Economizer calculator (enthalpy control decision)
Decide whether more outdoor air would save cooling right now. From the outdoor, return and coil leaving target states and the supply airflow it finds the coil load on minimum outdoor air, 100 % outdoor air and modulated outdoor air, and shows the differential enthalpy, differential dry-bulb and fixed dry-bulb decisions side by side.
How it works
An economizer opens the outdoor-air damper further while outdoor air is "cooler" than return air, cutting the coil load. What the coil must remove is an enthalpy difference, so when outdoor enthalpy is below return enthalpy, more outdoor air helps. Mixing blends enthalpy and humidity ratio linearly by dry-air mass.
| Item | Equation | Notes · basis |
|---|---|---|
| Dry-air mass flow | ṁ = V / v_SA | V is the supply airflow, v_SA the specific volume at the coil leaving target (Eq. 28). All three modes use the same ṁ |
| Mixed air | h_M = (1 − x)·h_RA + x·h_OA, W_M = (1 − x)·W_RA + x·W_OA | x = outdoor-air fraction (dry-air mass). Adiabatic mixing: enthalpy and humidity ratio blend by mass fraction |
| Coil load | Q = ṁ · (h_in − h(t_SA, min(W_in, W_SA))) | The coil cannot add moisture, so if the entering air is drier than the target the leaving humidity ratio equals the entering one (sensible cooling only). Negative means 0 (heating needed) |
| Modulated outdoor air (C) | t_M(x_C) = t_SA, x_min ≤ x_C ≤ 1 | When the outdoor air is colder than the supply target, the outdoor-air fraction that makes the mix dry bulb equal the supply (Brent's method). If its humidity ratio is at or below the target, coil load is 0 |
| Decision (ASHRAE 90.1 high-limit shutoff) | (h_OA ≤ h_RA, t_OA ≤ t_limit) · t_OA ≤ t_RA · t_OA ≤ t_limit | Differential enthalpy · differential dry bulb · fixed dry bulb. Above the threshold the economizer shuts off and returns to minimum outdoor air |
The three methods compared
Dry-bulb methods need simpler sensors, but on humid days when outdoor air is cool yet moist they can bring it in and raise the coil load. ASHRAE 90.1 tabulates which methods and limits apply in each climate zone (values below are for reference).
| Method | Outdoor air allowed when | Humid outdoor air (e.g. 24 °C, 90 %) | Sensors |
|---|---|---|---|
| Differential enthalpy + fixed dry bulb | h_OA ≤ h_RA, t_OA ≤ t_limit | Off (counts latent heat) | Temperature and humidity, outdoor and return (humidity sensors need calibration) |
| Differential dry bulb | t_OA ≤ t_RA | On (may bring in moisture and raise coil load) | Temperature, outdoor and return |
| Fixed dry bulb | t_OA ≤ t_limit | On below the limit (ignores humidity) | Outdoor temperature only (limit by climate zone, e.g. 24, 21, 18 °C) |
Worked example: humid rainy-season outdoor air
Outdoor air 24 °C, 90 %; return air 26 °C, 50 %; coil leaving target 14 °C, 90 %; supply 10,000 CMH; minimum outdoor air 20 %; fixed dry-bulb limit 24 °C; 101.325 kPa. Both tables below come from the same functions as the calculator on this page.
| Method | Outdoor air | Basis |
|---|---|---|
| Differential enthalpy + fixed dry bulb | Off (minimum outdoor air) | h_OA 67.27 > h_RA 52.91 kJ/kg(DA) · t_OA 24.0 ≤ 24.0 °C |
| Differential dry bulb | On (bring in outdoor air) | t_OA 24.0 ≤ t_RA 26.0 °C |
| Fixed dry bulb | On (bring in outdoor air) | t_OA 24.0 ≤ 24.0 °C |
| Mode | OA fraction % | Coil entering °C · kJ/kg | Coil load kW | RT | kcal/h |
|---|---|---|---|---|---|
| (A) Minimum outdoor air | 20.0 | 25.6 · 55.78 | 64.17 | 18.24 | 55,172 |
| (B) 100 % outdoor air | 100.0 | 24.0 · 67.27 | 102.82 | 29.23 | 88,408 |
| (C) Modulated outdoor air | Not applicable: the outdoor air is warmer than the supply target, so mixing alone cannot reach the supply temperature. | ||||
Both dry-bulb methods say "on" but the enthalpy method says "off". On 100 % outdoor air the coil load is 102.82 kW, 38.65 kW more than 64.17 kW on minimum outdoor air. On a humid day, going by dry bulb alone costs energy.
Limits
It compares one moment only. It does not estimate hourly or annual savings, recommend controller setpoints (changeover temperature, hysteresis), or include fan, damper or relief-fan power, coil bypass or heat recovery.
The coil leaving dry bulb is fixed at the target and its humidity ratio is the lower of the entering and target values. A real coil's leaving state varies with load, and dry outdoor air lowers room humidity, which changes the return state too.
Humidity sensor error changes the enthalpy decision a lot. When outdoor and return enthalpy differ by only a few kJ/kg, either choice makes little difference.
Modulated outdoor air (C) reaches the supply dry bulb by mixing alone and includes no reheat. If humid outdoor air needs dehumidifying, only the coil (latent) load is shown; reheat after overcooling is not calculated.
Sources
- ASHRAE Standard 90.1 section 6.5.1 economizer high-limit shutoff: allowed methods and climate-zone settings differ by edition; check the edition you apply
- ASHRAE Handbook, Fundamentals 2017 (SI), ch. 1 Psychrometrics: enthalpy (Eq. 32), specific volume (Eq. 28), adiabatic mixing
Frequently asked questions
When should the economizer run?
When cooling is needed and outdoor enthalpy is below the return air. The coil removes "entering enthalpy minus supply enthalpy", so mixing in more low-enthalpy outdoor air lowers the entering enthalpy. If outdoor air is even colder than the supply target, adjusting the outdoor-air fraction alone can let the coil almost rest.
How do dry-bulb and enthalpy control differ?
Dry-bulb control looks only at temperature; enthalpy control also counts moisture (latent heat). Rainy-season air at 24 °C and 90 % is cooler than a 26 °C, 50 % room but about 14 kJ/kg higher in enthalpy, so dry-bulb control lets it in and enthalpy control keeps it out. As the example on this page shows, letting it in raises the coil load that day.
What fixed dry-bulb limit should I use?
ASHRAE 90.1 tabulates the limit by climate zone: about 24 °C (75 °F) in dry climates and about 18 to 21 °C (65 to 70 °F) in humid ones, lower where it is more humid. The tool's default 24 °C is only a reference; check the table of the edition you apply. In humid regions differential enthalpy is the safer choice.
Last updated 2026-10-19.