Barry Trewhitt, Fan Coil Product Manager for TROX UK, discusses energy issues related to Variable Air Volume Fan Coils (VAV/FCU)
Fan coils have been one of the most popular terminal air conditioning systems for the past 40 years. They are a very forgiving system in that there are so many variables such as water flow, water temperature, fan speed, which could be adjusted to achieve a comfortable working environment.
Over the past few years we have seen the popularity of multi service chilled beams (MSCB) increase to such an extent in value terms the two market sectors, beams and fan coil units, are similar.
Much of this gain is due to the energy credentials of chilled beams and the change within the building regulations. Whilst the pros and cons of each system are much wider the focus for fan coils must be how they can be more energy efficient.
“With a conventional fan coil system the motor always runs at the same speed irrespective of whether the room needs to be cooled, heated or is at design temperature. Whilst constant air volume has the advantage of constant sound level and no air dumping from the diffuser it has a major disadvantage of consuming more energy than is required. EC motors are becoming increasingly popular, but instead of running at a constant speed greater savings will be achieved by adopting VAV principles. If the air volume for each fan coil was reduced from 100% to 80% (a small reduction in air flow and barely discernable to the room occupants) the motor energy would, based on The Fan Laws, be reduced by 50% - a significant saving in power.
If the air volume was reduced from 100% to 60% there would be an 80% reduction in motor energy.
The above theory needed to be confirmed by conducting a project type test in a special BSRIA approved test cell. Certain project design parameters were established and these are summarised below in Table 1.
Table 1 – VAV Fan Coils Design Parameters __________________________________________________________________________________
Cooling ~ 100 W/m2 Perimeter Zone
1 Fan Coil / 25m2
Air flow ~ 10-12 l/s / m2
CAV Fan Coil Motor 4 – 5 W/m2
In a typical zone the cooling load percentage against the time run in a year has been worked out as follows:-
% of load Hours/Year
100 475
80 385
60 2000
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Table 2 - The results are summarised as follows:-
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|
Energy Consumption Overall running for one fan coil for a year |
||||
|
Fan Type |
kVAh |
Kg CO2/ kWh *` |
Cost (£) ** |
Motor Consumption W/m2 |
|
AC CAV |
447 |
166 |
26.37 |
4.5 |
|
ECVAV |
149 |
42 |
8.79 |
1.11 |
|
MOTOR WATT CONSUMPTION REDUCED TO ¼ * Based on 0.43 kg CO2/kWh ** Based on 5.9p kVAh |
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As can be seen the reduction in motor energy went from 4.5W/m2 down to 1.11W/m2, a significant and worthwhile saving in energy.
So the potential pay-off of Variable air volume fan coils using high efficiency EC motors is enormous. However, 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.
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 take away the concerns of designers of air dumping with variable air volume fan coil units (VAV/FCU) and minimise the risks when designing these types of systems.
Starting with the original design, a system approach will ensure the best possible solution is developed. This will also allow for project cost savings by value engineering, off-site fabrication, and pre-commissioning of units. Indeed, I believe that offsite fabrication will become increasingly common as the shortage of skilled site labour starts to bite, and as demands grow for better build consistency and efficiency, reduced installation times and increased fast track programming.
Factory prefabrication and pre-commissioning offers two big benefits in particular:
• Fan coil controllers can be factory set for design air flow and pressure independent modulating water valves can be factory set for design water flow.
• Cost - Construction costs are significantly lower because the construction process is faster, there is a reduction in the number of trades, and the process is more predictable, of higher quality, less wasteful and safer.
On top of this a single point responsibility makes performance guarantees more easily available because of the focus on one manufacturer. Variable air volume fan coils (VAV/FCU) could give these systems an energy saving option to chilled beams, particularly with elevated chilled water temperatures.
Benefits of VAV Fan Coils.
• Energy efficient when compared with constant volume units.
• Exceed the requirements of the Building Regulations and reduce carbon emissions.
• High motor efficiency leading to lower specific fan power
• Efficient speed control
• Lower maintenance costs due to longer bearing and motor life
