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Fan Coil Units: Lifting the lid on airside fan coil units

For many years, airside fan coils have prompted a great deal of debate focused mainly on their energy efficiency attributes. However, a new development in the technology promises to change the terms of this debate, as Barry Trewhitt, product manager at Trox UK, explains
Fan Coil Units: Lifting the lid on airside fan coil units
FAN coils are a well-established terminal system for controlling room temperature. Until recently they have been the first choice for developers of large city-based office blocks. Although chilled beams have gained significant market share fan coils still remain the popular choice.

Waterside units are well established and pretty uncontroversial. However, airside fan coils provoke a great deal of discussion and tend to polarise views within the industry.

Some designers believe airside provides much better control of room temperature compared with waterside. The other significant benefit is that the dampers on airside units are relatively maintenance-free whereas the valves on waterside units require a higher level of maintenance.

Airside fan coil units account for approximately one third of the UK market and it was important for Trox to develop an energy-efficient solution.

During the initial research of the market it was established that very few clients/designers specify energy loss targets. Those that do normally state that a maximum of 100watts loss in either full heating or full cooling would be acceptable. While these figures may seem high, Trox, like many other manufacturers, found it difficult to keep losses within these limits. Extensive testing of a wide range of units revealed there was only one other manufacturer who could meet this specification.

During the product development phase it was essential to undertake thorough research on potential energy losses since as recently observed it 'seems now that every watt counts'.

The programme included:

· Theoretical calculations of energy loss by conduction, convection and radiation;

· Use of thermal imaging to pin point any potential conduction losses;

· Building a unit with Perspex cover and introducing smoke to establish air patterns;

· Installation of BSRIA certified fan coil test rig for very accurate measurement of watts consumed versus watts output;
Early results showed two significant design issues which were to provide Trox with some interesting challenges. These were:

· Using a single actuator and springs, the losses in the by-pass position were in the range 400 to 1000watts which was unacceptable;

· The air circulation in the fan coil showed how the air patterns could significantly influence the energy losses.

There are two basic airside fan coil designs available.

· Inlet damper arrangement (fig 1) uses dampers/flaps to divert air through heating, by-pass and cooling zones to control the temperature output.

· Outlet damper arrangement (fig 2) uses dampers/flaps to control the air through heating, by-pass and cooling zones to control the temperature output.

With the inlet arrangement, during heating mode, the air will be drawn
into the fan area but turbulence will cause heat to be transferred into the exposed face of the cooling coil (fig 3).

With the outlet arrangement, the inlet side of the heating and cooling coils are exposed to the air and thus energy will be taken from the heating coil and put into the cooling coil (fig 4). Typically this will be 500W. All units tested had a very high energy loss in the bypass position.

Trox' solution to these problems has been to create a unit comprising dampers on both the inlet and discharges of the heating and cooling coils. These dampers are driven via an externally-mounted linkage by 90deg actuators. These dampers are necessary to prevent radiant, convective and conductive heat being transferred to the air when in bypass, heating or cooling.

The new units which have been developed give energy losses of less than 100watts in both maximum heating and maximum cooling. In the by-pass position energy losses are less than 200watts. This compares with the 500 - 1000watts which is achieved by other units in the market.

Each of the new airside control fan coil units developed by Trox comprises a multi-speed fan with a damper providing airside control of its integral water to air cooling and heating coils. The controller will also control the position of the fcu's heating/cooling control damper which diverts varying proportions (0 to 100%) of the unit's air flow through either its cooling coil or (where fitted) its heating coil, or by-pass both coils altogether.

Revisions to Part L of the Building Regulations have highlighted the need for greater energy efficiency and the recently published Stern report on climate change hammered home the point that we cannot afford to do nothing in the face of a growing threat from global warming.

In his report, Sir Nicholas Stern said: 'The world does not need to choose between averting climate change and promoting growth and development. Changes in energy technologies and in the structure of economies have created opportunities to decouple growth from greenhouse gas emissions'.

On top of this, government plans to introduce new laws to help the UK reduce its carbon dioxide emissions. The Climate Change Bill will include moves to help the UK reduce emissions from 1990 levels by 60%, by 2050, says the environment secretary.

Through continuous design and innovation, Trox has made several improvements significantly to reduce the running costs of airside fan coil units and hence improve the commercial return of these projects to public and private investors.

1 February 2007

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