Coil Leaving Air Calculator

Solve for coil leaving conditions from entering air, total capacity, and SHR. Or solve backward from a known leaving temp to find required coil capacity.

❄️ Coil Inputs

Typical DX: 0.70–0.80; CHW: 0.65–0.85

📊 Leaving Air Conditions

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Leaving Dry Bulb (°F)
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Leaving W (gr/lb)
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Sensible Cap (tons)
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Latent Cap (tons)
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Temp Drop ΔT (°F)
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Leaving Enthalpy (BTU/lb)
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About This Calculator

This coil leaving air calculator predicts the dry-bulb temperature and humidity of air leaving a cooling coil from the entering condition, airflow, total capacity in tons, and sensible heat ratio. It also splits the load into sensible and latent tons, reports the temperature drop and leaving enthalpy, and is altitude-corrected for the lower air density at elevation.

Leaving air conditions set the supply temperature your diffusers deliver and the moisture the coil removes, so they drive both comfort and dehumidification. The tool flags leaving temperatures low enough to risk coil frost and high enough to suggest the coil or airflow is mismatched, helping you sanity-check a selection before specifying equipment.

Formula & Method

Load splitQsens = tons × 12000 × SHR,   Qlat = tons × 12000 × (1 − SHR)
Sensible ΔTΔT = Qsens ÷ (1.08 · f · CFM)
Leaving dry bulbLAT = EAT − ΔT
Latent ΔWΔW = Qlat ÷ (0.68 · f · CFM)  [gr/lb]
Altitude factorf = (1 − elev ÷ 145366)5.2561

Total capacity is split by SHR, then the sensible heat equation Qsens = 1.08 · CFM · ΔT is inverted to find the temperature drop and the latent equation Qlat = 0.68 · CFM · ΔW (gr/lb) is inverted to find the moisture drop. The 1.08 and 0.68 factors are standard sea-level constants; both are scaled by the standard-atmosphere density ratio f so results are altitude-corrected. Entering and leaving humidity ratio and enthalpy use the moist-air relations W = 0.622 · Pw ÷ (P − Pw) and h = 0.240 · T + W · (1061 + 0.444 · T), per ASHRAE Fundamentals Chapter 1.

Frequently Asked Questions

How is leaving air temperature calculated?
The tool splits total coil capacity into sensible and latent parts using the sensible heat ratio. The sensible temperature drop is the sensible BTU per hour divided by the sensible heat factor times CFM, where the factor is about 1.08 at sea level. Leaving dry bulb is entering dry bulb minus that drop. The latent portion is converted to a humidity-ratio drop the same way, giving the leaving moisture content.
What is sensible heat ratio (SHR)?
Sensible heat ratio is the fraction of total coil capacity that goes to lowering air temperature rather than removing moisture. SHR of 1.0 is pure sensible cooling, and lower values mean more dehumidification. Typical direct-expansion coils run about 0.70 to 0.80, and chilled-water coils about 0.65 to 0.85. A lower SHR drives a colder, drier supply air condition.
Does altitude affect the result?
Yes. The 1.08 sensible factor and the 0.68 latent factor both scale with air density, which falls with elevation. This calculator multiplies each factor by (1 minus elevation divided by 145366) raised to the 5.2561 power, the standard-atmosphere density ratio, so a coil at altitude shows the larger temperature and humidity drop that the thinner air actually produces.
Why does the tool warn about temperatures below 45 degrees?
A leaving dry bulb below about 45 degrees Fahrenheit suggests the coil surface may drop near or below freezing, risking frost or ice on a direct-expansion coil. The tool flags that case, and also warns when leaving temperature is above 65 degrees, which usually means the coil capacity or airflow is mismatched. Normal cooling-coil leaving air is roughly 50 to 60 degrees Fahrenheit.

Related Tools

Psychrometric Calculator Enthalpy Calculator Humidity Ratio Dew Point Calculator Wet Bulb / RH
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Results are design estimates for preliminary sizing. Verify final designs against applicable codes and standards — engineering judgment and a licensed professional engineer’s review are required.