﻿State-of-the-art fan-coil units tick all the right boxes
Today’s fan-coil units are very different from those of just three years ago — with major advances in their potential for energy-efficient air-conditioning and controllability, as our industry survey shows. Make sure you don’t miss the huge impact that variable-speed motors can have on reduce carbon emissions!
According to BSRIA, the market for fan-coil units in air-conditioning system is growing again. It saw a 15% increase in value in 2007 compared to 2006. BSRIA attributes the resurgence to fan coils becoming able to compete more effectively with the energy efficiency of chilled beams with the introduction of electronically commutated DC (ECDC) motors. Such motors use less power than conventional AC motors and can readily have their speed controlled. They are now routinely used by a number of manufacturers of fan-coil units.
But fan-coil units have seen a host of other improvements in the last few years, as this article will demonstrate.
Just how significant the energy savings to be obtained using fan-coil units with ECDC motors is revealed by work carried out by Trox. An important feature of ECDC motors is being able to control their speed, and this was taken into account by varying the airflow as a means of controlling cooling output. The fan cube law comes into effect, with fan power at 60% air volume being only 21.6% of that at 100% air volume.
Terry Farthing, technical consultant with Trox Design Bureau, explained how the company went about seeing how this theory might work in practice.
The first stage was to check the effect of controlling airflow by varying fan speed. This work was carried out in its BSRIA-approved laboratory at Thetford.
The next stage was to use EDSL/TAS software to simulate the performance of fan-coil units in a typical building over 12 months.
Technical expertise for the project came from Neil Pennel, head of sustainability and engineering with Land Securities, with guidance from the viewpoint of a property and building owner. Stuart East, managing director with John Noad (Building Environmental), provided input as a forward-thinking consultant.
EDSL simulated a 30 m-square open-plan office building to comply with Part L2 and with zones in accordance with the National Calculation Method. 17 constant-speed fan-coil units were compared with the same number of FCUs with variable-speed motors.
Terry Farthing says that the results were staggering. The constant-volume FCUs used 13 800 kWh per year compared with just 1405 kWh for the VAV FCUs [those figures have been checked several times, as have following figures —Ed.] — giving a payback on the premium for ECDC motors of less than three years.
That reduction of nearly 90% also has a very significant effect on total building carbon emissions.
A building with fixed-speed FCUs is predicted to have CO2 emissions of 30 kg/m2 compared with 23.5 kg/m2 for the same building with variable-speed FCUs. That reduction of over 20% is entirely due to reducing the CO2 emissions associated with the motors of the FCUs from 10 to 3.5 kg/m2. For comparison, the carbon emissions associated with lighting for both buildings is 12 kg/m2, 6 kg/m2 for actual cooling and 2 kg/m2 for heating and hot water.
Peter Lowther, managing director of Ability Projects, agrees with BSRIA that fan-coil units are regaining ground over current rival systems, including chilled beams. Backing up that view, he says, ‘Two or three substantial projects, originally conceived as chilled-beam systems, have recently been switched to a fan-coil system. The reasons behind these changes are probably varied, but taken together support the contention that, overall, the fan coil remains the most appropriate air-conditioning system for the UK.’
Peter Lowther expresses concern that discussion of the energy-efficiency claims of chilled beams have tended to drown out the energy-efficiency and other improvements that have taken place in fan-coil technology over the same period. He says, ‘Today’s state-of the-art fan coil is far removed from its predecessors of only two or three years ago.
The fundamental change from AC to ECDC fan motors brings two benefits, according to Peter Lowther. One is reduced electrical consumption, which, in turn, reduces the demand for chilled water. ‘However,’ he says, ‘the real revolution in fan coils stems not from the energy-saving potential of ECDC motors but from the controllability that they afford — the result being a system that can certainly compete and even surpass the performance of its contemporaries.’
Whereas the fans in fan coils used to operate continuously at design speed, with output controlled by modulating water flow, there is now the potential to control air flow according to occupancy and demand. And such control need not be expensive. Peter Lowther explains, ‘There is a misconception that these “fancy” control strategies are expensive to employ, but that is not the case, since a number of terminal controllers have already been pre-programmed for the task, and many others need only a simple strategy adjustment.’
