Enthalpy of Moist Air, Explained

Enthalpy is the one property on this calculator that bundles temperature and moisture into a single heat-content number — which is exactly why cooling coil selection and total-load calculations are built around it instead of temperature alone.

The formula

h = 1.006 × T_db + W × (2501 + 1.86 × T_db)

in kJ per kg of dry air, with T_db in °C and W as humidity ratio (kg water per kg dry air). The first term, 1.006 × Tdb, is the sensible heat — dry air's own heat content above 0°C. The second term is the latent heat carried by the moisture itself: 2501 kJ/kg is the latent heat of vaporization of water at 0°C, and the small 1.86 × Tdb addition accounts for water vapor's own slightly higher specific heat relative to dry air.

Worked example

At T_db = 25°C, 50% RH, sea level: W = 0.00988 kg/kg (from the humidity ratio page). h = 1.006 × 25 + 0.00988 × (2501 + 1.86 × 25) = 25.15 + 0.00988 × 2547.5 ≈ 50.3 kJ/kg. That ~50.3 kJ/kg is close to the enthalpy most HVAC references quote for typical indoor comfort conditions — a useful sanity-check number to remember.

Why coil selection uses enthalpy, not just temperature drop

A cooling coil removes both sensible heat (temperature drop) and latent heat (moisture removal) from the air passing through it. The coil's actual cooling capacity — the number on its spec sheet — is sized against the enthalpy difference between entering and leaving air, not the temperature difference alone, because two air streams with identical temperature drops can carry very different total heat loads depending on how much moisture each one is also shedding. This is also the basis for sensible heat ratio (SHR) — see the apparatus dew point page — which is literally the slope of the enthalpy-vs-humidity-ratio process line a coil draws on the chart.

Differential-enthalpy economizer control

Many commercial rooftop units use an economizer — a damper system that brings in outside air directly for "free cooling" instead of running the compressor — and the smartest version of that control doesn't just compare outdoor to indoor temperature, it compares outdoor to return-air enthalpy. Even on a day when outside air is cooler by the thermometer, if it's humid enough its enthalpy can exceed the return air's, meaning bringing it in would actually increase the total heat (sensible + latent) the cooling system has to remove. ASHRAE 90.1 explicitly allows differential-enthalpy economizer control for exactly this reason — it correctly captures load in humid climates where a temperature-only economizer would get fooled into "free cooling" that's actually a net loss.

FAQ

Why does enthalpy use dry-bulb temperature and not wet-bulb?

Because the formula already accounts for latent heat separately through the humidity-ratio term — mixing in wet-bulb as well would double-count the moisture effect. Dry-bulb plus humidity ratio fully and correctly describes the air's total heat content.

Can enthalpy be negative?

Yes, at low enough dry-bulb temperatures (well below 0°C) the sensible term goes negative and can outweigh the small latent term, since this formula's zero point is referenced to 0°C dry air, not absolute zero — this is a relative reference, not a true physical minimum.

Is differential-enthalpy economizer control always better than temperature-only?

In humid climates, yes, meaningfully — it avoids false "free cooling" triggers. In very dry climates the two methods tend to agree more often, since enthalpy and temperature track more closely when humidity ratios are low across the board.

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