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.
Worked example
Suppose a commercial AHU with return air at 75 °F dry-bulb and 50% relative humidity. Using the psychrometric formula, the humidity ratio works out to roughly 0.0093 lb/lb and return-air enthalpy comes to about 28.1 BTU/lb.
The outdoor air on a warm, muggy afternoon reads 74 °F dry-bulb and 80% relative humidity. Despite being cooler than many dry-bulb lockout setpoints, the higher humidity means outdoor enthalpy is roughly 36.7 BTU/lb — well above the return air. Differential-enthalpy control correctly locks out the economizer. A fixed dry-bulb controller set at 75 °F would have opened it and added latent load instead of saving energy.
Choosing the right control strategy
Fixed dry-bulb is the simplest and most reliable approach — one sensor, easy to calibrate, no enthalpy calculations. It works well in dry climates where humidity is rarely a problem. ASHRAE 90.1 permits it in climate zones where the mean coincident wet-bulb temperature is low enough.
Differential enthalpy is preferred in humid climates (zones 1–3 and parts of zone 4) because it correctly accounts for both sensible and latent heat. When outdoor air is cooler but moister than return air, dry-bulb control would unnecessarily bring in high-latent-load air, raising cooling coil load.
Fixed enthalpy (a single high-limit on outdoor enthalpy only, without comparing to return) is a middle-ground option where return-air sensors are not available or not trusted.
Common pitfalls in the field
- Humidity sensor drift. Capacitive RH sensors can shift 5–10% over a year, making the calculated enthalpy wrong in both directions. The economizer may enable on humid days (falsely “free cooling”) or lock out on ideal days. Calibrate or replace outdoor and return enthalpy sensors on a 12–18 month cycle.
- Stale-air bypass. If the outdoor air damper leaks when closed, actual outdoor air enthalpy bleeds into the return stream, artificially raising the apparent return enthalpy and triggering more economizer hours than are actually beneficial.
- High-limit setpoint too high. Some controls engineers use a single fixed enthalpy limit of 28 BTU/lb as a catch-all. For humid climates this is too permissive; for dry climates it cuts off valid free-cooling hours. Use the ASHRAE 90.1 climate-zone table to pick the appropriate dry-bulb high limit for your project’s location.