Historically a fan coil unit has been seen as a relatively uncomplicated device with a fan in a thermally and acoustically treated enclosure passing air over a cooling/heating coil.  Traditionally these units have been supplied to site in a basic form for on site fitting of electric controllers, temperature sensors, water valves, commissioning sets, etc.

With the increasing trend towards offsite prefabrication, contractors have become aware of the benefits that would accrue from this way of working which is changing the fan coil market.  For example fan coil controllers, transformers, relays and temperature sensors can be pre-assembled/wired in the factory. The fan coil controllers, especially if they are to operate over a network, can be factory programmed by the controls company.  Self balancing modulating water valves can be factory pre-set for the design water flow. Flexible hoses, isolating valves, commissioning valves can also be installed in the factory.  With the use of EC motors it is much easier to factory set the fan coil units to the correct design air volume.

Prefabrication within the construction industry now exceeds £1billion a year, according to some estimates, of which around £100million is spent on building services plant and pipework.

Off-site prefabrication enables a consistent product to be produced within a controlled environment, and it also results in a shorter production cycle and fewer trades on site, as well as delivering programme and cost certainty for the client.  Because prefabrication allows the manufacturer to test individual components, it can predict precisely how each unit will perform in a given space. This information is invaluable to the building services designer, who can take it into consideration when selecting components to create the optimum system.  Prefabrication has a dramatic positive impact on the performance of products because the build quality is tightly controlled and precisely measured.  Because of all of this prefabrication on fan coils will increase both in the numbers and the extent to which it covers other components

Another major change in the fan coil market is the utilisation of variable air volume fan coils, or VAV/FCU as they are now known.  With the increasing use of VAV/FCU where both the airflow and water flow are varied, special attention has to be paid to the diffuser selection and air distribution within the room. The air volume is normally varied from 100% to 60% where a significant saving in motor energy and carbon emissions can be achieved. Recent measurements and calculations from our own laboratories are suggesting savings of over 75% in motor power consumption.

However at 60% air volume there is a risk of the air dumping which would have a significant detrimental effect on comfort. Because of the “hysteresis effect” the air volume may have to be increased to over 80% before the diffuser stops dumping and the air reattaches to the ceiling making the need for some form of anti-dumping device imperative.

The TROX solution is The Varyset which is a simple, but clever, piece of technology that is ideal for deployment in systems with variable volume flows. It comprises a simple segmented plenum with an air operated flap that covers two thirds of the spigot.

The Varyset damper is adjusted to the desired volume flow range by means of a counterweight. When the air volume is at 100 per cent the flap is open and the Varyset operates in the same way as a standard plenum. As the air volume reduces, so the flap progressively closes until it reaches a point where the damper is closed. The air can still reach a small section of the diffuser and this constant velocity ensures no dumping and stable discharge behaviour, down to 25 per cent volume flow.

As VAV fan coils become more popular this will drive the market to a more systemised approach and enforce the importance of good air distribution engineering.

Air distribution engineering is a powerful ally in the battle to create good indoor air quality (IAQ). It can involve the use of computational fluid dynamics (CFD).  However, although CFD is a useful design tool, it still usually only offers approximate solutions. That is why there is no substitute for room testing. Indeed, the ability of manufacturers to demonstrate room air conditions via a mock-up is becoming increasingly important as designers and occupiers demand greater IAQ and higher comfort levels.

With fan coil systems the manufacturer is now taking more and more responsibility for both the fan coil unit and the air distribution through the ceiling diffusers. It is not unusual for the client to underpin the specified parameters by getting the specialist manufacturer to provide guarantees covering aerodynamics, thermodynamics and acoustics.

Download article (DOC format)

Download picture