Mixed Air Temperature Calculator
Blend up to 4 HVAC air streams by airflow (CFM-weighted average), reverse-solve the outside-air CFM your AHU needs for a target mixed air temperature, and generate an outside-air % vs mixed air temp table — with economizer presets for real air handlers. Works in °C, °F, or K.
✓ 100% private — all math runs in your browser, nothing is uploadedQuick presets
Working backwards: you know the target mixed air temperature (your economizer setpoint), the return-air and outside-air temperatures, and the total supply airflow. The solver tells you exactly how much outside-air CFM the damper must deliver.
Pick your return-air and outside-air temperatures and get the full lookup table: mixed air temperature at every outside-air percentage. Handy for commissioning sheets and damper checkout.
How the mixed air temperature math works
Mixing air streams is a straight energy balance: each stream carries heat in proportion to how much air it brings, so the mixed air temperature is the airflow-weighted average of the stream temperatures.
Worked example — two streams
1,000 CFM of outside air at 35 °C mixed with 1,000 CFM of return air at 20 °C:
Worked example — reverse solver
Target mixed air 25 °C, return air 22 °C, outside air 35 °C, total 2,000 CFM:
Assumptions (documented, not hidden)
- Equal density: the formula weights by volumetric flow (CFM). That is exact only if every stream has the same density and specific heat — an excellent approximation for air near atmospheric pressure across normal HVAC temperatures.
- Steady state, adiabatic mixing: no heat gain/loss in the mixing plenum and fully mixed streams.
- Dry-bulb only: humidity is ignored (see scope boundary below).
- Same unit in, same unit out: because the average is linear, you can mix in °F, °C, or K directly — just keep every stream in the same unit.
Scope boundary: temperature only
This tool computes the dry-bulb mixed air temperature — a fast, CFM-weighted average. It deliberately does not account for humidity ratio, enthalpy, wet-bulb temperature, or condensation on the coil.
When moisture matters — cooling-coil load, dehumidification, or dew-point risk — use the full psychrometric mixer instead:
→ Mixed Air Psychrometric Calculator — blends temperature plus humidity and enthalpy by true mass flow.
Mixed air temperature — FAQs
How do you calculate mixed air temperature?
Multiply each air stream's temperature by its airflow, add those products together, then divide by the total airflow: T_mix = Σ(CFM_i × T_i) / Σ(CFM_i). It is a weighted average — the bigger stream pulls the result toward its own temperature. Example: 1,000 CFM at 35 °C mixed with 1,000 CFM at 20 °C gives exactly 27.5 °C.
What is the mixed air temperature formula in HVAC?
In HVAC the standard form blends outside air and return air: T_mix = (CFM_OA × T_OA + CFM_RA × T_RA) / (CFM_OA + CFM_RA). This calculator generalizes it to up to 4 streams (for example outside + return + bypass air) and can also run the formula in reverse to find the outside-air CFM needed for a target mixed air temperature.
How do you calculate mixed air temperature with 3 air streams?
The same weighted-average idea extends to any number of streams: T_mix = (CFM₁·T₁ + CFM₂·T₂ + CFM₃·T₃) / (CFM₁ + CFM₂ + CFM₃). A common 3-stream case is outside air + return air + bypass air (for example 150 CFM at 91 °F + 1,550 CFM at 75 °F + 300 CFM at 55 °F = 73.2 °F). Use the Stream Mixer tab above — add a third stream and it handles the rest.
What is the difference between mixed air temp and supply air temp?
Mixed air temperature is measured in the mixing plenum before the coil — it is what the heating or cooling coil "sees" coming in. Supply air temperature is measured after the coil, once heating or cooling has been added or removed. Mixed air temp determines the coil load; supply air temp determines what the rooms feel.
How do I find the required outside-air CFM for a target mixed air temperature?
Use the Reverse OA Solver tab: CFM_OA = CFM_total × (T_target − T_RA) / (T_OA − T_RA). For example, targeting 25 °C with 22 °C return air, 35 °C outside air, and 2,000 CFM total needs 2,000 × (25−22)/(35−22) ≈ 461.5 CFM of outside air (≈ 23.1%). If the target lies outside the range between the return and outside temperatures, no damper position can reach it and the solver will tell you so.
Can I use °C, °F, or Kelvin?
Yes — all three units are supported and you can convert between them freely. Because the mixing formula is a linear weighted average, you can mix directly in whichever unit you enter as long as every stream uses the same unit; the tool also shows the result converted into the other two units.
What does the "equal density" assumption mean?
The calculator weights by volumetric airflow (CFM) rather than true mass flow. That shortcut is valid when all streams have nearly the same air density and specific heat — true for air at roughly atmospheric pressure across normal HVAC temperatures. The error from this assumption is far smaller than typical field-measurement error in airflow or temperature.
What is a typical mixed air temperature setpoint for an economizer?
Many commercial economizers target a mixed air temperature around 52–58 °F (11–14 °C) in cooling mode, floating the outside-air damper to hold that setpoint when outdoor conditions allow "free cooling". The exact setpoint depends on the system design and climate — always follow the equipment schedule or controls sequence for the specific AHU.
Why does the solver say my target temperature is unreachable?
A target mixed air temperature must lie between the return-air and outside-air temperatures — no blend of two streams can be hotter than the hotter stream or colder than the colder one. If your target is outside that range, check for sensor error, wrong stream assignment (swapped OA/RA), or a system that genuinely needs mechanical heating or cooling to get there.
What is outside-air percentage and how does it relate to mixed air temperature?
Outside-air percentage is OA CFM ÷ total CFM × 100. Mixed air temperature moves linearly with it: T_mix = T_RA + (OA% / 100) × (T_OA − T_RA). The OA% Table tab generates this full relationship — for example, with 72 °F return air and 95 °F outside air, 20% outside air gives 72 + 0.2 × 23 = 76.6 °F.
Does mixing air change its humidity?
Yes, but this temperature-only calculator does not track it. Blending humid outside air with drier return air changes the humidity ratio and enthalpy of the mixture, which matters for coil load and condensation. For the full picture use our Mixed Air Psychrometric Calculator, which mixes temperature, humidity, and enthalpy together.
What is a mixed air plenum?
The mixed air plenum (or mixing box) is the section of an air handling unit where the outside-air and return-air streams meet and blend before reaching the filter and coil. The mixed air temperature sensor usually sits just downstream of it, and its reading drives the economizer damper logic.
Classroom ventilation example — how does this work in practice?
Take a classroom unit bringing in 450 CFM of 32 °F winter air with 1,050 CFM of 70 °F return air: T_mix = (450 × 32 + 1,050 × 70) / 1,500 = 58.6 °F. That 58.6 °F is what the heating coil must raise to the supply setpoint — which is why cold-climate ventilation air carries a real heating cost, and why heat-recovery ventilators exist.
Is this calculator accurate enough for sizing heating or cooling coils?
It is accurate for the sensible (temperature) part of the load and for field checkout, damper setup, and economizer verification. For formal equipment sizing, latent (moisture) load must be included — use the psychrometric mixer linked above and have a licensed engineer review the selection. This tool provides estimates, not professional engineering advice.
Free tool by NRTechStudio