Please clean me
The specific fan power of fan-coil units incorporating filters will certainly fall off over time, but when is the ideal time to clean and replace those filters? Peter Lowther of Ability Projects describes how FCUs themselves can tell you.
Depending on which side of the fence one sits, designers have for a long time been specifying, and fan-coil manufacturers supplying, units based on thermal and acoustic performance. These days, however, there is a new parameter, an important new performance benchmark, that has to be taken into account — and that is specific fan power (SFP).
Whilst the regulatory limit for fan-coil units is 0.6 W/(l/s), many units can achieve, and many designers are looking for, figures of half that value — with SFPs of around 0.2 to 0.3 W/(l/s) being commonly discussed. Almost overnight there is another differentiator between fan-coil products and the designs into which they are applied. That said, there are a couple of facts about EC fan motors for which looking beyond the initial as-installed SFP can turn out to be just as important.
For example, the specific fan power of any product using both fans and filters is only correct when the product is practically brand new and clean — and this is certainly the case with fan coil units. Just two or three days on a typical site will dirty the filter, increasing the SFP.
This problem emphasises the need to hand over a building initially with a new, or at the very least clean, set of filters and for the facilities management team to keep to a fairly rigorous filter-cleaning regime from then on.
The first requirement is solved easily. Even now many fan-coil contracts are ordered with a new set of filters, which are exchanged for the ‘commissioning’ filter set at, or just before, handover to the client.
The second requirement is not quite so easy to meet. The reason is that while regular filter cleaning is simple to request, the practicalities of such a routine are inevitably more difficult and costly to implement. The answer is not to avoid cleaning the filters but to clean them only when required, rather than on some notional time-based schedule.
This is where the Ability Matrix Cubed accessory comes into its own. When applied to any EC-driven fan coil the Cubed sensors continually monitor the power that the fan coil consumes. Since the Matrix control strategy also senses the air volume being supplied, it can and does calculate a real-time SFP for the unit. Thus on any building with Matrix installed, the SFP can be recorded when the building is handed over, monitored over time and used as the trigger to clean the filters. There is no longer any arbitrary necessity to visit every filter to establish that half do not need cleaning whilst the other half are well overdue. Filter cleaning can in fact, be targeted.
This means that the building can be kept within SFP limits, disruption kept to an absolute minimum and the best compromise (because it always will be a compromise) achieved between cleaning the filters adequately while keeping costs to a minimum.
It is well known that filters that are regularly cleaned do not return to their as-new condition every time. In fact after some three or four cleans, they could be returning unacceptable SFPs very soon after the servicing team has visited them. This is why the Matrix strategy has a second programmable filter alarm. This recognises a filter that although a filter has been cleaned, an acceptably low SFP has not been restored. This routine triggers a separate flag that tells the service team that on the next visit the filter needs changing rather than simply cleaning.
One other rarely considered consequence of using EC fan motors is that, when they are disabled but left powered up, the fans and the harmonic filters continue to consume power, even in this idle state. The Matrix Cubed board, however, possesses another SFP-reducing feature, which recognises any idle fan or fan coil and prevents power from going to the unnecessary components. This situation can exist in low-occupancy areas or where a time schedule or similar shuts down sections within a larger installation. This effect and the amount of power consumed may seem inconsequential, but the average sized fan coil with this energy saving feature employed will save about a half of its normal standby consumption.
So the moral of the story is that EC fan-coil units represent one of the most efficient and flexible air-conditioning solutions available but can offer even bigger savings alongside an intuitive, far more focused and less disruptive maintenance regime. No developer or occupier would accept a reduction of 30% in their equipment’s thermal performance over time, so why should they be expected to be any more compromising with their energy consumption.
Peter Lowther is chairman of Ability Projects.