It is a common practice in the design of HVAC systems to select a fan coil unit according to the maximum cooling load of the air-conditioned room. The purpose is to ensure the room temperature during peak load.

That is a misunderstanding on a fan coil unit!

In fact, most of the time the air-conditioned room is running, it will not be at peak load, causing excess cooling capacity. Instead, it switches to medium and low-range operation to reduce the cooling output, thereby maintaining the thermal balance of the room.

The actual cooling output of a fan coil unit relies on variations in the air conditioning load and bears little correlation with the nominal cooling capacity of the unit.Therefore, cooling capacity is only a necessary condition to achieve air conditioning, but it cannot determine the effectiveness of air conditioning.
To evaluate the effectiveness of a fan coil unit, one is the closeness of the average room temperature to the set temperature; the other is the room temperature distribution (gradient) and change (fluctuation) amplitude. The greater the supply air temperature difference, the fewer the number of air changes, and the greater the room temperature gradient and fluctuation. Therefore, the supply air temperature difference and the number of air changes are the main factors affecting the accuracy and comfort of air conditioning.
The standard clearly stipulates the maximum air supply temperature difference and the minimum number of air changes for air-conditioned rooms with different precisions. The higher the accuracy of the air conditioner, the smaller the air supply temperature difference and the greater the number of air changes required.

When selecting a fan coil unit based on the maximum cooling load, it is essential to consider not only meeting the room temperature during peak load but also ensuring the appropriate air supply temperature difference and the required number of air changes to meet the comfort requirements of the room。