Fan Coil Unit Systems

  • 4 Jun 2015

What are they?

A fan coil unit (FCU) contains a fan which draws the air in a space into the unit then blows it over a cooling or heating coil. The air comes out of the FCU either cooler or hotter than before. They are used in some office buildings and shopping centres and typically specified where there are multiple small spaces requiring individual control.  Typically an individual FCU serves only up to 150m², so there can be tens or even hundreds in a building.  FCUs are, however, most commonly used as a supplement to a building for which other HVAC systems provide the majority of the air-conditioning.

FCUs will generally have a chilled water coil for cooling and either a hot water coil for heating or an electric heating element.  Chilled water is provided from a chiller located in the central plant, and hot water form a boiler.

Each FCU is provided with a small supply of outside air to ensure adequate ventilation.

Advantages / Disadvantages

  • High level of flexibility in terms of subdivision and rearrangement of space.
  • Poorly suited to open plan spaces, as adjacent units can operate in conflict
  • Does not generally have the ability to use outside air for free cooling, making the system less efficient in cooling particularly in spring and autumn.
  • Chilled water valves and hot water valves in ceiling can be a maintenance problem

Energy efficiency

FCU systems are of average efficiency only but if well maintained and operated can produce a good level of efficiency.  They are generally not as efficient as standard alternatives such as variable volume air conditioning, chilled beams and displacement systems.

Running costs

The age and condition of FCU systems will significantly affect running costs.  Older systems tend to have higher rates of temperature controller failure, poorly operating valves and inefficient central plant, all of which can increase costs.

As with all systems, efficiency will be improved by good management and  commissioning, especially in relation to the calibration of temperature sensors and the maintenance of valves.  A building management system (BMS) will provide a higher level of control and if well managed will help reduce running costs.

Retrofit / improvement opportunities

The major areas for improvement for FCU systems are:

  • Review and optimisation of zone temperature control for FCUs.
  • Recommissioning and maintenance of chilled water and hot water flow balancing and valve operation.
  • Upgrade of controls for boilers, chillers and associated pumps
  • Upgrade of chiller plant.  Chillers have 15-20 year lifespan, although they generally have been superseded in terms of efficiency well before the end of their operational life.  Replacement of chillers is best undertaken in winter.
  • Upgrade of FCUs.  Ageing FCUs can be replaced by modern units which are quieter, more energy efficient and more reliable.

Control improvements can be implemented with the tenants in-situ

Applicable buildings

FCUs are found as a secondary HVAC system in most building types but are less common as a primary HVAC system type, except in shopping centres and smaller offices.

Floor plate implications

FCUs are typically ceiling mounted and thus do not take up floor space.  Positioning of existing FCUs may adversely affect the subdivision  of space. Units can, however, be moved and / or supplemented to accommodate different floor space configurations.

Temperature control / Occupant comfort

FCUs are generally considered a lower grade solution for office air-conditioning due to fan noise.  They can deliver good control when well maintained but the small zone size and high component count often leads to occupant discomfort.

Maintenance implications

FCUs are simple to operate and are relatively straightforward to maintain, but the components that require most maintenance – the temperature controllers and the valves – are in the occupied space and therefore maintenance can be intrusive.

Identification

An FCU will typically be installed in the cavity above a ceiling. The visible parts will be either a pair of supply and extract grilles, or a ceiling cassette.  FCUs may also be found mounted on the wall, on the ground, or occasionally in a floor void.

Maintenance or operational staff will be able to advise whether a building uses FCUs.

Questions to ask

  • Is the heating electric or via hot water?
  • Is there a building management system (BMS)?
  • Is the system regularly maintained (at least annually) and seasonally commissioned?
  • How often are the filters cleaned?

More information

AIRAH Guide DA08 HVAC&R An Introduction

CIBSE Ventilation / Air conditioning / Technical Glossary