How this instrument works
Mixed air temperature is what a rooftop unit's coil actually sees before it does any heating or cooling: not the return stream alone, not the outside stream alone, but the two combined in whatever ratio the outside air damper is set to. The formula is a weighted average, T_mix = T_ret·(1 − f) + T_out·f, where f is the outside air fraction written as a decimal. Each stream contributes to the result in exact proportion to how much of the total supply it makes up, which is why a damper stuck at 20% open pulls the blend only a fifth of the way toward outdoor conditions.
The shape of the formula follows from conservation of mass flow: if fraction f of the supply comes from outside and the rest is recirculated, the resulting temperature falls on the straight line between the two source temperatures, positioned f of the way from return toward outdoor. This is why a 0% fraction simply returns the return air temperature unchanged, and a 100% fraction discards the return stream entirely — a fully open damper in mild weather sends the intake stream straight to the coil with no blending at all, which is the point of running an economizer for free cooling.
The formula assumes the two streams mix completely at the point of measurement and that no heat is gained or lost in the short plenum where they meet, a reasonable assumption for most air handlers. It also uses dry-bulb temperature only; it says nothing about moisture. When humidity swings the coil load as much as temperature does, engineers switch to enthalpy-based economizer logic instead, blending total heat content rather than temperature alone — the weighted-average structure of the math stays identical, only the quantity being blended changes.
- Enter the return air temperature — the value coming back from the conditioned space, in °F.
- Enter the outside air temperature measured at the intake louver, in °F.
- Set the outside air fraction to the damper's current open percentage, from 0 for fully closed to 100 for fully open.
- Read the mixed air temperature — the blended value the coil sees before any heating or cooling is applied.
Worked example — a 20% economizer damper in spring
A rooftop unit is returning air at 72°F while the outdoor sensor reads 35°F on a cool spring morning, and the economizer controller has the outside damper parked at 20% open. The blend is T_mix = 72 × (1 − 0.20) + 35 × 0.20 = 72 × 0.80 + 35 × 0.20 = 57.6 + 7.0 = 64.6°F. That 64.6°F stream is what the heating coil downstream actually has to raise to supply setpoint, not the 72°F return temperature a glance at the space thermostat might suggest.
This is exactly the arithmetic an economizer control sequence runs continuously, recalculating the mixed air temperature every time the damper position or either sensor reading changes, to decide how far to open the damper for free cooling before the mechanical coil needs to do any work. Push the same two temperatures to a 100% outside fraction instead and the coil would see 35°F straight — good for aggressive free cooling, but cold enough on many units to risk tripping a low-temperature freezestat protecting the coil.
Questions
Why is mixed air temperature a weighted average and not a simple average?
Because the two streams rarely arrive in equal amounts. A simple average would only be correct at exactly a 50% fraction; at any other damper position, the result has to be pulled toward whichever stream supplies more of the total flow, which is what multiplying each temperature by its own fraction and adding the results accomplishes.
What happens to the result at 0% or 100% outside air fraction?
At 0%, f is zero and the formula returns the return air temperature unchanged — the damper is fully closed and nothing outside enters. At 100%, f is one and the formula returns the outside air temperature unchanged, since the return path contributes nothing — the condition an economizer aims for during ideal free-cooling weather.
Why does a cold mixed air temperature matter for coil protection?
A mixed stream far below freezing can ice a coil face or trip a low-limit freezestat that shuts the whole unit down, even though the return reading alone was comfortably warm. Controllers run this same mixed air calculation to cap how far the outside damper is allowed to open on cold days, trading some free cooling for a coil that keeps working.
Does this formula account for humidity or just temperature?
Just temperature — it blends dry-bulb readings only. When latent load matters as much as sensible load, HVAC designers use an enthalpy-weighted version of the same calculation instead, combining total heat content of each airstream rather than temperature; the underlying weighted-average structure does not change, only which property is being blended.
How is outside air fraction related to the physical damper position?
On most economizers the two track closely but are not identical — damper position is a mechanical angle, while outside air fraction is the actual proportion of total mass flowing through it, which depends on duct pressures and damper characteristics too. Controls technicians calibrate this relationship during commissioning so a given damper command delivers a known, repeatable fraction.
Can I use this if I only know the return air fraction instead?
Yes — enter 100 minus the return fraction as your outside air percentage, since the two must add to 100%. The formula is symmetric: relabeling which stream's share you enter as f just swaps the roles in the weighted average, and produces the same mixed air temperature either way.