The new Building Regulations Part L 2010 are putting greater focus on the design of super energy efficient fan decks, says Mick Holland.
A building designed to meet the new 2010 Building Regulations will have very different proportions of carbon emissions from, say, a building designed 10 years ago. The big users of energy such as the chillers and lighting have already been targeted with significant reduction in carbon emissions. These 'big hits' were progressively 'designed in' to meet the 2002 and then the 2006 Building Regulations.
With these significant reductions being made, the total carbon emissions have obviously reduced, but the proportion of carbon emissions contributed by other items such as the main fans and terminal fans has increased.
If AC motors were used within a fan coil system of a 2010 compliant building then the energy consumption of the terminal fans could be around 15 per cent of the total building energy consumption. This figure can be proved through Dynamic Simulation Modelling of the actual building's projected energy usage.
In other words, all the saving in energy on the so-called major plant items (such as the chillers and lighting) have magnified the energy consumption proportions of the lesser items such as terminal fans.
The perception that the terminal fans are only a relatively small proportion of building energy usage is out of date and inaccurate. It means therefore that designers now need to look carefully at the fan deck design of the fan coil units if they are going to further reduce the total building emissions.
Until recently, fan decks have been designed by specialist suppliers using AC motors, either the permanent split capacity (PSC) type or external rotor motors (ERM).
The fan deck normally comprises a number of small fans - usually three but could be up to five vertically mounted across the width of the fan coil unit. This vertical mounting of the fans meant that multiple units were used to keep the height to a level where the complete fan coil assembly could easily fit into the ceiling void.
With the development of EC motors with a much higher efficiency this option became more and more popular and now many consultants will not consider AC fan decks.
Within the Building Regulations specific fan power (SFP) is limited to 0.6 W/l/s. SFP is a measure of the fan coil's ability to efficiently convert electrical power (Watts) into an airflow (litres/second). With an SFP of 0.6 this means that for every 1 l/s of airflow the fan deck will consume 0.6 Watts of electrical power. A comparison of the SFP for a range of fan coil options is shown in the graph.
The development of EC motors for use with fan coils copied the convention of vertically mounted centrifugal fans. The only European motor manufacturer of EC motors launched a range of small 75 Watt motors so that a motor for each fan shaft was required. Although these motors were highly efficient, there was a premium of around £35 per motor and with three motors in the most popular fan coil size the premium was over £100.
An alternative fan deck has also been created by my company with a much larger EC fan coil motor option (250 Watts compared to 75 Watts currently available in Europe). By cleverly turning the fan on its side and mounting it horizontally we were able to restrict the height to 280mm and only use one fan and motor for the most popular fan coil sizes.
The fan coil unit with the horizontally mounted fan could achieve the airflows with the single larger fan running at lower speeds compared to the conventional EC fan decks. This resulted in lower noise levels or for a given NR rating, say NR35, the unit allows a smaller sized fan coil to be used with a consequential reduction in price. The combined cost savings from the single motor and smaller sized fan coil unit means that the payback period for this fan coil could be measured in months, not years.
For the last 40 years virtually all fan coils have been constant fan speed and consequently constant air volume (CAV).
Any changes in cooling or heating were adjusted by the water valve, i.e. varying the water volume. However, with the fans running at full speed all of the time this is a needless waste of energy.
The EC fan deck has another great asset in that the fan speed can be easily varied usually from 100 to 60 per cent of maximum air flow.
By varying the air volume, significant reductions in energy can be achieved and typical values of SFP for a wide range of constant and VAV fan coils is shown in the energy comparison graph opposite.
This VAV option of fan coil with a single fan gives specific fan powers down to a very low 0.15w/l/s. With these highly efficient terminal fans the fan coil unit has similar carbon emissions to chilled beams and will make it the system of choice for many consulting engineers.