Sequence of Operations Calculator
Control sequence definition from design intent.
Inputs
%
%
Results
OA enthalpy (BTU/lb)
22.72
RA enthalpy (BTU/lb)
28.12
Dry-bulb economizer ON1
Enthalpy economizer ON1
Enthalpy advantage (BTU/lb)5.4
OA humidity ratio (lb/lb)0.01
RA humidity ratio (lb/lb)0.01
Annual dry-bulb free cooling hrs3,500
Annual enthalpy free cooling hrs3,800
Extra hrs from enthalpy method300
Mixed air temp at min OA (°F)73
Mixed Air Temp100pct OA65%
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) |
|---|---|---|
| -23 | -5.35 | 28.12 |
| 19 | 5.75 | 28.12 |
| 71 | 25.85 | 28.12 |
| 113 | 62.25 | 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%).
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.
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