Expected Supply Air Temperature Calculator
Work out what the supply air temperature should be from the return air temperature, airflow and the system’s capacity — and what your measured temperature implies.
Method reviewed October 26, 2026. Version 1.0.0. How it works
Result
- Expected supply air temperature
- 56.5°F
- Expected temperature drop
- 18.5°F
- Measured minus expected
- 0.52°F
Expect supply air at about 56.5 °F (18.5 °F drop). You measured 57 °F (0.5 °F warmer than expected).
- Sensible capacity used
- 24,000BTU/h
- Sensible capacity implied by the measurement
- 23,328BTU/h
Show the working
- ΔT = sensible capacity ÷ (1.08 × density ratio × CFM)24,000 ÷ (1.08 × 1,200) = 18.52 °F
- Supply = return − ΔT75 − 18.52 = 56.5 °F
- Implied sensible capacity = 1.08 × density ratio × CFM × measured ΔT1.08 × 1,200 × 18 = 23,328 BTU/h
The calculation
Sensible heat carried by air is 1.08 × CFM × temperature difference for standard air. Rearranged, the temperature difference is sensible capacity ÷ (1.08 × CFM). Cooling: supply = return − that difference. Heating: supply = return + it. The 1.08 factor is scaled for air density at your elevation.
If you know total cooling capacity, multiply by the sensible heat ratio to get the sensible part.
Comparing with a measurement
Enter a measured supply temperature to see the gap and the sensible capacity the measurement implies at your airflow. A gap of several degrees usually means the airflow or the capacity differs from what you assumed. Check airflow first — filter, blower speed, static pressure — and leave refrigerant and burner diagnosis to a licensed technician.
Worked example
These are the example values the calculator opens with, and the result the engine calculates for them.
Inputs
- Mode
- Cooling
- Return air temperature
- 75 °F
- Capacity I know
- Sensible capacity
- Capacity delivered to the air
- 24,000 BTU/h
- Airflow
- 1,200 CFM
- Measured supply temperature (optional)
- 57 °F
- Site elevation
- 0 ft
Result
- Expected supply air temperature
- 56.5 °F
- Expected temperature drop
- 18.5 °F
- Sensible capacity used
- 24,000 BTU/h
- Measured minus expected
- 0.52 °F
- Sensible capacity implied by the measurement
- 23,328 BTU/h
Expect supply air at about 56.5 °F (18.5 °F drop). You measured 57 °F (0.5 °F warmer than expected).
Assumptions
- Sensible heat equation Q = 1.08 × CFM × ΔT for standard air, scaled by the air-density ratio at your elevation.
Limitations
- Only as good as the capacity and airflow you enter; nominal ratings differ from capacity at real conditions.
- Measure supply air in well-mixed air away from radiant heat.
Frequently asked questions
What supply temperature should my AC produce?
It depends on capacity and airflow. A system delivering 24,000 BTU/h of sensible cooling with 1,200 CFM and 75 °F return air should supply about 56.5 °F.
Where do I get capacity at real conditions?
From the manufacturer’s expanded performance data at your indoor and outdoor conditions, not the nominal rating.
Why is my supply air warmer than expected?
Common reasons include higher airflow than assumed, humid return air (more capacity goes to moisture removal), or reduced capacity. The measurement narrows it down; it does not diagnose it.
Sources
- Regional Line Card — air-side formulas — Trane. Sensible = 1.08 × CFM × ΔT; latent = 0.68 × CFM × Δgr; total = 4.5 × CFM × Δh, with the derivation of each constant.
- PsychroLib — psychrometric functions after ASHRAE Handbook—Fundamentals (2017) ch. 1 — PsychroLib (open source, MIT licence). Humidity ratio W = 0.621945·pw/(p − pw), wet-bulb, enthalpy and standard-atmosphere equations as published by ASHRAE.
Spotted an error? Report a problem with this calculator — see our corrections policy.