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Fan Coil Units: Meeting conflicting demands on FCUs

The demands placed on fan coil units in modern commercial and industrial applications can be particularly challenging. Are they up to the job? Ian Thomas has the answer.
Fan Coil Units: Meeting conflicting demands on FCUs
Demands on air conditioning systems continue to grow. Systems must be economically viable, while also meeting the latest energy requirements laid down in Part L of the Building Regulations. They must provide good indoor air quality, and high levels of comfort. In addition there must be sufficient flexibility within the system design to accommodate changes in layout and usage of the occupied space in the future.

Given the demanding nature of the task, can long-established technologies, such as fan coils, keep pace with today's requirements?

I believe the answer is yes. The fan coil equipment available some years ago was undoubtedly too static to meet these challenges. But fan coils have evolved significantly over recent years, making it possible to design dynamic demand based systems that only use energy when required, and respond much more effectively to fluctuations in building load and occupancy of individual spaces.

An important step towards identifying best practice when designing fan coil systems, is to consider each of the following challenges equally:

• Energy efficiency.
• Future-proofing for office churn and change of use.
• Fresh air supply.
• Thermal performance.

This article reviews the relevant standards governing these demands, and suggests potential system design solutions.


Trox pioneered the development of VAV (variable air volume) EC fan coils and has welcomed the gradual migration of the UK market towards this highly energy efficient generation of units. EC motors with SFPs capable of less than 0.2 w/l/s have reduced the gap between fan coils and chilled beams from an energy perspective. Harnessing the EC motor's infinitely variable speed control with the use of a 0 to 10 volt signal further enhances the energy savings by turning down the air volume to a specified level 80 or 60 per cent as rule of thumb depending on the diffuser choice.

The energy efficiency benefits of this type of technology are no longer purely theoretical. Today they can be quoted confidently with supporting data derived from use of the latest building simulation software. For example, information gained using EDSL TAS software, analysing the energy efficiency performance of Trox fan coils installed in real room mock ups, has given the hvac industry a robust benchmark from which to quantify and substantiate carbon reduction claims made to clients.

The perennial challenge for the system designer is to achieve impressive performance in energy efficiency while also maintaining acceptable conditions within the space to maximise the health and productivity of occupants.


Two national standards help to define what indoor thermal comfort actually means for the system designer. CIBSE provides guidance on thermal comfort criteria, and BS EN 13779:2007 provides a definition of the occupied zone in which these conditions must be maintained. The zone is usually defined as 1.8 m high and a minimum of 150 mm from any wall.

The comfort conditions are governed by how the space will be used. For example, a typical office application would require an air velocity of not more than 0.25 m/s in the occupied zone and a temperature differential not greater than +/- 1.5 K in cooling. These indices are used by Trox when measuring indoor thermal comfort in our air distribution laboratories, during development of new products, and when testing system designs on behalf of customers.

While there may be a conflict between the demands of reducing energy consumption and the achievement of indoor thermal quality, there is also a synergy in the case of fan coil systems. Fan coil turn down has obvious energy efficiency benefits, but by turning down the fan coil where it will run for the longest period, we can achieve a quieter system which will also achieve higher levels of comfort as a result of the lower air volume.

How much to turn down? This depends on the inherent performance of the diffuser chosen and how it is sized. A correctly sized diffuser will turn down further than one which has been oversized, avoiding dumping where comfort conditions in the space are not maintained. Ceiling diffusers such as our own VSD linear slot diffuser SDW high induction swirl will enable the best turn down on the VAV fan coil system before dumping occurs.

Fan coils often come into their own in applications where system flexibility is needed to deal with office churn or changes of use. The ability to easily adjust diffuser positions can make it possible to adapt the existing system for new office layouts and partitions, without the need for further capital outlay or costly refurbishment.


But what about the need to supply fresh air? The minimum amount of fresh air required in the building is governed by Part F of the Building Regulations. In many applications, such as educational buildings and offices, it is essential to get this aspect of the indoor environment right to enable occupants to concentrate and remain productive.

In this energy-conscious age however, supplying more fresh air than is needed can waste energy and contribute to unnecessary carbon emissions.

A solution is to use a pressure independent VAV controller. Used in conjunction with an air quality sensor, these can manage the indoor air quality by varying the quantity of fresh air supplied to the occupied zone. This guarantees the provision of fresh air, when it is needed, but reduces it to a minimum level when it is not required.

While the demands placed upon air conditioning systems are increasing, the latest generation of fan coils are designed ideally to meet the challenges. By considering system design as a whole - balancing energy, comfort, cost and flexibility - there is no reason why longer-established technologies, with a proven track record, cannot deliver the air conditioning requirements of discerning clients into the future.
11 February 2011

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