Case study: Hospital surgical suite HVAC system

The following case study examines unoccupied setback of an air handling unit dedicated to a surgery suite

By Roger Koppenheffer, LEED AP, and Kevin Miller, PE, LEED AP, Certus Consulting Engineers, Carrollton, Texas February 25, 2020

Unoccupied setback of an air handling unit dedicated to a surgery suite was examined in a hospitalIn this particular case, the hospital wanted the entire surgery suite system to be in the same mode (occupied or unoccupied) at once, with a single override switch located at the control desk. Such a system can easily be set up to allow only a portion of the rooms to be overridden, provided that override buttons are installed for each room. 

The local authority, the Texas Health and Human Services Commissionwould not permit the outside air quantity to be reduced during unoccupied modes. The additional preheat required during unoccupied mode in the winter months to maintain an acceptable mixed air temperature, is included in the overall energy numbers. 

The system consists of a single 20,000 cubic feet per minute AHU dedicated to serving eight operating rooms and the sterile core. Outside air is constant at 4,000 cfm (20%). During unoccupied mode, the airflow to the ORs is reduced from 20 to 5 air changes per hour. Pressure-independent air valves on the supply and return are set for a constant offset to help ensure positive room pressure is always maintained. 

The ORs are generally unoccupied 7 p.m. to 5 a.m. Room temperature setpoint was maintained at 62°F at all times.  

The energy consumption specific to the OR heating, ventilation and air conditioning can be broken down into five main components: 

  • Supply fan energy kilowatt. 
  • Return fan energy kilowatt. 
  • Cooling plant energy kilowatt. 
  • Preheat energy 1,000 Btu/hour (aka MBH). 
  • Terminal reheat energy 1,000 Btu/hour. 

Note that there is also energy consumed for humidification. However, the difference between occupied and unoccupied modes is negligible because outside air is constant and is not included in this analysis. 

The analysis included a heating and cooling load analysis on an hourly basis for an entire year (“8,760 hourly analysis”). Table 3 illustrates the annual sum of energy consumption of each of these components, with the base assuming 24/7 operation with no airflow setback, as well as the energy savings associated with the setback sequence. 

Author Bio: Roger Koppenheffer is a founding principal of Certus Consulting Engineers and brings 24 years of experience in the consulting field to manage and engineer a multitude of projects specializing in health care. Kevin Miller has 21 years of experience, specializing in health care. He is a founding principal of Certus Consulting Engineers and continues to bring technical expertise and provide creative solutions to the industry.