The process of selecting the most appropriate motor for a fan coil unit needs to take a wide range of factors into consideration. Peter Faruqi of Dunham Bush explains why there is more to it than following the crowd
AS an industry, we have a responsibility to ensure that our clients get the solution that best suits their needs. Unfortunately, that does not always happen. Choices are sometimes influenced by trends or the marketing skills behind a particular product rather than the basic engineering criteria that ought to prevail.

A prime example of this is the selection of motors for fan coil units - an area where choices have expanded in recent years. From an engineering perspective, there are essentially three types of motor from which to choose.

These are an AC external rotor motor, a DC external rotor motor or an AC internal rotor motor. In our experience of supplying fan coil units with all three types of motor, there is considerable variation in the way that the decision is made and what criteria it is based on.

For example, with the current - and laudable - emphasis on energy efficiency, DC motors are currently popular. And these certainly offer greater efficiencies compared with AC motors.
At the same time though, it is true to say those projects that are driven solely by energy and emissions are few and far between. In the majority of projects, energy efficiency is just one criterion (albeit an increasingly important one) which needs to be taken into account during the design process. Consequently, most building services designs end up as a compromise between capital costs and whole-life costs.

In fact, there are still plenty of projects where the return on investment in energy efficiency needs to meet stringent criteria set by a cost consultant or financial director. In a recent survey of facilities managers, only 20% said their organisations were prepared to accept a longer payback for energy-efficient plant.

Short-term payback

Potentially, this situation could have serious ramifications for the efficiency of installed plant in our building stock. We all know that there are many old and inefficient systems out there which are nursed along because their owners do not want to fork out for a more efficient replacement.

Often, the deal maker or breaker will be the payback period. Because of their extra cost, DC motors tend to offer a longer payback.

For instance, Dunham Bush was recently involved in a project where the extra cost of using DC motors instead of correctly-sized AC motors would have added a further 10 years to the payback period, based on the additional energy which would have been saved by DC motors.

This sort of situation often arises and there are real benefits from opting for a lower-cost solution that allows the project to go ahead and get rid of the ancient, energy-guzzling system. For instance, use of a Part L-compliant, correctly-sized AC fan motor will often give a faster payback, and a more acceptable solution, when all of the criteria are taken into account.

In such cases, this 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 this is still well within the limiting 0.8W/(l/s) specified by Part L. And, of course, the important thing is that the project goes ahead and an inefficient system is replaced with a more efficient alternative, so everyone is a winner.

Universal principles

These same principles can be applied to the specification of external rotor motors, whether they are AC or DC. There are many situations where the high level of performance offered by external rotor motors simply are not required, and where a correctly sized internal rotor motor may be more appropriate.

Generally, the size of an external rotor motor ensures that it will overcome some significant external static pressures. For instance, a typical fan coil unit equipped with four external rotor motor assemblies, giving a combined power rating of 800W, will easily cope with 150Pa.

But most fan coil systems are designed so that the system pressure does not exceed 30Pa. As a result, there is a good chance that the fans will be throttled back all of the time, operating at a point well below their optimum efficiency.

Clearly, 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, giving a significant saving in electrical power consumption

As technologies move on and present specifiers with increasing choice, there is a growing need to consider all of the variables that affect a project, encompassing performance, life cycle costs and capital budgets.

Things are moving too fast to simply copy specifications from one job and paste them into the next to save time. Our industry's clients have a right to expect that we use our specialist expertise to deliver the best solution - and it is up to us to do just that.

www.dunham-bush.co.uk