It is possible to achieve significant energy savings on the ubiquitous fan coil unit, as 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 blatant 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.

In diagram 1 the control sequence is represented for a constant volume fan coil.  The water valves for heating or cooling are adjusted according to room temperature but the fan is allowed to run at full speed all of the time, an obvious needless waste of energy.   A few years ago maybe we wouldn’t switch the lights off when we left the office but I feel sure more effort is made now to control lighting and if you see an empty office with all the lights on you now realise this is an unnecessary waste of energy.  This is the same with constant speed, constant volume fan coils.

Since the motor power consumption follows a cube relationship to fan speed and air volume a small reduction in speed gives a big reduction in energy consumed as show in the simple table below:-

So varying the fan speed with a VAV fan coil  gives significant savings in motor power consumption.  The control sequence for a VAV fan coil unit is illustrated in Diagram 2 and shows that as the heating or cooling is reduced so the air volume is reduced proportionally and at the same time.  A limit of 60% of rated air volume will generally avoid “dumping” but selections of diffusers should be checked by air distribution engineers.  The trend is now to make the fan coil a complete package with the diffusers and ductwork design giving the benefit of single point responsibility.  There are antidumping devices on the market, such as the TROX Varyset,  a simple but clever device in the form of a segmented plenum and air operated diverter flap which allow the air volume to be turned down to 25% without dumping.

Based on a building that would meet or exceed the 2006 regulations the saving in total building carbon emissions with VAV/FCU compared to conventional FCU would exceed 20%.

With regard to running cost 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.

Test work in a dedicated BSRIA approved fan coil test cell was undertaken by TROX at its R & D centre in Thetford and the results are summarised in Table 1 and Table 2.

Table 1 – VAV Fan Coils Design Parameters

The results are summarised in Table 2 as follows:-

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 and carbon emissions reduced by a factor of 4 to 1.

So using VAV fan coils gives a readily available saving in carbon emissions which is going to be a progressively key factor in the choice of the terminal air conditioning system.

Currently Part L 2006 sets a target of 28% reduction in carbon emissions from the base in Part L 2002.  But we all know it doesn’t stop there, in fact industry representatives are confirming the new 2010 Part L will target a further 25% reduction in carbon emissions with a view to getting commercial buildings carbon neutral by 2019.

A further important issue is the transparency that will exist initially in the calculated energy consumption of a building.  Combine these two issues and you have a significant market driver for the property developer to ensure his buildings are utilising the most energy efficient systems that are currently available and not necessarily those that just comply with the existing building regulations.

The transparency created by display energy certificates (DECs) will lead to a better informed tenant who can compare the energy performance of buildings that he may be considering renting.

By using VAV fan coils this will certainly give the whole building much better energy credentials which will be a key factor in determining rentable values.

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