The changing face of fan coils

The demand for energy saving fan coils has increased significantly due to Part L 2006 of the Building Regulations setting a target of 28% reduction in carbon emissions from the base in Part L 2002.  We all know that it doesn’t stop there; 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.

Prior to the focus on promoting the improvement of energy performance of buildings in the community fan coil development had been relatively slow, however recently there have been several developments to the fan coil market.

Energy efficient motors

Traditionally AC motors have been used in fan coils. They have operated at a constant speed. DC motors have been experimented with, due to them inherently being more energy efficient, but problems persist about how to distribute DC around the building or the cost of local transformers, etc.

For a given size fan coil, the difference in power consumption could be up to 50%. Electrically commutated motors have the energy benefits of DC combined with the ease of being able to be directly connected to the 230 VAC power circuit. AC motors require a multi-tapped transformer to adjust the speed/air volume in increments. One of the key benefits of EC motors is infinitely variable motor speed using a 0 – 10v dc signal.

VAV fan coil

The natural progression, once using an infinitely variable speed motor is to use this facility instead of leaving the fan running at a constant speed, irrespective of load, etc. Since the power consumption of the EC motors 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 table 1.

So varying the fan speed with a VAV fan coil gives significant savings in motor power consumption.  A limit of 60% of rated air volume will generally avoid “dumping” but selections of diffusers should be checked by air distribution engineers.

How VAV Waterside fan coils operate

Isothermal mode – this is where the room temperature is at the design value (set point) and no heating or cooling is required. In this mode the water control valves, hot and cold, will be closed but the fan will be running typically at 60% volume.

Cooling mode – as the room air temperature increases a signal from the sensor to the controller starts to open the chilled water control valve. Care must be taken with the coil air off temperature such that the air entering the room is not too low and thus cause potential air distribution issues. It may be prudent to introduce a temperature sensor to monitor the coil air-off and at the specified minimum temperature, start to increase the air volume. If the room air temperature continues to increase, the controller opens the chilled water valve further and then increases the motor speed.

Heating mode – as the room air temperature decreases a signal from the sensor to the controller starts to open the hot water control valve. If the room air temperature continues to fall, the controller opens the hot water valve further and then increases the motor speed.

All of this is captured in the control diagram Figure 1 which also shows a 2°C dead band where the fan coil operates in the isothermal mode.

Airside fan coils

Waterside fan coils as opposed to Airside fan coils have always enjoyed the lion’s share of sales. Due to the focus being on energy saving the sale of Airside fan coils is reducing. Recent specifications ensure that there is a minimum heat transfer between the heating and cooling coils with a typical limit of 100 Watts. This limit can be achieved if the fan coil is designed with insulated flaps either side of the coils controlled by electrical actuators. Badly designed airside fan coils can waste in excess of 500 Watts.

The system approach

Using VAV FCU the importance of air distribution must be emphasised to ensure comfort conditions are met at all times. At minimum air volumes adequate air movement is maintained and the diffuser coanda effect is sustained. At maximum air volumes, the diffuser does not overthrow, create excessive noise or external static resistance. This re-enforces the complete system approach.

This cuts energy consumption and reduces emissions. On top of this, by varying the motor’s speed to suit the heating or cooling requirement, the motor running cost is reduced and energy is saved.

Example

The total building services system carbon emissions can be evaluated using real fan coil test data with a bespoke software package such as TAS EDSL.

A simple building model has been used to provide basic space heating and cooling demand hourly for a typical London weather set (TRY). The model is to Part L2 to standards.

The open plan first floor of the model (30m by 30m) forms the basis of the analysis. It has been subdivided into perimeter and core zones according to the NCM.

Thermostat controls have been set at 18ºC heating and 24ºC cooling with 2ºC proportional control ranges and a 2ºC dead band (as per figure 1).

A basic set of internal heat gains has been used for all zones.

The simulation of the building provides hourly profiles of heating and cooling demand.

A comparison of a CAV Fan Coil, which includes SFP of 0.8, 10 ACH of room air, 0% fan turndown and design margin; against a VAV Fan Coil, which includes SFP, fan of 0.25, 10 ACH room air, 60 % fan turndown and design margin.

Savings in total carbon emissions for a well designed building due to using a VAV FCU compared with CAV FCU could be around 15%.

Summary

With fan coil systems the manufacturer is now taking more and more responsibility for both the fan coil unit and the air distribution through the ceiling diffusers. It is not unusual for the client to underpin the specified parameters by getting the specialist manufacturer to provide guarantees covering aerodynamics, thermodynamics and acoustics.

It is now possible to design buildings using fan coils which will exceed the requirements of the building regulations changes planned for 2010.

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, improved room comfort condition
  • Lower maintenance costs due to longer bearing and motor life
  • Lower room air velocities and reduction in draughts
  • Reduction in sound levels providing overall greater comfort
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