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The volume of outdoor air being introduced through the economizer/damper.
The flow actually passing the damper at its current position, not the minimum ventilation rate the design calls for. An economizer modulates between the two, and the mixed temperature is the number that tells you which position it is in — a reading that never moves with the weather usually means a stuck damper rather than a stable one.
The dry-bulb temperature of the incoming outdoor air.
Measured after the intake louvre and before the coil, in the airstream rather than on a wall nearby — a sensor in the sun or in an exhaust plume reads a building that is not there. Dry bulb alone also decides nothing in a humid climate: an economizer sequenced on dry bulb will happily draw in air that is cool and saturated, which costs more to dehumidify than it saved on cooling.
The volume of return air being recirculated from the conditioned space.
Return plus outdoor should equal the supply the fan is moving. Where they do not, air is leaking into or out of the system between the two, and the mixed temperature predicted here will not match the one the unit's own sensor reads — which makes the disagreement a useful diagnostic rather than an error.
The dry-bulb temperature of the return air stream.
The temperature of the air coming BACK from the space, which is not the room setpoint — it picks up fan heat, plenum heat and any duct gain on the way, and in a ceiling-plenum return it can run several degrees above the room it left. Using the setpoint here is the commonest reason a calculated mixed temperature disagrees with the sensor in the unit.
Resulting mixed air temperature
62.74 °F
- Total combined airflow
- 2,000 CFM
They open the calculator with your figures already in it
Mixed Air (Economizer) Temperature Calculator: 62.74 °F — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- Mixed air temperature = (Outdoor Air CFM × Outdoor Air Temp + Return Air CFM × Return Air Temp) ÷ (Outdoor Air CFM + Return Air CFM), the standard air-stream mass-balance (weighted average) formula used in HVAC economizer and mixed-air control design.
Inputs used
- Outdoor Air Flow
- 500 CFM
- Outdoor Air Temperature
- 35.06 °F
- Return Air Flow
- 1500 CFM
- Return Air Temperature
- 71.96 °F
Intermediate steps
- Total combined airflow
- 2,000 CFM
What this calculation does not cover
- Returns the fully-mixed average, and a mixing box rarely mixes. Cold outdoor air crossing the damper does not blend with the return in the short distance before the coil — it arrives as a cold layer across part of the face, ten degrees or more below the average reported here — so a freeze stat trips on the coldest foot of its element and a hydronic coil bursts at its coldest tubes, not at the mean. Blenders, opposed-blade dampers and the geometry of the plenum are what close that gap, and none of them are inputs.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-09-06 · in the site-wide review of 2026-09-06 · v1.1.1
Regulatory standards & verification citations1
- Mixed air temperature = (Outdoor Air CFM × Outdoor Air Temp + Return Air CFM × Return Air Temp) ÷ (Outdoor Air CFM + Return Air CFM), the standard air-stream mass-balance (weighted average) formula used in HVAC economizer and mixed-air control design.
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