Rating System

LEED BD+C: Core and Shell

Rating System Version

v2 - LEED 2.0

Inquiry

Our question deals with selecting the most appropriate HVAC system baseline for our project. Our project is a 3-story, 40,000 sf office building. It includes 10,000 sf parking, and 30,000 sf tenant improvement office and core space. Most of the offices and all core spaces will be served by a single VAV system consisting of a rooftop packaged air conditioning unit and electric resistance reheat at series fan-powered boxes in each zone. The rooftop unit delivers low-temperature supply air to the spaces, reset to a temperature dictated by the zone requiring the greatest amount of cooling. The rooftop unit also contains a gas furnace, which provides heating for the following infrequent scenarios: 1. During night low limit control, when three perimeter zones and one interior zone on any floor fall below the night low limit temperature setpoint. 2. During morning warmup, when outside air temperature is below an adjustable setpoint. Virtually all other heating needs are served by the electric resistance heaters at the series fan-powered boxes. Based on this predominant use of electric resistance heat, we have categorized our heating source as "electric" and have selected a "packaged rooftop heat pump" (System 4) for our baseline HVAC system per ASHRAE 90.1-2004 Appendix G Table G3.1.1A. Four considerations affected our interpretation of which baseline to use: 1. Heating source for the design. The two scenarios in which heating is provided by the gas furnace, identified above, do not comprise a significant proportion of the total heating required by the building. Rather, the majority of the building heating (approximately 90% of the total annual heating energy), is provided by electric resistance reheat at the fan-powered boxes. 2. Avoidance of fuel-switching. Although a "packaged rooftop air conditioner" with fossil fuel furnace (System 3) is specified for design buildings using a "fossil/electric hybrid" HVAC system, use of a System 3 baseline appears to create a situation of fuel-switching. Our design building heating source is predominantly electric, whereas the System 3 baseline heating source is a fossil fuel furnace. 3. Consideration of reheat strategy for both design and baseline models. The System 3 "packaged rooftop air conditioner" with fossil fuel furnace might be considered a reasonable baseline for comparison to our design building if the baseline system included electric reheat at each zone. However, based on Appendix G rules and guidance provided in the ASHRAE 90.1-2004 User\'s Manual, the System 3 HVAC system must be represented with a separate packaged single zone unit for each thermal zone in the baseline model, which implies that no zone level reheat is necessary. 4. Consequence of baseline requirement on future building HVAC designs. If the intent of Table G3.1.1A is to require a System 3 baseline for our particular design, the baseline has the effect of encouraging replacement of infrequently-used gas furnace heaters in rooftop units with less energy-efficient, electric heaters. Such a design change could occur relatively easily, especially during value engineering. These reasons - lower initial cost of an electric heater and the opportunity to show higher energy cost savings relative to an electric heat pump baseline - would seem to provide a compelling reason for changing the design by eliminating the gas furnace. It does not seem reasonable that this type of small design change to a rooftop packaged unit, replacing gas heat with electric, should become a pivotal issue in determining the correct baseline for LEED calculations. For these reasons, we consider System 4 to be the appropriate baseline for our project. Is this an acceptable baseline selection?