Our project consists of a two story retail building. The building is open seven days a week from 9:00 am to 9:00 pm for business. We are requesting clarification regarding the modeling of lighting power density for the Proposed Design Case. In an effort to limit lighting energy usage, the project has developed two completely separate ambient lighting systems, which will NEVER operate simultaneously. A Building Automation System will be used to control the lighting systems, and to ensure that these systems do NOT ever operate in combination with one another. - System 1 consists of the general illumination (ambient lighting) having an overall LPD of 0.53 watts/sf, operational during business hours. - System 2 consists of the emergency/cleaning lighting system having an overall LPD of 0.9 watts/sf (using the ASHRAE 90.1-2004 Building Area Method). This system is a completely separate bank of lighting fixtures, which will only operate during the night for a few hours for housekeeping and maintenance, and during emergencies to provide code required egress lighting. Since the dual lighting systems have been designed for the purpose of limiting energy usage, and since these systems will never operate at the same time, we believe it would be unfair to require the project to model the cumulative lighting power density for the two systems for the "Proposed Lighting Power Density". Instead, we propose to model the lighting power density for the proposed case as two independent interior lighting systems for the building. The lighting power density for each system will then be compared to the base case ASHRAE 90.1-2004 building having a LPD of 1.5 watts/sf, based on the Building Area Method. For example, during store hours, the regulated lighting power would be modeled as 0.53 W/sf versus 1.5 W/sf allowed, and during cleaning hours the regulated lighting power would be modeled as 0.90 W/sf versus 1.5 W/sf allowed. We believe this modeling strategy meets the intent of the LEED v2.1 EAc1 credit by encouraging energy efficient design and controls, and by limiting total building energy consumption and costs.
The proposed modeling strategy does not comply with ASHRAE 90.1-2004. Per ASHRAE 90.1-2004, Section 9; "Exception to 9.1.3: If two or more independently operating lighting systems in a space are capable of being controlled to prevent simultaneous user operation, the installed interior lighting power shall be based solely on the lighting system with the highest wattage." Therefore, the proposed design must be modeled using 0.9 W/sf, for the purpose of the Energy Cost Budget calculation.