Selecting air conditioning fan coil units is crucial in system design. Designers match units to the room’s peak cooling load to ensure proper temperature during maximum demand. However, rooms rarely stay at peak load, leading to excess cooling capacity. Units switch to lower gears, reducing cooling output and maintaining thermal balance. Cooling capacity must meet the calculated load, while sensible and latent heat must match the room’s heat-humidity ratio. Air volume must meet temperature, ventilation, and airflow needs.
Many people assume coil technology is similar across brands and focus only on price. However, power consumption can vary significantly. The difference between maximum and minimum power consumption of similar products can exceed 20W. The following discusses selecting fan coil units, key considerations, and the impact of parameter changes on performance.
fan coil unit
How to select fan coil units
Air conditioning Fan coil have two key parameters: cooling capacity and air supply capacity. Designers use two selection methods:
Select by the room’s circulating air volume:
Calculate the circulating air volume by multiplying room area, floor height, and ventilation rate. Determine the model using the high or medium-speed air volume of the fan coil unit.
Select by the room’s cooling load:
Calculate the required cooling load using the unit area load and room area. Determine the model using the cooling capacity matching the room’s cooling load.
Notes on fan coil selection
Cooling capacity
The main reason for insufficient cooling capacity is that many companies do not have their own testing methods, the parameters on the samples are also copied from other manufacturers, and the thermal performance of the coils they produce is poor (mainly caused by the fin form, expansion tube quality, production process, etc.). Therefore, it is recommended to pay attention to the manufacturer’s testing facilities and methods when conducting project inspections. It is difficult to imagine that a manufacturer without its own testing equipment can produce good products.
Cooling capacity/heating capacity: Determine the required cooling capacity/heating capacity based on the cooling load/heating load of the room, generally calculated at 100-150W/㎡.
Air volume
How to consider the air volume of the coil is a problem. Most manufacturers in the domestic market have only one type of coil with three rows of tubes, and some manufacturers also provide coils with two rows of tubes. In fact, for most civil building air conditioning systems, it is more advantageous to choose coils with two rows of tubes (except for high humidity occasions). This is because the two-row products have a larger air volume under the same cooling capacity, which will increase the number of ventilation times in the air-conditioned room, which is conducive to improving the air conditioning accuracy and comfort. Under the same cooling capacity, using a small temperature difference and a large air volume to supply air will achieve a better air conditioning effect than a large temperature difference and a small air volume.
Air volume: Determine the required air volume based on the room area and the number of people, generally calculated as 15-20㎡/h·person.
External residual pressure
The national standard requires testing fan coil air volume, cooling, and noise under zero external static pressure. In practice, air ducts and shutters often connect to the coil’s air outlet. Some projects also add return air boxes. As a result, the actual air volume is less than the nominal value. This reduces room air volume and increases supply air temperature difference. It also lowers air conditioner comfort. To prevent this, some designers choose the coil’s mid-range air volume. This avoids insufficient air volume but raises initial project costs. When standards remain unchanged, designers should select units with residual pressure of 10-15Pa.
For air pressure, designers calculate it based on air duct resistance and outlet layout. Typical values range from 50 to 100Pa.
Noise
When we examine a manufacturer’s products, we should check the noise test report of the product issued by the national authoritative quality inspection department (it must be the products we are going to order. For projects with large batches, we should take samples on site and send them to the relevant quality inspection department for testing.
Noise: The noise level of the fan coil unit is determined according to the environmental requirements of the place of use, generally calculated as 35-50dB(A).
Effect of parameter changes on performance
When the fan coil air volume is constant and the water supply temperature is constant, the cooling capacity changes with the change of water supply. According to the performance statistics of some fan coil products, when the water supply temperature is 7 degrees and the water supply is reduced by 80%, the cooling capacity is about 92% of the original, indicating that the effect on the cooling capacity when the water supply changes is relatively slow
When the fan coil water supply and return water temperature is constant, the cooling capacity decreases when the water supply temperature increases. According to statistics, when the water supply temperature increases by 1 degree, the cooling capacity decreases by 10%. The higher the water supply temperature, the greater the reduction, and the dehumidification capacity decreases.
When the water supply conditions are constant and the fan coil air volume changes, the cooling capacity and the air conditioning treatment enthalpy difference change. Generally, the cooling capacity decreases, the enthalpy difference increases, and the power consumption per unit cooling capacity level does not change much.
When the inlet and outlet water temperatures of the fan coil increase, the water volume decreases, and the thermal coefficient of the heat exchange coil decreases. In addition, the heat transfer temperature difference also changes. Therefore, the cooling capacity of the fan coil decreases with the increase of the water supply temperature. According to statistics, when the supply water temperature is 7 degrees and the supply and return water temperature increases from 5 degrees to 7 degrees, the cooling capacity decreases by 17%.
