Rating System

LEED BD+C: New Construction

Rating System Version

v2 - LEED 2.2

Inquiry

Our question is in regards to 1) the baseline HVAC system selection and 2) modeling a variable air volume strategy for a bus maintenance and bus storage facility. The facility is a 20,500 square foot bus maintenance and 128,600 square foot bus storage facility (plus administrative space which is outside the intent of this question). The bus maintenance is heated to 68 F; the bus storage is heated to 45 F. The exhaust requirement by code is 1.5 cfm per square foot. The bus maintenance area is ventilated by heating and ventilating units and exhaust fans with heat recovery. Additional heat is supplied to the bus maintenance with radiant heat from an underslab hydronic system. The sequence of operation for supply and exhaust fan control uses a combination of space temperature and carbon monoxide and nitrogen dioxide sensing to vary the fan speed and the amount of outside air of the heating and ventilating units and fan speed of the exhaust fans. The heating and ventilating units will operate most of the time in a recirculating mode with 10% to 20% outside air and at reduced fan speed, generally 50% of design air flow. Exhaust fans do not run continuously. The heating and ventilating units are estimated to run for nine hours per day in the bus maintenance. Significant outside air quantities are estimated to be required less than one hour per day on a cumulative basis. Bus Storage is served by dedicated make-up air units which are off unless high carbon monoxide or nitrogen dioxide levels exist. Bus storage is heated by ducted combustion air and high efficiency unit heaters. Based on typical times that buses run in the storage area, the make-up air units will likely run for less than 6 hours per day on a cumulative basis which includes a purge cycle of about five minutes each hour during regular operating hours. Per ASHRAE Standard 90.1-2004, the baseline HVAC type is assumed to be:  Bus maintenance: System 3 - Packaged Single Zone - AC with heat recovery  Bus storage: System 5 - Packaged VAV with Reheat (no heat recovery). We are using a DOE-2 based software to vary air volume (except the baseline for bus maintenance which is constant volume) based on space temperature for both the baseline and proposed building performance as a reasonable approximation of the space temperature, carbon monoxide and nitrogen dioxide profile which is unknown. Please confirm that  the baseline system selection for bus maintenance and bus storage is correct  the schedule for HVAC equipment in bus storage can use a reasonable assumption based on typical times that bus engines will be running for both baseline and proposed building  the air flow and fan speed for the maintenance area can be modeled based on space temperature for both the baseline and proposed building performance.