Tools · Economizer Ratio

Economizer Ratio Helper HVAC

How far do I open the outdoor-air damper to hit a mixed-air setpoint at these conditions?
Dry-bulb tab is the calc a tech runs at the panel. Enthalpy tab adds the full mixed state and the OA-vs-RA enthalpy-changeover verdict a high-end BAS economizer uses to decide whether free cooling is worth running in the first place.

Inputs are seeded with an example — edit them to your numbers.

Input

Output

Enter values to see feasibility.
Required outdoor air —

Worked example

Summer cooling on an economizer-favorable morning — OA 60 °F, RA 75 °F, mixed-air setpoint 65 °F.

  1. Setpoint sits between OA and RA → feasible.
  2. The damper has to bring in enough OA to drag the mix down to setpoint:
    % OA = (MA − RA) ÷ (OA − RA) × 100
  3. Plug in: (65 − 75) ÷ (60 − 75) × 100 = 66.7 %.
  4. Two-thirds outdoor air, one-third return — that's the damper command.

When the calc lands between 0 and 100 %, that's the damper command. Outside that band the mix is physically impossible, so the readout blanks to — and the feasibility line carries the why: below 0 % means OA is hotter than the setpoint AND than the room — minimum-OA only, no economizer; above 100 % means OA is cooler than the setpoint but not cold enough to carry the whole load alone (drop the setpoint to 55 °F to see it — the damper goes wide open and the coil makes up the rest).

Input

Outdoor air

Return air

Mixed-air target

Atmospheric pressure fixed at sea level for v1; humidity and enthalpy match the chart tool at 0 ft altitude. Open the chart tool if you need an altitude-adjusted answer.

Output

Enter values to see feasibility.
Required outdoor air —

Resulting mixed-air state

MA dry-bulb (°F)—
MA wet-bulb (°F)—
Humidity ratio (gr/lb)—
Rel. humidity (%)—
Enthalpy (Btu/lb)—

Enthalpy changeover

OA enthalpy (Btu/lb)—
RA enthalpy (Btu/lb)—
Enter values to see the changeover verdict.

Worked example

Mild-but-muggy morning — OA 78 °F / 68 °F WB, RA 75 °F / 63 °F WB, MA setpoint 65 °F.

  1. Dry-bulb is what the damper modulates to. % OA = (65 − 75) ÷ (78 − 75) × 100 = a negative number — OA is hotter than RA, so even on dry-bulb alone the economizer can't help. Minimum-OA position.
  2. The mixed-state readouts show what the coil would see at the calc's %OA — useful when the answer lands between 0 and 100.
  3. The enthalpy comparison is the real reason a high-end BAS is more conservative: an OA that's two degrees cooler than RA but soaked with moisture carries more heat per pound than RA does. Free cooling on dry-bulb alone, latent penalty on the coil.

Most commercial economizer logic is dry-bulb with a humidity high-limit; enthalpy economizer logic compares hOA vs. hRA to gate the damper before any dry-bulb modulation runs.

A %OA answer is a setpoint theory, not a safe command. Use it to check whether a sequence’s arithmetic holds together and to learn the changeover logic — then let the low-limit protections decide how far the dampers actually open. In cold weather the same math that finds free cooling can call for enough outdoor air to freeze a coil; the freezestat and the mixed-air low limit govern, not the calculator.

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