How to Size a Fan Coil Unit

Best Practices to Ensure Comfort and Efficiency

Fan coil units are used in HVAC systems to provide heating, cooling and dehumidification for individual zones or spaces. They work by using a heat exchanger to transfer energy between water and air, coupled with a fan to circulate conditioned air throughout the room.

Typically, cold water from a chiller is used to cool and dehumidify air, while hot water from a boiler is used to warm air. Properly sizing a fan coil unit is essential to ensure comfort, efficiency and quiet operation.

What Determines Fan Coil Sizing?

For most regions in North America, the sensible cooling capacity is the primary factor for sizing a fan coil. Sensible cooling refers to reducing the air’s dry-bulb temperature without changing its state (e.g., condensation). The fan coil’s sensible cooling capacity must meet or exceed the space’s sensible cooling load to maintain the temperature set point.

How to Calculate the Sensible Cooling Load

To calculate the cooling load for a room, consider these key factors:

• Environmental conditions: local climate, outdoor temperatures and solar loads
• Building construction: walls, roof, insulation and windows
• Occupant activity: the number of occupants and their activity levels
• Infiltration: air leakage and ventilation rates
• Internal gains: heat generated by lighting, appliances and equipment

HVAC software tools such as Trane TRACE, Carrier HAP and Revit with the Ripple HVAC tool are commonly used to perform these calculations. Additionally, ASHRAE-approved methods such as the heat balance method or the radiant time series method provide accurate load assessments.

Sizing a Fan Coil Using Sensible Cooling Load

Once the sensible cooling load is calculated, we can determine the required air supply volume in cubic feet per minute (cfm) using the sensible heat equation:

For example, say our project has a cooling load of 8,500 BTUH, a supply air temperature of 55°F and a return air temperature of 75°F. We can calculate the required air supply volume as follows:

Rule-of-Thumb Alternative

As a quick estimate, you can assume 400 cfm per ton of cooling (12,000 BTUH). Note, however, that this approach may result in colder supply air temperatures than desired for some commercial applications.

Selecting a Fan Coil Unit Based on Supply Airflow

Once the design day supply airflow is calculated, we can select a fan coil using the manufacturer’s selection tools, such as the Price Excel Toolkit, or by consulting a local sales representative. Alternatively, we can analyze manufacturer-provided fan performance curves to confirm that the unit meets the required supply airflow at acceptable noise and energy levels.

 Best practices for selection include the following:

• Choose a fan coil that operates at less than 75% of its maximum rated airflow for optimal efficiency and quieter operation.
• Confirm compatibility with design conditions, including coil type (two-pipe versus four-pipe systems), and optimize the coil rows and circuits to achieve the best chiller and pump operation.

This simplified approach ensures accurate sizing of fan coil units, delivering comfortable and efficient performance for your space.

Price’s project and application engineers are ready to assist design engineers, building owners, architects and mechanical contractors with product choices that can optimize air distribution system efficiency.

Click here to find a sales representative or technical expert near you, or reach out to our team at airmovement@priceindustries.com.