The evolution of the FCU

With a track record for being reliable and cost-effective, fan coil units (FCUs) have been used to air condition thousands of commercial buildings across the UK since the 1970s. Over recent years the controllability of FCUs has evolved, ensuring they only use energy when required and can better respond to fluctuations in demand. Andrew Saxon, Marketing Manager at Biddle Air Systems, explains how these advancements are making FCUs more energy efficient, but have also led to the development of innovative commissioning and maintenance methods.

Being widely used since the 1970s, fan coil technology is well established and remains a popular way of providing air conditioning in commercial buildings, whether in new build developments or refurbishment projects.

However, even though the fan coil unit (FCU) has earned an enviable reputation and an excellent track record for reliability and cost effectiveness, several years ago it was recognised that the technology needed to move on in terms of energy efficiency, as of course has been the case with many HVAC products.

To be energy efficient, air conditioning must be responsive, adaptable and flexible, preferably throughout its life.  In addition a building manager wants a system to be future proofed, so they can alter or add to it without major disruption or expense.

Limited control

Despite their many benefits, traditional FCUs cannot answer all these needs.  Drawing unconditioned air over a cooling/heating coil, traditional FCUs deliver a constant air volume, set in accordance with maximum requirements.  However, in reality, a constant air volume isn’t always required as internal environments are continually affected by changing weather patterns, as well as building occupancy levels, the reconfiguration of space and any increase/decrease in heat-generating office equipment such as PCs, printers and photocopiers.

Although the cooling and heating output of traditional FCUs can be adjusted via four-port water control valves, the constant water flow rate required means the pump always needs 100% of pump design power.

This limited control means that traditional FCUs are now regarded as being largely inefficient. Therefore, understanding that FCUs need to move with the times, and that in today’s energy focused world the installation of inefficient products isn’t acceptable, or permitted in accordance with Part L2 of the Building Regulations, manufacturers are working hard to ensure their products provide air conditioning in a more energy efficient, controllable and flexible way.

Time to change

The revised Part L2A and Part L2B documents rate the efficiency of FCUs in relation to their specific fan power (SPF), stating that this should not exceed 0.6W/ls-1 when measured as the rating-weighted average of the installation. Reducing the amount of energy a fan motor consumes to produce an air flow rate is therefore a critical matter in the fan coil industry – and manufacturers have quickly realised that improvements are required. In addition, the ErP (Energy related Products) directives being introduced in 2013 and 2015 have put some urgency to the issue.

Traditionally FCUs have incorporated two-pole AC fan motors, which have an inefficient SFP of 0.9W/ls-1 or higher.  A more efficient approach is the use of four-pole AC fan motors, which have SFPs ranging between 0.5 and 0.75W/ls-1.

However, this still delivers a constant air volume, when in truth we know the key to any significant reduction in energy usage is having a variable air volume that can be easily controlled.

So a more preferable approach to meeting the requirements of Part L2 A and B is the modern electronically commutated (EC) DC motor.  EC FCUs operate with slower fan speeds, which make them more efficient, but the real benefit is that the air volume can be altered in line with cooling/heating demands.  This makes them much more efficient and claims suggest they use up to 70% less energy than typical AC motors, and will operate for twice as long, resulting in lower maintenance costs.  In addition, they can be coupled with intelligent controls, programmers and building management systems to optimise energy efficiency. 

New lease of life

The EC DC motor has really given FCUs a new lease of life, and alongside this, the increased use of two-port water control valves means the water flow rate is varied and in turn the pump can run at variable speeds, making significant energy reductions.

However, it must be remembered that when a number of FCUs are fitted with two-port valves, some will be opening at the same time as others are closing, meaning the system will struggle to maintain stable control. To reduce these problems, two-port valves are combined with a differential pressure control valve (DPCV) or, better still, a pressure independent (PI) dynamic flow control valve which combines a DPCV, a two-port valve and a balancing valve all in one assembly – reducing installation work and most importantly giving a valve authority of one.

Two-port PI valves allow design water flow rates to be quickly and easily adjusted or pre-set from a remote location in relation to heating/cooling demands, without the need for specialist skills.

So, it is clear that FCUs have evolved to only use energy when required and to respond to changes and fluctuations in demand. This obviously improves energy efficiency but there is another important benefit because  the fan coil controls available today enable quicker and easier commissioning and maintenance, and have allowed more intelligent, open-minded, innovative and flexible approaches to be developed.

For example, a manifold system can be used, saving vast amounts of time. All the valves for a group of around six terminal units are housed in a single, insulated manifold box, rather than individual valve assemblies being installed at each terminal unit.

And, if a manifold system fitted with PICVs is combined with the single station balancing (SSB) technique, further benefits can be achieved. With conventional proportional balancing the commissioning engineer needs to balance the entire system before problem circuits can be identified, and again after they are solved.  This is a lengthy, disruptive process, which involves gaining access to numerous ceiling voids.

The innovative SSB technique, meanwhile, uses a ‘subtraction’ method to identify problem valves, and it is highly appropriate for variable water volume systems. It is based on knowing the design flow rates for each individual valve and therefore the total flow rate for that fan coil group. Assuming all valves are functioning correctly, isolating each valve will have a predictable effect on the total flow rate of the remaining valves. Where an unexpected result is observed, the valve causing the problem will be identified by the process of elimination. Single station balancing takes full advantage of new valve technologies and is quick, cost effective and practical.

Then there is wireless commissioning. If the valve characteristic and design flow is known then wireless control of the valve actuator can set the position required for the design flow rate. With this approach there is no need to access ceiling voids and a considerable amount of time can be saved.  It is also easy to alter the valve position if occupancy levels change or if the FCUs are reused elsewhere.

In recent times FCUs have evolved, and the development of variable air volume products has not only improved energy efficiency but has also paved the way for innovative commissioning and maintenance – undoubtedly securing the future of the FCU.