Remp:
The economizer air handler compares the return and outdoor enthalpies (ideally) or temperatures. When the outdoor air has lower thermal energy than the return air, the economizer dampers (exhaust/relief damper, mixing damper, and outdoor air dampers) modulate to maintain the supply air temperature setpoint.
The enthalpy economizer is better than the temperature model because it accounts for total thermal energy of the air (sensible and latent). For example, a temperature economizer set to enable at 65°F outdoors might actually waste energy as outdoor air with high moisture content can have more total energy than the warmer return air. Without accounting for humidity, there are times when the air handler will economize when it shouldn’t, or not economize when it should.
Any time outdoor air has less thermal energy than return air, it is preferable to maintain the discharge temperature setpoint using the three-damper set, controlling the ratio of return and outside air, than to use mechanical cooling or heating (chilled water/DX or steam/gas/hot water). Mechanical cooling or heating in these systems should not be used unless the economizer dampers are at either minimum or 100% outside air and unable to achieve discharge temperature setpoint.
When outdoor air thermal energy exceeds return air, the economizer dampers default to the minimum outside air position. The minimum outside air setting is usually done by a size difference between the supply and return fans. This airflow setting correlates with 1) a volume equal to or slightly higher than that of the air exhausted from the space, or 2) the volume necessary to satisfy ventilating requirements/standards.
This didn’t end up being too concise, but hopefully it’s clear. Reference the sketch in this link:
(courtesy of I'm not sure who - found on Google search).
-CB
