FAN COIL ENERGY SAVING MADE SIMPLE
26/08/2008
It is possible to achieve significant energy savings on the ubiquitous fan coil unit. Barry Trewhitt , Fan coil Product Manager at TROX UK, explains
The fan coil unit is a well-established, well-liked terminal system for controlling room temperature because it is relatively simple air conditioning technology with a reputation for consistent and dependable performance. But there is a problem. With a conventional constant volume fan coil system, the motors always run at the same speed and the fan continues to run at full blast whether the room needs to be cooled, heated or is at the right temperature. This is a terrible waste of energy.
But it doesn’t need to be this way. With a variable air volume (VAV) system, the motors are not running at 100 per cent all the time – they vary in speed depending on the demand for cooling or heating. This cuts energy consumption and reduces emissions significantly.
By using VAV fan coils you can save something like 3.5W/m² and that could represent as much as a 5 per cent saving on emissions from the building. The Part L2 saving target is, of course, higher than this – 28 per cent better than the 2002 levels – but these savings do not come about all at once; they are inevitably made up of a whole series of savings which incrementally add up to or exceed the target.
Only a small speed reduction of 20 per cent can cut in half the energy consumption from the fan motor. On a building of, say, 10,000m² with 400 fan coils this could equate to an energy saving of £15,000 to £20,000 per annum.
That makes VAV fan coils an important energy saving development. However, it comes at a price – if they vary the air volume, designers must be extra careful to select diffusers that match. The speed variability of VAV fan coils itself makes the selection of the grilles and diffusers more critical because they need to work at a range of volumes. That is why it makes good commercial sense to adopt a systems approach to match the grilles and diffusers to the fan coils.
Systems engineering involves providing fan coil units, diffusers and grilles from a single source, with guaranteed noise levels, thermal performance and air distribution. Essentially, this means looking at the overall system in terms of duct design, joint scheduling of fan coils and grilles/diffusers, with a view to offering guarantees on performance. This will offer peace of mind to the designers and minimise their risks when designing fan coil system.
Starting with the original design, a systems approach ensures the best possible solution is developed. This also allows for project cost savings through a process of value engineering, off-site fabrication, and pre-commissioning of units.
Nonetheless, there is another potential problem with VAV fan coil units that does not apply to constant air volume systems; if you vary the volume and go below a certain airflow limit you run the risk of air coming down off the ceiling because it doesn’t sustain enough energy to remain up there. This is called “dumping”.
The hysteresis effect means that, once the dumping has started, to ensure that the air returns to the ceiling, the air volume must be raised above what would normally be required to keep the air at ceiling level. And that means using more energy.
Dumping can have serious consequences for the comfort in a space and that is why it pays to fit anti-dumping devices in VAV applications, such as our own Varyset plenum. This 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. See picture 1.
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.
Fan coil performance data
There is a lack of trust among designers in some fan coil manufacturer’s data, particularly sound levels. When TROX conducted our own market research, it contacted over 400 design engineers. Seventy nine per cent said they would specify a mock up test to ensure that the fan coil system performed according to their specification and more than half claimed to have experienced serious noise problems on at least one of their projects.
Fan coil manufacturers have to win back the trust of design engineers and the only way this can be achieved is if manufacturers are willing to offer guarantees on performance of the system acoustically, thermally and aerodynamically.
