It is too early to sound the death knell of fan coil units, but change is needed on several fronts if they are to maintain their market position, says Barry Trewhitt, product manager at TROX UK

Fan coils have maintained their position as the most popular terminal system for more than four decades, but the technology has, over that time, enjoyed little in the way of innovation. Now, with the introduction of tough new energy requirements under Part L of the Building Regulations and the growth in sales of multi-service chilled beams, pressure is building on fan coil manufacturers to introduce innovations to ensure the continued popularity of this system.

However, a lack of innovation is not the only challenge facing the fan coil sector. There is also a perception among many that much fan coil sales information, particularly relating to performance, is unreliable and that many buildings have been supplied with noisy fan coil systems.

So  if fan coils are to maintain their popularity, changes must be introduced in the form of:

•        Urgent product development.

•        Greater performance guarantees.

•        An overhaul of the way fan coils are designed, specified and procured using a “complete system” approach.

PRODUCT DEVELOPMENT

There has been much publicity concerning the energy benefits of electronic commutation (EC) motors compared with alternating current (AC) motors, but this has focused primarily on running the motors at a constant speed. In fact, variable air volume (VAV) systems can be used with fan coils resulting in several solid advantages (see the box “Benefits of VAV fan coils”).

A variable air volume (VAV) system contributes greatly to fan coil efficiency because it means that the motors are not running at 100% all the time. The use of lower fan volumes during periods of reduced room cooling demand also achieves savings in annual demand for energy because the input power is proportional to the cube of the air flow rate. So, for example, if the air volume is reduced to 80% there will be a 50% power saving and a 30% saving in apparent power.

In a typical room the cooling load percentage against the fan coil running time in a year is as follows:

% of load                     Hours/year

100                               475

 80                               385

 60                               2,000

* Based on 0.43 kgCO2/kWh.

** Based on 5.9p kVAh

As you can see, this technique is better used on fan coils with EC motors rather than AC motors.

The EC motor used in the VAV fan coil is a brushless direct current (DC) motor that connects directly to the 230V AC mains supply. The AC-to-DC conversion and motor commutation are integrated within the motor body and this ensures increased efficiency which provides lower running temperature and higher life expectancy than a traditional motor. EC microprocessor-based commutation also provides other features including alarm outputs and speed monitoring.

EC motors typically boost motor efficiency from 45 to 80 per cent and therefore achieve lower specific fan power. However, while higher efficiency motors are available for terminal units incorporating fans, the forward curved fan impeller is not particularly efficient so the overall motor and fan efficiency is 37 per cent for EC and 21 per cent for AC (see table 1).

Table 1

An AC motor’s speed is controlled by voltage reduction and tapped transformers are normally used to achieve this. However, this voltage reduction reduces the motor efficiency. EC motors, on the other hand, use a low voltage reference signal to provide a command to the commutation to change the speed. The control signal can be either 0 – 10V analogue, or a PWM signal, and motor efficiency is maintained from high speed down to low speed.

Essentially, the EC motor is more efficient than the AC motor because:

•         It does not need to induce a current in the rotor.

•         The EC motor has a simpler winding which results in reduced copper losses losing less heat to the air. At 320l/s, AC motors will increase the air temperature by 0.345°C compared with 0.174°C for EC motors.

•         The specific fan power of EC motors is lower and exceeds the requirements of the latest Building Regulations.

As standard, the EC motor has an input terminal for a 0 to 10Vdc signal from a building management system (BMS) controller to vary the set volume on the fan in relation to the room temperature at minimal additional cost to the BMS package.

PERFORMANCE GUARANTEES

TROX has undertaken extensive market research from which we have established the pros and cons of fan coil units and these are summarised in the box “Advantages and disadvantages of fan coil units”.

However, engineers have experienced noise problems from fan coil units and three quarters of the engineers interviewed by TROX said they would test the fan coil units for major projects.

BSRIA product testing specialist Phil Stonnard agrees that there is a problem with fan coil performance figures. He has said: “From its extensive experience in fan coil testing, BSRIA have found that some of those products had the performance which is 25-30% down on catalogue data.”

We therefore need to look at the overall system in terms of duct design, joint scheduling of fan coils and grilles/diffusers, with a view to offering guarantees on performance. This will take away the concerns of designers and minimise their risks when designing fan coil system

COMPLETE SYSTEM APPROACH

To improve the confidence of consultants still further, I believe there needs to be a total change in the system of design, specification and procurement where all the components within the fan coil system – diffuser, plenum box, fan coil unit and associated controls – come from a single source.

Starting with the original design, a system approach will ensure the best possible solution is developed. This will also allow for project cost savings by value engineering, off-site fabrication, and pre-commissioning of units.

Offsite fabrication will, I believe, become more prevalent due to the shortage of skilled site labour, improved build consistency and efficiency in the factory, and reduced installation times and increases fast track programming. “Just in time” delivery of factory-finished assemblies will streamline the construction process.

The benefits of factory prefabrication and pre-commissioning include:

•         Fan coil controllers can be factory set for design air flow and pressure independent modulating water valves can be factory set for design water flow.

•         Cost – Construction costs are significantly lower because the construction process is faster, there is a reduction in the number of trades, and the process is more predictable, of higher quality, less wasteful and safer.

Besides, a single point responsibility makes performance guarantees more easily available because of this focus on one manufacturer.

So, to summarise, I believe fan coil units are at a crossroads. They face mounting competition from other technologies and faith in their performance figures has suffered a series of hammer blows.

However, provided sufficient care is taken with how fan coils are developed, how they are tested and how they are specified, I see no reason why fan coil units should not only survive, but thrive.

Benefits of VAV fan coils

Fan coils that use VAV technology offer a range of benefits. For example, they are:

•         Energy efficient compared with constant volume units.

•         Exceed the requirements of the Building Regulations and reduce carbon emissions.

•         Have a lower specific fan power because of their high motor efficiency.

•         Capable of efficient speed control

•         Able to demonstrate lower maintenance costs due to longer bearing and motor life.

Advantages and disadvantages of fan coil units

Download picture (original)

Download article (DOC format)