1. Design and Construction /
  2. Vol. 3 New Construction Guidelines /
  3. 6.0 Heating Ventilating and Air Conditioning /
  4. 6.2 Design Criteria

6.2.3 System Sizing

Load Calculations

Loads: Load calculations shall be performed at the temperatures indicated which are different than the conditions to which the space will be controlled. Detailed calculations shall be performed on all projects. Copies of detailed calculations (inputs/outputs) shall be provided to CPS upon request. Refer to 6.2.1 Design Conditions for outdoor and indoor conditions. 

Boilers and associated pumping shall be sized based upon the design-day building load for the space served and not on the maximum connected equipment load or existing equipment sizes in renovation projects. Calculate and provide the peak load and percent diversity to CPS during the design phase. The percent diversity is the ratio of the connected loads (equipment gpm) to the distribution pump system load (gpm). Chillers and associated pumping shall be sized incorporating diversity among the systems served (i.e. the Auditorium and Classrooms are never fully occupied at the same time; the peak building occupancy is significantly less than the sum of individual room occupancies). Calculate and provide the peak load and percent diversity to CPS during the design phase. The percent diversity is the ratio of the connected loads (equipment gpm) to the circulating pump system load (gpm).

Water Systems

For all new systems, including full replacement/new installation systems in existing buildings, size water systems based upon the following:

  • Chilled Water: 44°F supply/58°F return. Chilled water supply reset with values set in the Sequence of Operation.
  • Condenser Water: 85°F supply/95°F return. Condenser water has supply reset with values set in the Sequence of Operation.
  • Heating Hot Water (condensing boilers): 140°F supply with 30°F delta main coils, unit heaters and cabinet unit heaters, 20°F delta on terminal equipment (VAV/FTR/etc.). Hot water has supply reset with values set in the Sequence of Operation.
  • Heating Hot Water (non-condensing boilers): 180°F maximum supply with 30°F delta main coils, unit heaters and cabinet unit heaters, 20°F delta on terminal equipment (VAV/FTR/etc.). Hot water has supply reset with values set in the Sequence of Operation. Approval by CPS required to provide new non-condensing boilers on projects.
  • Coils and Terminal Units: All shall be selected for turbulent flow or 3 ft/sec, whichever is greater. Minimum flow shall not be less than 0.5 GPM.
  • Dual Temperature Systems: Design temperatures shall match chilled water temperatures for cooling and heating hot water temperatures for heating.
  • The Fluid in Heating Hot Water, Chilled Water and Dual Temperature Systems with Components (coils, chillers, unit ventilators, etc.) exposed to outside air shall be 30% propylene glycol.
  • Low Pressure Steam Systems: Shall operate at 5 psig.

Energy Model

All new construction and major renovation projects shall have conceptual design (“box”) models and full energy models developed in preparation of being submitted for LEED certification, permit, and shall be compliant with the local Energy Code. All other projects shall include an energy model to demonstrate compliance with the Energy Code and as required for permit. The model will be performed on software accepted by the USGBC for energy analysis submissions. Detailed model calculation inputs/outputs shall be provided to CPS upon request. Refer to Appendix "X" for Integrative Design Process and details on the energy modeling process.

Additional Calculations and Information

  • Pump Calculations: For all new pump systems or the replacement of main distribution pumps, static pressure loss calculations shall be performed. These calculations shall be available for review along with the 90% and 100% review submittals and provided to CPS upon request.
  • Fan Calculations: For all new fan systems and existing fan systems where the airflow is being altered, air static pressure loss calculation shall be performed to verify the total external/total static pressure of the system to ensure proper fan selection. Calculations shall include a dirty filter condition (Approximately 0.6” W.C above clean). These calculations shall be available for review along with the 90% and 100% review submittals and provided to CPS upon request.
  • Brake Horsepower/Horsepower: Shall not exceed governing Energy Code requirements.
  • Code Required Information/Tables and System Diagrams: For all projects, the required Tables shall be provided per the City of Chicago Building Code. In addition to schedules required for permit, the following schedules and diagrams shall be provided within the Design Drawings
    • Ventilation Schedule
    • Heat Loss Schedule
    • Equipment Schedules
    • Hydronic Piping System Flow Diagrams
    • Steam/Steam Condensate Piping System Flow Diagrams
    • Air Flow System Diagrams
    • Refrigerant Piping System Diagrams (variable refrigerant systems when approved for project use by CPS)

Safety Factors

No additional safety factors shall be added to the design conditions or parameters noted in this document.