Using the AEDG in large hospitals


Graphics packages

  • Graphics are required for operator use and understanding of system performance. The adage “a picture is worth a thousand words” could never be more applicable in this case
  • Consolidating related system performance information on graphic screens enhance operator understanding of where energy is used, and what is setting the requirement for increasing the output of plant equipment. 

Exhaust air

  • Figure 3: In this mixed-air section configuration, use of dual (opposing) outside air entrances with air blenders (in cold climates) to eliminate stratification and minimize the use of preheat coils. Courtesy: Engineering Economics Inc.Less exhaust air can be achieved through better kitchen layouts with smaller kitchen hoods, no isolation exhaust when isolation rooms are not in use, and separate AHU exhaust air damper control.
  • Less exhaust requires less minimum outside air, less heating, and less cooling, with the more significant benefits to be obtained in high humidity areas where dehumidification requirements are prevalent.
  • Closer compliance with actual requirements, in lieu of adding a little extra to make sure. To this end, we highly recommend larger variable speed drive exhaust fans with static pressure control, in lieu of fixed speed, constant volume exhaust fans. Exhaust air requirements can be more accurately balanced, and quantities are ensured by varying fan speeds to exactly what is needed at the inlet grills, not just a proportional balance of fixed exhaust air quantities. 


  • Less fittings, less devices
  • Wyes in lieu of tees
  • Full port ball or butterfly valves
  • Two-way valves, more diversity. 


  • Less humidification—this is often overdone, and not required or desired. Refer to local codes to determine the amount of relative humidity required in the winter.
  • Smaller zones versus entire AHU applications.
  • Better control, and shutoff when not required. 


  • Less intake of humid outside air
  • Desiccant wheels in lieu of sub-cooling
  • Heat recovery. 

Heat recovery

  • Heat recovery is usually disappointing—extra air side static pressure losses, contamination of heat transfer surfaces, performance short of expectations, and/or no demand for recovered heat (hospitals really need more cooling or cooling enhancement in very hot or humid climates).
  • If exhaust air streams are minimized (the first step), the impact is either too small or too contaminated to justify heat recovery. The goal is to minimize exhaust air and then apply heat recovery, if at all.
  • Heat recovery can only be justified in very cold or very hot climates. Mild weather locations, or where the total number of hours when heat recovery will actually pay, may be limited in comparison to the total number of operating hours. 


  • Right-sizing of domestic hot water heating—the requirements are much less than the number of fixtures in a hospital would lead one to believe. A hospital has many fixtures, but few are used.
  • No booster pumps. Excessive use of booster pumps has been a major issue for lower multi-story buildings.
  • Centralized domestic hot water recirculation and balancing of recirculation loops. Better flow, less waste at the faucet.
  • Mixing valves with good check valves. Integral check valves have been less than satisfactory more often than not, with excessive hot and cold domestic water use to compensate. 

Building envelope

The building envelope and orientation of the building have significant impact on HVAC design, comfort, and operating practices. The orientation of large hospitals on the site is more often limited by the available site, parking, and other factors. However, the building envelope can be optimized by:

  • Eliminating thermal bridging. Continuous wall insulation between the structure and the exterior skin, and window frames, and more specifically, window sills must be thermally broken.
  • Providing a complete vapor barrier, with emphasis on the integrity of window and door openings, and the junctions of the wall assemblies with the floors and roof assemblies.
  • Less glass and better shading.
  • Exterior doors that limit infiltration. 


  • Less lighting, better located. General, overhead lighting versus task lighting has been a continual battle.
  • More efficient fixtures and lamps. Fewer watts, more lumens per watt, and better light quality to enhance medical diagnostic procedures.
  • Lighting controls, so lights are off when not needed. This has been challenging, with the lack of light when needed. Occupancy sensors do not always sense occupancy, particularly if there is little motion in the room. The approach may be some combination of daylighting, motion sensors, time of day control, or local switching in smaller zones.

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