A fan coil unit (FCU), also known as a Vertical Fan Coil Unit (VFC), is a device consisting of a heat exchanger (coil) and a fan. As part of an HVAC (Heating-Ventilation-Air-Conditioning) system found in residential, commercial and industrial buildings, a fan coil unit is usually connected to ductwork and a thermostat to regulate the temperature of one or more spaces and assists the main air-handling team in each space. The thermostat controls the fan speed and/or water flow to the heat exchanger using a motorised control valve.

Fan coil units, due to their simplicity and flexibility, can be more economical than ducted 100% fresh air systems (VAV) or air handling units or cold beam central heating systems. Various unit configurations are available, including horizontal (ceiling-mounted) or vertical (floor mounted).

Noise output from FCUs, like all other types of air conditioning, is mainly due to the design of the unit and the surrounding building materials. Some offer noise levels as low as NR25 or NC25

The output from an FCU can be established by looking at the temperature of the air entering the unit and the temperature of the air leaving the unit, together with the volume of air being moved through the unit. This is a simple statement and there is further reading on sensible heat rates and the specific heat capacity of air, both of which affect thermal performance.

Design and Management

Fan Coil Unit covers a range of products and has different meanings for users, specifiers and installers in different countries and regions, especially regarding product size and output capacity.

Fan Coil Unit is mainly divided into two main types: blowing and pulling. As the names suggest, fans of the first type are installed behind the heat exchanger and fans of the other type are installed in front of the coil to draw air through it. Pull units are considered thermally superior, as they normally make better use of the heat exchanger. However, they are more expensive as they require a chassis to hold the fans, whereas a blow unit typically consists of a series of fans bolted directly to a coil.

A fan coil unit can be concealed or exposed within the room or space it serves.

An exposed fan coil unit may be wall-mounted, freestanding or ceiling mounted and will typically include a suitable enclosure to protect and conceal the fan coil unit with a set of return air grille and supply air diffuser to distribute the air.

A concealed fan coil unit will typically be installed in an accessible ceiling void or service area. The return air grille and supply air diffuser, typically located flush to the ceiling, shall be ducted to and from the fan coil unit, thus allowing a great degree of flexibility for the grilles to match the ceiling layout and/or partition layout within a cavity. It is quite common for the return air not to be ducted and the ceiling void to be used as a return air plenum.

The coil within the fan coil receives hot or cold water from a centralised plant and reduces or increases the heat in the air through heat transfer. Traditionally fan coil units may include their internal thermostat or be wired to work with a remote thermostat. However, and as is common in most modern buildings with a Building Energy Management System (BEMS), control of the fan coil unit will be by a connected local digital controller or external station (with associated room temperature sensor and control valve actuators). The BEMS can be set and controlled via a communication network and therefore from a centralised point such as a supervisor’s head computer.

Fan coil units circulate hot or cold water from a coil to condition a space. The team receives hot or cold water from a central plant or mechanical room containing equipment to remove heat from the closed loop of the central building. The equipment used may consist of machines used to remove heat, such as a chiller or cooling tower, and equipment used to add heat to the building’s water, such as a boiler or commercial water heater.

Fan coil units are divided into two types: Two-pipe fan coil units and four-pipe fan coil units. Two-pipe fan coil units have one (1) supply and one (1) return pipe. The supply pipe supplies cold or hot water to the unit, depending on the time of year. Four-pipe fan coil units have two (2) supply pipes and two (2) return pipes. This allows hot or cold water to enter the unit at any given time. Since it is often necessary to heat and cool different areas of a building simultaneously due to differences in internal heat loss or heat gains, a four-pipe fan coil unit is most commonly used.

Fan coil units can be connected to piping networks using a variety of topology designs such as “direct return”, “reverse return” or “split series”. See ASHRAE Handbook “2008 Systems and Equipment”, Chapter 12.

Depending on the chilled water temperatures selected and the relative humidity of the neighbourhood, it is likely that the cooling coil will dehumidify the incoming air stream, and as a by-product of this process will produce condensate, which must sometimes be carried to the drain. The fan coil unit will incorporate a purpose-designed drip tray with a drain connection for this purpose. The simplest means of draining condensate from more than one fan coil unit will be a network of pipework laid in such a way that it falls at a convenient point. Alternatively, where space is limited for such gravity pipework a condensate pump can be utilised.

