When it comes to the selection of fan coil motors are we, as an industry, doing enough to provide the solution best suited to the requirements of the end user, asks Peter Faruqi of Dunham Bush
IT'S probably true to say that the knowledge and expertise embedded in the building services industry is not always fully appreciated or utilised by end clients. I would suggest that one of the ways that we can improve our standing is to use those skills to make sure clients are supplied with the best solutions for their needs. However, I am not convinced that we, as an industry, always do that.

A case in point is the selection of fan motors for fan coil units. Basically, there are three options - an AC external rotor motor, a DC external rotor motor or a correctly sized AC internal rotor motor. As a company that offers all three options, Dunham Bush sees much variation in selection criteria and, indeed, selection suitability.

For instance, there is an increasing call for DC motors at present, in response to demands for greater efficiency.

DC motors will certainly offer efficiency improvements over AC motors. However, in the majority of projects, while energy consumption is justifiably important, it is not the only consideration. Of course, there are some projects where carbon footprint is all and cost is immaterial, but they are few and far between.

In my opinion, this is where suppliers need to be careful. For example, consider a potential refurbishment project where the fan coils are operating acceptably, but the end client is considering upgrading to a newer and more efficient model.

In most cases, the deal maker or breaker is going to be pay back, and there tends to be a relatively long pay-back period for DC motors. On a recently quoted project, for example, it would have taken around 10 years to cover the extra cost of DC motors through the energy savings achieved while operating during typical office hours, compared with the correctly-sized AC equivalent.

In these cases, it may be better to opt for a lower priced, Part L-compliant correctly sized AC fan motor that gives a fast pay-back and ensures the project goes ahead rather than a solution that will rule itself out by virtue of an unacceptable pay-back. It may mean opting for a specific fan power approaching 0.45W/(l/s), rather than one approaching 0.25W/(l/s), which is typically achieved using DC motors, but it is still well within the limiting 0.8W/(l/s) specified by Part L. If the project goes ahead and inefficient products are replaced with more efficient products, everyone is a winner.

The same principle applies to the specification of external rotor motors, of the AC or DC variety. This high level of performance may not always be necessary and a correctly sized internal rotor motor may be more appropriate.

In general, the size of an external rotor motor ensures that it will overcome some very hefty external static pressures. A typical fan coil unit equipped with four such assemblies, with a combined power rating of 800W, will easily cope with 150Pa.

The majority of air conditioning systems are designed such that system pressure does not exceed 30Pa and so the fans will be throttled back all the time and operating at a point well below their optimum efficiency. So, for systems with average external static pressures of around 30Pa, it will often be more efficient to use critically sized fan motors rated at about 80W.

Underlying all of these points is an observation that specifiers don't always look as carefully as they might at the particular requirements of the project, and specifications often get copied from one job to the next because of time pressures. At the end of the day though, this is not giving the clients what they are paying for - and what they have a right to expect from our industry.