A Brief Commentary on Part L: 2014

13-08-2014

The way that Part L requires us to assess energy efficiency in lighting changed in April 2014, with the long-awaited launch of new Part L regulations. Previously, it was the relatively simple case of working within efficacy limits, either the efficacy of the light source itself, in the case of domestic premises, or that of the combination of source+luminaire in non-domestic installations.

The lighting design community’s main complaint at the time was that Part L didn’t take into account how the lighting installation was to be used. Did it matter how efficient a lighting installation was if the lighting was allowed to burn all through the night, or when rooms were unoccupied? Well, it matters now, and we have LENI to thank for that.

The latest Part L Regulation takes lighting usage onboard. Not much has altered in the way that they deal with domestic premises; there’s a single change in the way that switching should be planned, requiring fewer light fittings on a single switch line – thus suggesting that residents might prefer to switch some of their room lighting off, at the very least. The problem here is that there is only one scenario where this really works and that is a consequence of the parlous state of much home lighting design relying on downlights, where the real answer is not to go down that design route in the first place.

Part L is far more creative in the way that it addresses the idea of Light In Use for non-domestic premises. For the first time, the scheme planners have been offered an alternative assessment method. This new method is very prescriptive but it offers the designer/building user far greater flexibility in the lighting design approach. This alternative method is known as the Lighting Energy Numeric Indicator (LENI) which is possibly an acronym too far. But first, let’s look at what’s happened to the original assessment method.

The lighting for domestic premises is based on efficacy figures of the light source (lamp lumens/circuit Watt), so enabling efficient lamps to be fitted into desperately inefficient lamps shades. The domestic environment is, of course, uncontrollable as regards the type of fixtures in use, so there’s not much more that can be expected. The non-residential environment is very different and the efficacy requirement is for luminaire lumens / circuit Watt. Inefficient light fixtures are thereby frowned upon, which is the good news, but it also brings with it another risk; that the visual environment can suffer badly in the hunt for better efficiency in use. And this is where LENI comes in.

LENI is based on a target energy figure for the lighting, based on energy consumption (kWh) per square metre of floor area. Overshadowing the entire LENI metric is an assumption of what makes an energy-efficient lighting design, given the current state of luminaire and light source technology. Put crudely, it works out to be something like an efficacy figure of 60lm/W, which is the starting point for the method described above. Clearly, they don’t just throw this stuff together. But, as we say above, this is not a particularly generous figure and can require some very careful luminaire design and scheme planning to get that kind of efficacy working in a friendly fashion. Inevitably, the lighting industry is reacting by trying to increase the light output of its sources, both fluorescent lamps and LED modules and – frankly – some of the numbers that are being thrown around are quite intimidating (current expectations for a T5 fluorescent lamp is around 100lm/W; manufacturers are talking about double that. A similar situation applies within LED development. This way lies disaster and mayhem.)

So LENI requires three quantifiable pieces of information; the installed electrical load of the lighting installation, the number of hours that the installation will be in use and the size of the room. A number of factors are then introduced that can further influence the calculated LENI metric. These include occupancy control, daylight linking and the availability of ‘constant illuminance monitoring. Each of these mechanisms can help reduce the actual energy consumption of a lighting installation, and because LENI simply references a ‘typical installation’ benchmark figure, its possible to use luminaires that are less efficient, but maintain a LENI-friendly assessment by introducing effective illuminance control. It’s a way of saying that control is at least as important as energy-efficient sources.

Now, in that previous paragraph we slipped in the dangerous part; “the number of hours that the installation will be in use”. Remember that Part L is a part of the Building Regulations and, as such, has legal powers behind it. So when design indices require a time metric, the designers and scheme planners need to be certain of their parameters, because a Building Control approval sits behind this decision. Part L has been strengthened in another way. Following a string of (Part L) design failures in building design, it has now become a requirement that building performance is tested and monitored to ensure that actual performance matches those design figures. Now that’s going to get very interesting if energy consumption is far higher than planned, or those fancy lighting controls turn out not to be so effective after all. And who will be the final arbiter in all of this – who will decide that the impact of poor weather and low summer daylighting doesn’t constitute a breach of building regs?