Can fan coil unit fan speed impact FCU heat output?
Altering the speed of the fan in a fan coil unit (FCU) can affect the unit’s heat output; however, this is only possible when the water circulating through the fan coil is at an adequate flow rate. The water supplied to the FCU provides the energy required for heat transfer and may be either chilled water (CHW) or low temperature hot water (LTHW). The fan within the unit moves air across the heat exchanger (coil), enabling energy transfer and delivering either cooled or heated air into the conditioned space.
When chilled water passes through the coil, it removes energy from the warmer air drawn across it by the fan. This allows the FCU to deliver comfort cooling to the space and is typically the primary requirement when selecting FCUs. Conversely, when hot water flows through the coil, the air moving across it is warmed, enabling the unit to provide space heating. This dual functionality is increasingly common in modern developments, where FCUs are used to deliver both comfort cooling and heating within apartments.
2- and 4-pipe fan coil units for heating and cooling
A two-pipe FCU incorporates a single coil and two pipes—one for flow and one for return—connected to a single water circuit. This arrangement is commonly used where only cooling is required, or where heating and cooling operate on a seasonal changeover basis (2-pipe changeover). Typical applications include residential apartments and hotels. Where simultaneous heating and cooling are required, a four-pipe FCU is used. This configuration includes separate coils and pipework for heating and cooling and is commonly specified in office environments.
The water supply to both two-pipe and four-pipe FCUs must be maintained at the required flow rate and temperature to achieve the unit’s design heating or cooling capacity, referred to as its thermal duty. The fan speed governs the airflow rate and therefore the volume of air delivered by the unit. Adjusting the fan speed can be used to regulate the level of heating or cooling provided, alongside modulation of the water flow rate through 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 fan determines the air volume flow rate, measured in litres per second (l/s), that is delivered into the space. For HVAC designers, it is the air volume delivered rather than the fan speed itself that is the key performance parameter. The required airflow will depend on several factors, including the design of the FCU, the pressure conditions within the space, the thermal duty and the acoustic requirements of the installation.
The fan speed must be sufficient to enable effective heat exchange between the water in the coil and the air passing across it, ensuring the desired thermal performance is achieved.
Historically, FCUs were often fitted with AC motors, where fan speed could only be adjusted in fixed steps. Modern units are typically equipped with EC (electronically commutated) motors, which operate using a 0–10V DC control signal. This allows continuous modulation of fan speed across the full operating range. The control voltage corresponds directly to the airflow delivered by the unit, from zero output at 0V to maximum airflow at 10V. This enables the system to be set to deliver the precise airflow required to meet the design thermal duty.
Because the control signal is variable, fan speed can be finely adjusted to increase or decrease airflow and, in turn, influence heating or cooling output. However, as noted earlier, the impact of fan speed adjustments is limited by the amount of energy available from the water supplied to the FCU.
Where can I find more information on designing effective fan coil solutions?
Ability offers a comprehensive range of fan coil units designed to provide quiet, efficient and cost-effective solutions across residential, commercial and hotel applications. We regularly design, manufacture and test bespoke FCU solutions to meet specific project requirements.
Our experienced technical team is available to support you in identifying the most appropriate comfort cooling or space heating solution for your project. If you would like further information, please get in touch, and we will be happy to assist.
