Sequence of Operations Calculator
Control sequence definition from design intent.
About this calculator
This calculator evaluates two economizer (free cooling) control strategies side by side. Dry-bulb control enables the economizer purely on temperature: outdoor air temperature must sit below both the Dry-Bulb High Limit and the Return Air Temp. Enthalpy control is more sophisticated -- it compares outdoor air enthalpy against return air enthalpy, each computed from a simplified psychrometric model that factors in temperature AND humidity, so it correctly recognizes that hot, dry outdoor air can sometimes carry less total energy than cooler, more humid return air.
Outdoor Air Temp is the dominant input on OA Enthalpy: raising it increases both the sensible heat term (0.24 x T) and, through a higher saturation vapor pressure, the latent heat term contributed by moisture -- so hotter outdoor air always reads a higher enthalpy at a given relative humidity, never a lower one. Return Air Temp and Return Air RH set the enthalpy the outdoor air is being compared against but don't change how OA Enthalpy itself is computed. The calculator also reports annual free-cooling-hour estimates by Climate Zone, and consistently shows more available hours under enthalpy control than dry-bulb control -- because enthalpy control can enable free cooling on humid days dry-bulb control would wrongly reject, and never rejects a day dry-bulb control would accept.
Inputs
Results
OA enthalpy (BTU/lb)
22.72
RA enthalpy (BTU/lb)
28.12
How to Use This Calculator
- Enter Outdoor air temp (°F), Outdoor air RH (%), and Return air temp (°F).
- Set Return air RH (%), Dry-bulb high limit (°F), and Climate Zone.
- Adjust Hot/humid, Mixed as needed.
- Review OA enthalpy (BTU/lb) and RA enthalpy (BTU/lb).
- Use Dry-bulb economizer ON and Enthalpy economizer ON to inform your decision.
How the result changes with Outdoor air temp (°F)
| Outdoor air temp (°F) | OA enthalpy (BTU/lb) | RA enthalpy (BTU/lb) |
|---|---|---|
| 33 | 10.04 | 28.12 |
| 49 | 15.72 | 28.12 |
| 98 | 45.34 | 28.12 |
| 130 | 90.48 | 28.12 |
What each input means
- Outdoor air temp (°F)
- Current outdoor air dry-bulb temperature in °F.
- Outdoor air RH (%)
- Outdoor air relative humidity. Critical for enthalpy-based economizer decisions.
- Return air temp (°F)
- Return air (building) dry-bulb temperature.
- Return air RH (%)
- Return air relative humidity.
- Dry-bulb high limit (°F)
- Dry-bulb economizer high limit setpoint. Economizer locks out above this temperature.
- Climate Zone
- Select climate zone affecting free cooling hour estimates
What each result means
- OA enthalpy (BTU/lb)
- Outdoor air enthalpy calculated from temperature and humidity.
- RA enthalpy (BTU/lb)
- Return air enthalpy for comparison.
- Dry-bulb economizer ON
- 1 = Economizer enabled (OAT < high limit and OAT < RAT), 0 = Disabled.
- Enthalpy economizer ON
- 1 = Economizer enabled (OA enthalpy < RA enthalpy), 0 = Disabled.
- Enthalpy advantage (BTU/lb)
- Energy savings per lb of air from using outdoor air vs. recirculated air when economizer is active.
- OA humidity ratio (lb/lb)
- Outdoor air moisture content in lb water per lb dry air.
- RA humidity ratio (lb/lb)
- Return air moisture content.
- Annual dry-bulb free cooling hrs
- Estimated annual hours of free cooling using dry-bulb economizer control.
- Annual enthalpy free cooling hrs
- Estimated annual hours of free cooling using enthalpy economizer control.
- Extra hrs from enthalpy method
- Additional free cooling hours gained by using enthalpy vs. dry-bulb control.
- Mixed air temp at min OA (°F)
- Mixed air temperature with minimum outdoor air damper position (20%).
- Mixed air temp at 100% OA (°F)
- Mixed air temperature when the economizer damper is fully open to outdoor air -- equal to the outdoor air temperature.
How this is calculated
Worked example, using the default values
- Identify Input Parameters4 parametersOutdoor air temp (°F) = 65, Outdoor air RH (%) = 50, Return air temp (°F) = 75, Return air RH (%) = 50 = 6 input(s) provided
- Calculate OA enthalpyOA enthalpy = 0.24 * oaTemp + oaW * (1061 + 0.444 * oaTemp)22.72 = 22.72
- Calculate RA enthalpyRA enthalpy = 0.24 * raTemp + raW * (1061 + 0.444 * raTemp)28.12 = 28.12
- Calculate Dry-bulb economizer ONDry-bulb economizer ON1 = 1
- Calculate Enthalpy economizer ONEnthalpy economizer ON1 = 1
Engine last updated . Checked against 2 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
What raises outdoor air enthalpy the most?
Outdoor Air Temp. It drives both terms of the enthalpy formula -- the sensible heat term directly, and the latent (moisture) term indirectly, since warmer air can hold more water vapor at the same relative humidity. Raising Outdoor Air Temp always raises OA Enthalpy; it never lowers it.
Why would the enthalpy economizer be off when the dry-bulb economizer is on?
That happens when outdoor air is cooler than return air (satisfying the dry-bulb test) but carries enough extra moisture that its total heat content (enthalpy) still exceeds the return air's enthalpy. Dry-bulb control only checks temperature, so it would wrongly bring in that humid air; enthalpy control checks total heat content and correctly locks the economizer out.
Does enthalpy control ever provide fewer free-cooling hours than dry-bulb control?
No, structurally it cannot in this model. Dry-bulb control is a subset of conditions enthalpy control also recognizes as favorable, plus it additionally catches humid-but-cool days that pass the temperature test yet fail the true energy-content test. That is why the Annual Enthalpy Free Cooling Hrs figure is always at or above Annual Dry-Bulb Free Cooling Hrs for the same climate zone.
What is the mixed air temperature output actually showing?
Mixed Air Temp at 100% OA equals the outdoor air temperature directly, since at full outdoor-air economizer operation no return air is being blended in. Mixed Air Temp at Min OA blends a fixed 20% outdoor air fraction with 80% return air, which is the temperature the supply air handler sees when the economizer is not active and the system is running at minimum ventilation.
Why does return air humidity matter if the building is being cooled?
Return Air RH sets the return air's own enthalpy, which is the benchmark outdoor air is compared against under enthalpy control. A building with humid return air (e.g., from process loads or high occupancy) has a higher enthalpy benchmark, which makes the enthalpy economizer more willing to accept outdoor air that a drier building would reject as not worth bringing in.
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