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.
The applicant is requesting verification regarding their selection of Baseline System Type for the project, as well as their intended method for modeling the HVAC equipment schedules, air flow, and fan speed. 1. If the bus maintenance facility and bus storage facility are part of the same building, they should be modeled with the same system type, using any applicable exceptions listed in G3.1.1. The total square footage reported for the bus maintenance and bus storage facility is 149,100. If the administrative spaces result in the total building conditioned area exceeding 150,000 square feet, the project would be modeled using System Type7 - VAV with Reheat. 2. If the bus maintenance facility and storage facility are two separate buildings, the System Type may be modeled differently for each building, based on the total conditioned square footage of each individual building modeled. If the total square footage of the Bus Maintenance building is less than 25,000 square feet (if using Addendum a or 2007 Appendix G) or less than 75,000 square feet (if using Appendix G without Addenda), the appropriate system type would be System Type 3 - PSZ-AC. If the Bus Storage building is less than 5 stories, and less than 150,000 square feet, then the appropriate system type would be System Type 5 - Packaged VAV with reheat. For spaces that do not have cooling installed, cooling must be modeled in the Proposed Case using the identical cooling system type, cooling efficiencies and temperature schedules modeled for the Baseline Case (Table G3.1#10(Proposed)). Exhaust air energy recovery shall be modeled the Baseline Case of the Bus Maintenance facility wherever G3.1.2.10 applies (Note: if desired, the project may use the language from ASHRAE 90.1-2007 Appendix G in place of ASHRAE 90.1-2004 Appendix G, which would allow heat recovery to be excluded for the Baseline case for the bus storage facility systems since these systems are not cooled and are heated to less than 60 deg. F. If this option is selected, the project must follow the language of 2007 Appendix G in its entirety). It is reasonable to model HVAC equipment schedules based on a six-hour per day expected run time as indicated above. The schedules should be modeled identically in the Baseline and Proposed Case. It is unclear how modeling space temperature variations to meet temperature loads will adequately approximate the space temperature, carbon monoxide and nitrogen dioxide profile. Further information is required regarding this strategy in order to verify its appropriateness for this application, or to verify that any credit can be documented using this Exceptional Calculation Method strategy. Applicable Internationally.