Being able to control fan speed also brings advantages for designers and installers — as such units can be balanced through fan speed alone. Peter Lowther again: ‘By adjusting the speed of each fan, the air volume down each duct is set, removing the need for any mechanical volume-control device and their attendant problems of cost, regenerated noise and wasted energy.’
Leigh Stimpson, managing director of Diffusion, says that the pressure on fan-coil units from chilled beams, chilled ceilings and VRF systems has mainly been due to the perceived energy savings associated with those other products.
ECDC motors have transformed the situation, and Leigh Stimpson refers to Diffusion’s latest range of controllers winning an award as H&V product of the year. These controller can combine harmonic filters, where required, and power-factor correction up to 0.95. Indeed, Diffusion can offer FCUs with specific fan powers as low as 0.16 W/l/s.
There is also a move away from systems with constant water volume towards variable water volume, with obvious benefits on pump power consumption.
Associated with this trend says Martin Lowe, technical manager with Marflow Hydronics, ‘There has been an increase in the use of pressure-independent 2-port control valves (PICVs) in the design of these variable-flow systems. However, in many cases, the advantages offered by PICVs are not being exploited to the full — and this is particularly true of the addressable pressure-independent control valves that are now available.
Such valves can be linked to a BMS or BACnet system can set precisely from a remote location.
Martin Lowe explains that a whole new approach to the design and operation of fan-coil systems is made possible, as valves are no longer ‘fixed’ and difficult to adjust. Being able to adjust valves remotely makes possible continuous commissioning so that fan-coil systems can be adjusted in response to changes in office layout and temperature changes.
More possibilities come up when heating and cooling are provided by a heat pump, with addressable valves enabling water volumes to be adjusted in direct response to changes in heating and cooling throughout the year. The same coil can then be used for heating or cooling for most of the year, with water volumes adjusted to compensate for the varying outputs of the heat pumps.
Another benefit of the remote control of valves is adjusting fan-coil systems when the layout of a space changes. With manual valves, such changes are costly and disruptive — and may not be done.
Another problem for which remote control offers a solution is clearing dirt and air that can become trapped by valves operating nearly closed. Martin Lowe explains, ‘Conventionally, the only way to free any dirt particles larger than the set orifice is to manually open the valves to allow the dirt through. However, with a dynamic system, the simple expedient of setting valves to open fully for a few seconds once a week will eliminate blockage problems by flushing through any accumulated dirt particles.
Martin Lowe is keen that the potential for addressable pressure-independent valves to bring about a sea-change in the operation of fan-coil units is recognised. He says, ‘The key is for specifiers to recognise the potential and take advantage of it.’
Peter Lowther also focuses on the benefits of self-balancing fan coils that can be preset in the factory with individual air volumes, chilled-water flow and heating design flow rates. Once installed, such units set themselves. ‘Commissioning can now be considered a “checking” rather than a “setting” task,’ he says.
The flexibility of fan-coil units is highlighted by Leigh Stimpson and Peter Lowther.
Leigh Stimpson argues that the popularity of fan-coil units was due to the flexibility they offered in partitioning open-plan office space, providing zone control and accepting layout and load changes. ‘They are also,’ he says, ‘the most forgiving product in terms of design, as variables such as water temperature, flow rates and fan speed can be adjusted to achieve design conditions. For hotels, offices and public buildings, they have remained the preferred terminal unit. For smaller applications, they are a cost-effective, low-maintenance and flexible solution.’
Another reason for the currently popularity of fan-coil units is put forward by Peter Lowther. ‘The building-services industry is going through a fairly torrid period, with some construction projects being cancelled, some slowed and some progressing only as far as the shell-and-core stage. In times like this, it makes sense to employ a system that can be installed in stages, moved around if necessary, returns the best output per capital pound spend and retains complete flexibility with regard to a building’s ultimate use.’
Leigh Stimpson has another important reassurance about fan-coil units: ’During periods of peak load, fan-coil units can be relied upon to achieve higher cooling capacities, where in certain applications chilled beams and ceilings fall short in their ability to deliver a comfortable environment.’
Just as other sectors of the air-conditioning sector have been advancing, so too have fan-coil units. Let Peter Lowther have the last word: ‘Why look further than a fan-coil system when it ticks all the boxes.’