Oversized fan coil units short-cycle, dehumidify poorly and waste energy. Undersized units never reach setpoint on design days and generate complaints that persist for the life of the building. This guide walks through the sizing process for a chilled water fan coil unit in the order that avoids the most common and most expensive mistakes — with a fully worked example at the end.
Sizing from floor area alone is the single most common error in FCU selection. A south-facing glazed office and an internal meeting room of identical size can differ in cooling load by a factor of two.
A defensible load calculation accounts for:
Sensible heat gains
Latent heat gains
Total the sensible and latent loads separately. You will need both figures: total capacity determines model size, and the sensible-to-total ratio determines whether the unit can meet the space's humidity requirement.
A note on diversity. Do not simply sum peak loads for every room and size the plant to that figure. Peak solar gain on the east facade occurs at a different hour than the west. Apply diversity at plant level while sizing each terminal for its own peak.
This step is placed second deliberately. Most selection processes leave acoustics until last, then discover at commissioning that the chosen unit only meets its rated capacity at a fan speed that violates the room's noise criterion.
Reverse the order. Establish the noise limit first, and only the fan speeds that comply are available to you.
Typical noise criteria by space type:
Now check which fan speeds satisfy that limit. The Aura exposed series, for example:
For a hotel room requiring below 30 dB(A), the usable speeds are M2 up to Speed 3 (1,430W), M3 up to Speed 3 (2,120W), M4 up to Speed 2 (2,230W), M5 at Speed 1 only (1,730W).
Read that last line carefully: the M5 — the largest unit — delivers less usable capacity than the M4 in a sub-30 dB(A) room. Bigger is not better once acoustics bind. This is exactly the trap that noise-last selection walks into.
Concealed ducted units must overcome the resistance of ductwork, filters, grilles and any attenuators. Undersized static pressure is a leading cause of airflow shortfall discovered on site.
Add up the resistance of:
EC motor units provide adjustable static pressure, which gives commissioning headroom that fixed-speed AC units cannot:
Note how power consumption rises with static pressure — roughly 50% from 12Pa to 50Pa. Excess duct resistance is paid for every operating hour, so duct design deserves attention rather than being compensated for by a bigger fan.
Two figures must be cross-checked against the hydronic design: water flow rate and water pressure drop.
Water flow rate determines whether the pump can deliver enough water to every terminal on the circuit.
Water pressure drop accumulates across the circuit. A coil with high pressure drop placed at the end of a long run can be starved of flow, and no amount of fan speed will recover the lost capacity.
Sample water-side data across the cassette range:
Notice the jump from 25 kPa at FP-136KM to 39 kPa at FP-170KM. Where several large units sit on one branch, that step change can dominate pump head. Consider balancing valves or PICVs, and confirm the pump selection against the worst-case circuit rather than the average.
Capacity and acoustics narrow the choice; the building fabric decides the final format.
For retrofits where plenum depth is the binding constraint, ultra-thin concealed units with integrated motorised dampers can serve multiple rooms from a single unit — one air inlet, ducted supply to several zones, with independent thermostat control per room.
Room parameters
Step 1 — Load: 1,440W total.
Step 2 — Noise first: below 30 dB(A) restricts each model to specific speeds. Available capacity within that limit:
Step 3 — Static pressure: zero, as the unit is exposed with no ductwork.
Step 4 — Hydraulics: M3 requires 0.441 m³/h at 6.7 kPa; M2 requires 0.286 m³/h at 10.2 kPa. The M3 has the lower pressure drop despite the larger capacity, which favours it on long circuits.
Step 5 — Format: exposed Aura series confirmed.
Selection: Aura M3.
Rationale — the M2 at Speed 3 delivers 1,430W against a 1,440W load, leaving no margin and forcing the unit to run at its top compliant speed continuously. The M3 meets the load at Speed 2 (1,810W at 26 dB(A)), giving both acoustic headroom and reserve capacity for hotter-than-design days, while consuming only 6W at that speed. Its lower water pressure drop is a further advantage on a multi-storey circuit.
Lesson: the correct selection was not the smallest unit that technically met the load. It was the unit that met the load at a comfortable operating point.
Sizing from floor area. W/㎡ rules of thumb ignore orientation, glazing and internal gains. Use them for early budget estimates only, never for final selection.
Selecting at maximum fan speed. Rated capacity is usually quoted at top speed, which is often acoustically unusable in the actual space.
Ignoring the sensible/latent split. A unit with adequate total capacity but the wrong sensible ratio will leave a humid space in tropical climates.
Forgetting dirty-filter static pressure. Design for the dirty condition, not the clean one, or performance degrades between service intervals.
Overlooking low-temperature heating capacity. On heat pump projects at 45°C supply, heating capacity is roughly 60% of the 60°C figure. Check both.
Adding a blanket safety margin. Applying an arbitrary 20% oversize on top of an already conservative load calculation produces short-cycling and poor dehumidification. Trust the calculation.
How much oversizing is acceptable when selecting a fan coil unit? Aim for 5–10% above calculated peak load. Beyond that, short-cycling degrades both comfort and dehumidification. If the calculation itself was conservative, no additional margin is needed.
Should I size on total or sensible cooling capacity? Both. Total capacity must exceed the total load; sensible capacity must exceed the sensible load. In humid climates the sensible check often governs, because a unit meeting total capacity may still leave the space uncomfortably humid.
What air flow rate do I need per kW of cooling? Roughly 150–200 m³/h per kW is typical for chilled water FCUs at standard conditions, but this varies with coil design and entering conditions. Always use the manufacturer's published air flow rather than a generic ratio.
Can one fan coil unit serve multiple rooms? Yes, with ducted concealed units and motorised dampers. Ultra-thin concealed units with integrated dampers allow independent thermostat control per room from a single unit — useful where plenum space prevents installing one unit per room.
How do I size for a heat pump system at 45°C supply water? Use the 45°C heating capacity figure, not the 60°C figure. Capacity retention is typically around 60%, so terminals must be upsized accordingly. Confirm the manufacturer publishes both temperatures before specifying.
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