What impact does fan coil unit fan speed have on heat output?
Altering the speed of the fan in a fan coil unit (FCU) can influence the unit’s heat output; however, this will only happen if the water passing through the fan coil is moving at a sufficient flow rate. The water supplied to the fan coil unit provides the energy for heat transfer and can either be chilled water (CHW) or low temperature hot water (LTHW). The fan within the FCU passes air over the heat exchanger (coil). This facilitates an exchange of energy and provides either cooled or warmed air to a room or space.
Chilled water flowing through the coil extracts energy from warm air being drawn over it by the fan. This allows the FCU to provide comfort cooling to the space and is the most common requirement for FCU selection. However, if heated water is flowing through the coil, air passing over it will be warmed up, allowing the FCU to supply space heating. This is increasingly seen in modern developments where FCUs supply both comfort cooling and space heating throughout apartments.
2- and 4-pipe fan coil units for heating and cooling
A two-pipe FCU uses one water coil and two pipes—one for supply and one for return—connected to a single water source. This is the most common, where only cooling is required or in buildings where heating and cooling do not occur at the same time, but rather seasonally (2-Pipe Changeover). Residential apartments and hotels are the most common applications. Where both heating and cooling are needed simultaneously, a four-pipe FCU is used. It has separate coils and piping for heating and cooling. These types of systems are commonly used in office buildings.
The water supply to either a two-pipe or a four-pipe fan coil unit should be kept available to meet the design flow rate and temperature to achieve its required cooling or heating capacity, known as its thermal duty. The fan speed of the unit controls the rate of air flow and, therefore, the air volume that the FCU can deliver. Varying the fan speed can therefore be used to adjust the level of comfort cooling or space heating provided by a fan coil unit, along with varying the water flow rate via the control valves.
How is fan speed of a fan coil unit linked to its air volume flow rate and thermal duty?
The speed of the FCU’s fan will determine the air volume flow rate, measured in litres per second (l/s), that the FCU can deliver to a room or space. It is the air volume output that a fan delivers, rather than the speed at which it runs, that is the key metric for HVAC designers. The optimum FCU air volume flow rate will depend on a number of factors. These include the design characteristics of the FCU itself, the pressure within the room being conditioned and the thermal duty required along with the design acoustic level. The fan speed needs to be sufficient to facilitate enough energy transfer between the water in the coil and the air passing over it, to meet the required thermal duty.
Historically, many fans in FCUs had AC motors where the fan speeds could be changed, but only by set stepped amounts. Modern FCUs use EC (Electronically Commutated) motors. These typically run with a 0 to 10V DC control signal that allows variable fan speed control throughout the whole voltage range. The voltage relates to the air volume flow rate that the FCU can deliver, from no air being delivered at 0V, up to the maximum capable air volume for the unit when the control is set to 10V. An optimum voltage can be determined to deliver the air volume flow rate required to meet the designed thermal duty.
As the control voltage to the fan is variable, it allows fine adjustments to be made to its speed. This can increase or decrease the air volume flow rate of the FCU and therefore affect its cooling or heating output. Although, as discussed earlier, there will be limits on the effect that fan speed adjustment alone can have, as it is dependent on the energy available from the water supplied to the FCU.
Where can I find more information on designing effective fan coil solutions?
Ability has a range of fan coil units that offer quiet, highly efficient, and cost-effective solutions for residential, commercial and hotel applications. We routinely manufacture and test bespoke FCU solutions for our clients to ensure their project objectives are met.
We have a vastly experienced team on hand to help you find the best comfort cooling or space heating solution for your project. If you would like to learn more, please contact us and we will be happy to help.
Troy Chambers
