cooling and 2 to 18 kW of heating. Selecting 5°C chilled water and 45°C hot water is common; a 1,000 m³/h unit cools roughly 7 kW. Noise is a key limit: well-designed FCUs run at 30 to 45 dB(A) on low speed, so fan speed and mounting isolation matter in bedrooms and meeting rooms.
Controls and Fresh Air
Most FCUs use a 3-speed fan and a room thermostat, with a motorised valve that modulates water flow to hold setpoint. Because an FCU recirculates room air, it does not supply fresh air by itself; buildings pair FCUs with a central AHU or dedicated outdoor-air system providing 20 to 60 m³/h per person. Without that fresh-air link, an FCU-only design fails ventilation codes.
FCU Type Comparison
| Type | Pipe config | Airflow | Best use |
|---|---|---|---|
| Two-pipe | 1 circuit | 200–2,000 m³/h | Seasonal, low cost |
| Four-pipe | 2 circuits | 200–2,000 m³/h | Mixed heating/cooling |
| Cassette | 2 or 4 pipe | 400–1,800 m³/h | Ceiling, 4-way throw |
| Ductable | 2 or 4 pipe | 300–1,500 m³/h | Hidden grilles |
Selection Checklist
- Choose a two-pipe FCU for budget projects with clear seasonal heating or cooling.
- Choose a four-pipe FCU when zones need heating and cooling on the same day.
- Pick a cassette unit for open ceilings and even four-way air distribution.
- Keep low-speed noise at or below 35 dB(A) for bedrooms and quiet offices.
- Pair every FCU zone with a fresh-air source meeting 20–60 m³/h per person.
Maintenance and Water Quality
FCU coils foul with dust on the air side and scale on the water side; a 1 mm scale layer can cut heat transfer by 20% to 30%, so treat condensate and use softened water where hardness exceeds 150 ppm. Clean the air filter every 1 to 3 months in occupied rooms, and flush the coil annually. The condensate pan must slope and drain freely, because a blocked trap causes overflow, ceiling stains, and mould. A neglected FCU can lose 30% of capacity before anyone notices the comfort complaint and calls for service.
Controls and BMS
Each FCU links to the room thermostat or a BACnet/Modbus point that opens the water valve and sets fan speed (low, medium, high). A pressure-independent valve holds flow despite main pressure swings of ±10%, keeping capacity stable. For large buildings, a master controller can shed load by stepping fans down 10% to 20% during peak electricity tariff windows, cutting demand charges. Fault detection flags a stuck valve or a frozen coil when the room temperature drifts beyond 2°C from setpoint for more than 30 minutes of occupied time.
Choosing Coil Rows and Face Velocity
More coil rows (3, 4, or 6) raise capacity but also pressure drop and cost; a 4-row coil at 1.0 m/s face velocity is the common balance for 5 to 10 kW duty. Face velocity above 2 m/s risks water carry-over from the coil into the room, so size the coil area for 0.8 to 1.5 m/s. Oversizing rows wastes pump energy through higher water-side pressure, while undersizing leaves the room warm, so match rows to the load calculation rather than to a generic rule for the whole building type blindly.
Retrofit and Replacement Tips
When replacing an old FCU, measure the recess or ceiling grid (600 × 600 mm cassette is standard) before ordering, and confirm the water connection side (left or right) to avoid re-piping. Match the new capacity within 10% of the old to keep the existing pump and pipe sizes valid. A modern IE-class fan motor and EC fan option can cut FCU electrical use by 20% to 50% versus a legacy unit, a quick payback in a hotel with hundreds of rooms running 24 hours a day, every day of the year.
Summary for Specifiers
An FCU gives cheap, local control but needs a fresh-air partner and clean water to perform. Choose two-pipe or four-pipe by the building's mode needs, keep face velocity in range to avoid carry-over, and plan filter and coil service. With EC fans and proper control, a modern FCU cuts energy 20% to 50% versus legacy units, paying back fast in hotels and offices where hundreds run around the clock and drive the building's biggest electricity draw.
Quick Reference Numbers
Handy FCU figures: 200 to 2,000 m³/h per unit, 1.5 to 15 kW cooling, 2 to 18 kW heating, 30 to 45 dB(A) on low speed, and 20 to 60 m³/h of fresh air per person from a separate system. Four-row coils at 0.8 to 1.5 m/s face velocity avoid water carry-over, and EC fans cut power 20% to 50%. Keeping these numbers in view during layout stops both oversizing, which short-cycles, and undersizing, which leaves rooms above setpoint and occupants complaining about comfort and noise levels daily.
Keruite manufactures this equipment at an 80,000 m² facility in Dezhou, China, with an in-house AMCA-210-style test laboratory; every unit is balanced to G2.5 and performance-tested before shipment.
FAQ
What is the difference between two-pipe and four-pipe FCU?
A two-pipe FCU has one water circuit shared by heating and cooling, so the whole building switches mode together seasonally. A four-pipe FCU has separate hot and chilled coils, letting one room cool while another heats on the same day. Four-pipe costs more but is essential for mixed-mode or perimeter-zone buildings with simultaneous loads.
Does an FCU provide fresh air?
No, a standard FCU only recirculates and conditions the air already in the room; it has no outdoor-air intake. Buildings must supply ventilation separately through a central AHU or a dedicated outdoor-air unit at 20 to 60 m³/h per person. Treating an FCU as a fresh-air source will breach ventilation and indoor-air-quality codes.
How noisy is a fan coil unit?
A well-designed FCU runs about 30 to 45 dB(A) on low speed and can reach 50 dB(A) on high. Bedrooms and meeting rooms should use the low or medium setting and include flexible anti-vibration mounts; ceiling cassette units with insulated casings are quieter than wall units mounted on hard partitions without isolation.
How do I size an FCU for a room?
Match the unit's cooling capacity (1.5 to 15 kW typical) to the room load from heat gain, and pick airflow of 200 to 2,000 m³/h so the coil delta-T stays in range. Oversizing raises first cost and causes short-cycling; undersizing leaves the room above setpoint, so base the choice on a proper load calculation rather than floor area alone.