An air-side economizer saves energy by drawing in cool outdoor air for free cooling instead of running the compressor. The challenge is deciding the exact point at which outdoor air stops being a bargain. This tool computes the enthalpy of both the outdoor and return air streams and applies the differential-enthalpy rule plus a dry-bulb high limit.
How it works
Total heat, or enthalpy, is what matters because it captures both temperature and moisture. The tool builds it up from psychrometrics. First it finds the saturation vapour pressure at the dry-bulb temperature, scales it by relative humidity, and computes the humidity ratio:
W = 0.621945 x Pw / (P - Pw)
h = 0.240 x Tdb + W x (1061 + 0.444 x Tdb)Free cooling is beneficial when outdoor enthalpy is below return enthalpy AND the outdoor dry-bulb is below the high-limit lockout. If either test fails, the economizer should close to minimum outdoor air and let mechanical cooling carry the load.
Notes and gotchas
Differential enthalpy is the preferred strategy in humid climates precisely because it rejects warm, moist outdoor air that dry-bulb control would wrongly accept. ASHRAE 90.1 sets the dry-bulb high-limit lockout by climate zone, so confirm the right value for your location. The single most common reason economizers misbehave in the field is sensor drift, especially on humidity sensors, so make calibration part of every commissioning and maintenance visit.