Building-Sim users,

Perhaps somebody could shed some light on this issue for me.

In 90.1 Appendix G, Table G3.1, Section 6.f. states that credit can be taken in the proposed building for automatic controls due to daylight utilization. However:

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In order to know how much of an effect daylight should have on the power used, a target luminance/lux level is required. If this target is met by daylight alone, the lighting consumption would drop to 0 (except for emergency lighting). Where daylight falls between 0 and the target lux level, lighting power can be scaled linearly.

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The documentation gives no target lux levels and they cannot be derived from the baseline lighting power densities (from 90.1 Sections 9.5.1/9.6.1) without knowing the efficiencies associated with these values.

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The only related luminance levels available are from the IESNA Lighting Handbook but these levels do not seem to sensibly correlate with the power densities of 90.1 Sections 9.5.1/9.6.1. Take, for example, a drafting office. The IESNA Lighting Handbook Section 10, Quality of the Visual Environment, suggests a lighting level of around 100fc (1076 lux). Achieving this 100 fc with an LPD of 1.1W/ft? (11.83W/m?), i.e. that quoted for an Open Plan Office, requires a lamp with an efficiency of approximately 91 lumens/watt. Using the same Lighting Power Density, and illuminance of 30fc for simple office tasks, we require lights that produce 27 lumens/watt. This gives us two spaces that are classified identically, but are subject to massively different limitations when it comes to selecting lamps.

Could somebody please provide some clarification on whether it is wise to assume these lux levels while performing a Building Performance Method simulation, bearing in mind that it could affect results in an unfavourable way if lux level is overestimated (as LPD will be constant in the baseline buildings).

Regards,

Jonathan Catterall