These examples demonstrate the importance of engineers examining room or zone loads at conditions at which economizer operation is limited, with the expected internal loads (people, lights, etc.). The room-load process line then can be plotted on a psychrometric chart. Supply-air conditions under economizer operation fit into one of three cases:

  • Blended outdoor and return air, as shown in figures 4 and 7.
  • 100-percent outdoor air when room load equals the capacity of the economizer alone, as shown in figures 4 and 7.
  • 100-percent outdoor air with air-conditioning-unit operation (chilled water or DX) to achieve room-temperature set point, as shown in figures 5, 8, and 9.

Final room conditions can be estimated by assuming supply air will follow the slope of the room-load process line as it picks up heat and moisture.

The examples discussed here illustrate that using dry-bulb-temperature limit alone in humid climates can result in loss of room humidity control.

With integrated control, single-capacity DX systems are appropriate only with dew-point or electronic enthalpy control and loads having a high sensible-heat ratio. In all cases, the most increments of capacity control available should be specified. Even with capacity control, integrated operation of DX units will not deliver constant room-air temperature.

With integrated operation of chilled-water systems, chilled-water flow can be regulated to provide constant room-air temperature. However, when conditions are warm and humid, as depicted in figures 5 and 8, the same problems of moisture control occur because, at reduced chilled-water flow, the supply air may not be cooled sufficiently for dehumidification. A study published in 20082 offers further insights into economizer control of chilled-water systems.

Psychrometric analysis allows designers to evaluate economizer operation with different high-limit controls and to select controls and capacity modulation to avoid occupant discomfort during economizer operation.


ii) Point 1’ is the lowest humidity level possible in this example. Point 3 is the blended return and outdoor air, but the return used in the example is the design return. The actual return is Point 1’, which moves Point 3 up on the chart. The final achieved room condition will be at a higher humidity than is shown by Point 1’, but cannot be determined without a building simulation.

iii) To estimate the part-load performance of a 5,000-cfm face-split air handler operating with one of two 7.5-ton condensing units, the software inputs were the conditions at Point 2A, a hypothetical smaller air handler at 2,500-cfm airflow with a 7.5-ton condensing unit. The resulting total and sensible-heat capacity was applied to the chart using the full 5,000-cfm supply air.


2) Zhou, J., Wei, G., Turner, W.D., & Claridge, D.E. (2008, October). Airside economizer – comparing different control strategies and common misconceptions. Paper presented at the Eighth International Conference for Enhanced Building Operations, Berlin, Germany. Retrieved from


Fred W. Dougherty, PE, BAE, MME, is a consulting engineer with more than 30 years of experience designing building mechanical systems, including HVAC, plumbing, and fire protection. Currently, he works independently designing HVAC systems for small commercial and institutional projects. He can be contacted at


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