In an ideal world, every cooling system should be specified on its merits in relation to each project. John Lightfoot (right), sales and marketing director of Quartz's TEV and chair of HEVAC's Fan Coil Unit Group, argues that fan coil chilled water systems deserve a fairer crack of the whip
Have you ever wondered why most of us continue to drive around in vehicles powered by an old-fashioned internal combustion engine? It has been around for years, yet for most situations it is still the best technology for the job.

Of course, internal combustion engines have improved greatly over the years, with higher performance and greater energy efficiency. But the principles remain the same.

There are strong parallels with providing cooling for buildings: chilled water fan coil systems are tried and tested and have improved considerably but the underlying principles remain the same. Yet, a few years ago, you could have been forgiven for thinking chilled water was on its way out, in favour of direct expansion (dx) systems such as VRF (variable refrigerant flow).

Despite the marketing efforts, engineers and informed end-users have recognised the benefits of chilled water, and are gradually restoring a more sensible balance.

Of course, VRF is a good solution for some jobs but, like an electric car, it has inherent limitations which must be considered when choosing the right tool for the job.

On smaller projects, where just a few indoor units are required, single or multi-split dx systems are nearly always the most viable option. It is when you get to the larger projects that the decision often comes down to chilled water or VRF. Making the right decision depends on a number of factors.

Of all these, environmental considerations are becoming increasingly important - from duty of care to energy consumption. From a health and safety perspective, it is worth noting that refrigerants are covered by control of substances hazardous to health regulations, while water most certainly is not. Furthermore, the installation of refrigerant systems should be in accordance with the requirements of EN378 (the European version of the old BS4434) and the system maintenance and checking will come under the requirements of the F-Gas regulation.

These are some of the reasons that have led to reluctance by many engineers and end-users to pipe refrigerant around a building in a VRF system. In fact, many large organisations have a policy forbidding this. In these cases, a chilled water system, where the refrigerant is contained within a factory-built and tested product, and less refrigerant is required per kilowatt of cooling, is the most viable option.

Energy efficiency is also a major consideration. And, on larger projects where one chiller is servicing the entire installation, this will be considerably more energy efficient than a number of condensing units of similar total capacity. Chilled water systems are also able to provide higher sensible heat ratios and to exploit the opportunities for free cooling.

In some cases, a chilled water system may also be combined with thermal storage, using the chiller to charge an ice or sub-zero store overnight to supplement the cooling in the day. The result is that the building operator can reduce the building's peak electrical load and thereby avoid hitting maximum demand, in addition to making maximum use of off-peak tariffs.

Linked to overall efficiency is the ability to vary capacity in response to changing cooling requirements. Much has been made of this in relation to VRF systems. And it is true that they have become increasingly sophisticated and efficient.

But chilled water has been able to offer a much greater degree of capacity control for many years and is still the most efficient way to achieve variable capacity in larger systems. Like variable dx systems, multi-speed or multi-stage compressors can be used in the chiller to vary the capacity of the system.

Beyond this, though, is the additional flexibility of being able to vary the capacity of each fan coil from zero to 100% by reducing the flow of water through the coil. As a result, each area can be controlled to fine tolerances simply through the correct operation of the chilled water valves, thus reducing energy use.

This flexibility of chilled water systems has been enhanced by the tendency to operate with a higher flow temperature. Traditionally we used to look for chilled water at 5°C but are now frequently designing for temperatures nearer to 10°C. This provides much better control of the air from the fan coils, especially where a modulating damper is used on the air off-side. And there is greater potential for running the fan at lower speeds as there is no potential for freezing if the minimum airflow is not maintained - and noise can be reduced.

Much has been made of the ability of VRF to provide simultaneous heating and cooling. It is worth noting, therefore, that the same results can be achieved with a water-based heat pump system.
The more sophisticated of these make use of a split-coil design within the indoor units, which ensures that only the required heating or cooling capacity for each zone is provided. In addition, such a system should ensure that the installed outdoor capacity matches only the net heating and cooling requirements of the building. The system efficiencies at part load conditions are thereby maintained. Some installations using this configuration have demonstrated remarkably close control, ±0.4°C of the set point, of the occupied space. Again, the application of such highly efficient and sophisticated systems will enable the designer to meet Part L requirements.
All of these factors combine to provide greater energy efficiency, greater flexibility and a lower cost for the owner. And on the subject of cost of ownership, it is also important to consider installation factors and their contribution to life costs.

In many projects using VRF systems, the final solution represents a compromise dictated by the limitations of the overall size of each system and the need to keep pipe runs and vertical risers within strictly defined limits.

Where longer pipe runs are required because of the layout of the building, the overall performance of the system is derated. In contrast, chilled water systems are not limited by pipe run length or rise, and there is no limit to the overall size of the system or the number of indoor units it incorporates.

Because a chilled water system does not require refrigerant to be piped around the building, there is no need for refrigerant pipework. The requirement for hot working is eliminated, and pipework for chilled water and water-based heat pump systems can be installed by operatives without specialist refrigerant handling qualifications, thus reducing the need of labour and the overall cost of installation. There is now a trend towards the use of plastic pipe which shows a substantial saving in installation labour cost.

Total life costs are also affected by changes to the system after installation. With higher churn rates, many organisations require flexibility in their buildings to adapt to changes of layout and staffing levels.

With chilled water, it is easy to add fan coils to the system and change the position of existing units. With VRF systems, changes require the refrigerant to be removed from the system before reconfiguring the pipework and then operating parameters have to be recalculated in relation to changed pipe lengths.

Once the system has been recharged with refrigerant, it needs to be recommissioned - amounting to an expensive and disruptive procedure to achieve simple changes.

Amid all the hype of recent years' generated marketing, some of the virtues of chilled water have been lost. VRF has an important role to play in the armoury of comfort conditioning but each horse performs best on a particular course.