Speed control of the fan motors within a fan coil unit is used in part to control the desired heating and cooling output from the unit. Some manufacturers accomplish speed control by adjusting taps on an AC transformer that supplies power to the fan motor. Typically this requires adjustment at the commissioning stage of the building construction process and is therefore set for life at a constant speed. Other manufacturers provide specially wound Permanent Split Capacitor (PSC) motors with speed stages in the windings, set to the desired speed levels for the fan coil unit design. A simple speed selector switch (Off-High-Medium-Low) is provided for the occupant in the local room to control the fan speed. Typically this speed selector switch is integrated into the room thermostat and is either manually set or automatically controlled by the digital room thermostat. Building Energy Management Systems can be used for automatic fan speed and temperature control. Fan motors are typically AC Shaded Poles or Permanent Split capacitors. More recent developments include brushless DC designs with electronic commutation. Compared to units with asynchronous 3-speed motors, fan coil units with brushless motors will reduce power consumption by up to 70%. [1]

DC / EC Motorised Units

These motors are sometimes called DC motors, sometimes EC motors and sometimes EC/DC motors. DC stands for Direct Current and EC stands for Electronically Modified.

DC motors allow the speed of the fans in a Fan Coil Unit to be controlled by a 0-10 Volt input ‘Signal’ to the motor(s), transformers and speed switches associated with AC Fan Coils are not required. Up to a signal voltage of 2.5 Volts (may vary with different fan/motor manufacturers) the fan will be in a stopped state, but as the signal voltage increases the fan speed will increase smoothly until the maximum value is reached at a signal voltage of 10 Volt voltage. Fan Coils will generally operate between about 4 Volts and 7.5 Volts because air volumes below 4 Volts are ineffective and above 7.5 Volts the Fan Coil will probably be too noisy for most commercial applications.

The 0-10 Volt signal voltage can be adjusted with a simple potentiometer and left or the 0-10 Volt signal voltage can be delivered to the fan motors by the terminal controller on each of the Fan Coil Units. The former is very simple and inexpensive, but the latter offers the opportunity to continuously vary the fan speed depending on various external conditions/influences. These conditions/criteria can be ‘real time’ demand for heating or cooling, occupancy levels, window switches, time clocks or any number of other inputs from the unit itself, the Building Management System or both.

The reason why these DC Fan Coil Units are becoming more popular, despite their apparent relative complexity, is their improved levels of energy efficiency compared to their AC motorised counterparts of only a few years ago. A direct changeover from AC to DC will reduce electricity consumption by 50%, but the implementation of Demand and Occupancy dependent fan speed control can take the savings up to 80%. In areas of the world (such as the UK) where there are legally enforceable energy efficiency requirements for Fan Coils, DC Fan Coil Units are fast becoming the only option.

Areas of Use

In high-rise buildings, fan coils can be stacked vertically, stacked floor to floor, and all interconnected by the same piping circuit.

Fan coil units are an excellent distribution mechanism for hydronic chiller boiler systems in large residential and light commercial applications. In these applications, fan coil units are installed in bathroom ceilings and can be used to provide unlimited comfort zones with the ability to close off unused areas of the structure to save energy.

Installation

In high-rise residential construction, typically each fan coil unit requires a rectangular passage in the concrete slab on which it sits. There are usually 2 or 4 pipes made of ABS, steel or copper running through the floor. The pipes are usually insulated to prevent condensation from forming with cooling insulation such as acrylonitrile butadiene/polyvinyl chloride (AB/PVC) flexible foam (Rubatex or Armaflex brands) in all pipes or at least in the chilled water lines.

Apparatus

A Unit Ventilator is a fan coil unit used predominantly in classrooms, hotels, apartment buildings and condominium applications. A unit ventilator may be a wall-mounted or ceiling-suspended cabinet and is designed to use a fan to blow outside air along a coil, thereby air conditioning and ventilating the space it serves.