How to Size a Chilled Water Fan Coil Unit (FCU)

nexoradesign.net
3 days ago
4 min read

Selecting a chilled water fan coil unit by tonnage alone can result in poor cooling, high noise, excess energy use or humidity problems.

The selected FCU must meet the room’s total cooling load, sensible cooling load, required airflow and chilled-water flow at the project’s actual operating conditions.

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Information Required for FCU Sizing

Before selecting the fan coil unit, collect:

Room total cooling load in kW
Room sensible cooling load in kW
Indoor dry-bulb and wet-bulb temperatures
Chilled-water supply and return temperatures
Required supply airflow
Required external static pressure
Fresh-air load, if handled by the FCU
Maximum acceptable sound level
Available ceiling or installation space

Step 1: Determine the Room Cooling Load

Calculate both:

Total Cooling Load = Sensible Load + Latent Load

The calculation should consider:

Heat transfer through walls, roof and glazing
Solar heat gain
Occupants
Lighting
Electrical equipment
Ventilation air
Infiltration

Do not select an FCU using floor area alone. The accuracy of the FCU selection depends directly on the accuracy of the cooling-load calculation.

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Step 2: Calculate the Required Airflow (How to Size a Chilled Water Fan Coil Unit (FCU))

Airflow is normally estimated using the room’s sensible cooling load:

Airflow (m³/s) = Sensible Load (kW) ÷ [1.2 × Air Temperature Difference (°C)]

Where:

1.2 is the approximate volumetric heat capacity of air
Air temperature difference = Room temperature − Supply-air temperature

Airflow Example

Assume:

Sensible cooling load = 6.4 kW
Room temperature = 24°C
Supply-air temperature = 14°C

Therefore:

Airflow = 6.4 ÷ [1.2 × (24 − 14)]

Airflow = 0.533 m³/s

This is approximately:

533 L/s
1,130 CFM

The selected FCU should deliver approximately this airflow at the intended operating fan speed.

Step 3: Calculate the Chilled-Water Flow

Use the total cooling load to calculate the required chilled-water flow:

Water Flow (m³/h) = 0.86 × Total Cooling Load (kW) ÷ Water Temperature Difference (°C)

Water-Flow Example

Assume:

Total cooling load = 8.0 kW
Chilled-water supply temperature = 7°C
Chilled-water return temperature = 12°C
Water temperature difference = 5°C

Therefore:

Water Flow = 0.86 × 8.0 ÷ 5

Required Water Flow = 1.38 m³/h

Once the water flow is known, the pipe, control valve and pump must be sized for the complete chilled-water circuit. (How to Size a Chilled Water Fan Coil Unit (FCU))

👉 CALCULATE YOUR CHILLED-WATER PUMP HEAD

The Pump Head Calculator helps organize pipe-friction, fitting, valve and equipment pressure losses when determining the required pump duty.

Step 4: Select the FCU from Manufacturer Data

Use the manufacturer’s selection software or performance tables. Confirm that the FCU provides the required performance at the project’s actual conditions.

Check:

Total cooling capacity is equal to or higher than the total load
Sensible cooling capacity is equal to or higher than the sensible load
Airflow matches the calculated requirement
Chilled-water flow matches the design flow
Entering chilled-water temperature matches the project
Entering-air dry-bulb and wet-bulb temperatures match the design
Capacity is available at the intended fan speed
External static pressure is sufficient

Do not rely only on the nominal capacity printed in the model name. Nominal ratings may be based on different air and water conditions.

Step 5: Check External Static Pressure

For a ducted fan coil unit, the fan must overcome the resistance of the external air-distribution system.

Consider:

Supply and return duct friction
Bends, transitions and branch fittings
Supply diffusers and return grilles
Volume-control and fire dampers
Flexible duct connections
External filters or accessories

Confirm what is already included in the manufacturer’s published FCU rating to avoid double-counting pressure losses.

👉 EXPLORE THE HVAC DUCT PRESSURE LOSS DESIGN SUITE

Use it to calculate duct and fitting pressure losses, identify the critical air path and check the required fan static pressure.

Step 6: Complete the Final Checks

Before approving the FCU, verify:

Coil water pressure drop
Control-valve size and authority
Pipe connection size
Sound level at the selected fan speed
Condensate drain and trap arrangement
Electrical supply and motor power
Thermostat and control requirements
Filter access and maintenance clearance
Unit dimensions and ceiling-space availability
Fresh-air connection, where applicable

Selecting the duty at medium fan speed can provide quieter normal operation while keeping high speed available as a reserve.

Quick FCU Sizing Example

For a room with:

Total cooling load: 8.0 kW
Sensible cooling load: 6.4 kW
Room/supply-air temperatures: 24°C/14°C
Chilled-water temperatures: 7°C/12°C

The approximate requirements are:

Airflow: 533 L/s or 1,130 CFM
Chilled-water flow: 1.38 m³/h
Total FCU capacity: At least 8.0 kW
Sensible FCU capacity: At least 6.4 kW

These capacities must be confirmed using manufacturer data at the stated air temperature, water temperature, water flow, fan speed and external static pressure.

A small selection margin may be used when required by the project, but avoid excessive oversizing or adding multiple safety factors.