⚡ Air State Inputs
°F

Regular thermometer reading. The most common input.

50%

Percentage of maximum moisture the air can hold at this temperature.

ft

0 = sea level. Higher elevation = lower pressure = affects calculations.

— BTU/lb
Total specific enthalpy (live)
—
Sensible BTU/lb
—
Latent BTU/lb
—°
Dew Point

Calculating enthalpy…

Enthalpy in HVAC — What It Is and Why It Matters

Enthalpy is the total heat content of air. It’s the single most important number in HVAC load calculations. Here’s everything explained simply.

⚡

What Is Enthalpy?

Enthalpy is the total heat energy in moist air per pound of dry air. It has two parts: sensible heat (related to temperature — what you feel) and latent heat (related to moisture — the energy stored in water vapor). Together they give you the full energy picture. The unit is BTU per pound of dry air, or kJ/kg in metric.

🌡️

Sensible Heat Component

Sensible heat is the part of enthalpy tied to air temperature. When you change the air temperature without changing its moisture content, you’re adding or removing sensible heat only. In Imperial units: sensible enthalpy = 0.240 × dry bulb temperature (°F). A higher temperature means more sensible heat — simple and direct.

💧

Latent Heat Component

Latent heat is the energy stored in water vapor. It doesn’t change the thermometer reading — but it represents a huge amount of energy that your AC must remove to dehumidify the space. In a humid climate, latent heat can be 30–50% of the total cooling load. That’s why oversized AC units that cool quickly but don’t run long enough fail to dehumidify properly.

🏭

Using Enthalpy in Load Calculations

The total cooling load formula is: Q = 4.5 × CFM × Δh. Where CFM is airflow in cubic feet per minute and Δh is the enthalpy difference between return and supply air (BTU/lb). This single formula captures both sensible and latent load simultaneously — that’s why it’s more accurate than separate sensible-only calculations.

📊

Sensible Heat Ratio (SHR)

The Sensible Heat Ratio is the fraction of total load that’s sensible. SHR = Sensible Load ÷ Total Load. A typical office has an SHR of 0.7–0.8. A humid climate or high-occupancy space has lower SHR (more latent load). Your HVAC equipment must be selected to match the SHR — a mismatch causes either overcooling or inadequate dehumidification.

📐

Enthalpy vs Temperature

Two air states can have the same temperature but very different enthalpies. 90°F air at 20% RH has an enthalpy of about 34 BTU/lb. But 90°F air at 80% RH has an enthalpy of about 55 BTU/lb. That’s 60% more energy that your cooling system must remove — even though the thermometer reads exactly the same. This is why humidity matters so much in HVAC design.

📐 The Enthalpy Formula — ASHRAE Fundamentals

h = 0.240 × T + W × (1061 + 0.444 × T)   [BTU/lb dry air, T in °F] h = 1.006 × T + W × (2501 + 1.805 × T)   [kJ/kg dry air, T in °C] W = 0.62198 × Pw / (P − Pw)   [lb water / lb dry air] Total Load (BTU/hr) = 4.5 × CFM × Δh Sensible Load (BTU/hr) = 1.1 × CFM × ΔT Latent Load = Total Load − Sensible Load

Where: h = specific enthalpy, T = dry bulb temperature, W = humidity ratio (lb water per lb dry air), Pw = partial pressure of water vapor, P = atmospheric pressure (14.696 psia at sea level). The factor 4.5 = 60 min/hr × 0.075 lb/ft³ (standard air density). At altitude, use corrected density: 4.5 × (altitude correction factor).

Enthalpy Reference Chart (BTU/lb, Sea Level)

Common air conditions with their specific enthalpy, sensible and latent components.

ConditionDB (°F)RH (%)Total h (BTU/lb)SensibleLatent
Cold winter air40°F30%10.69.61.0
ASHRAE winter comfort70°F40%26.516.89.7
Typical AC supply air55°F90%22.913.29.7
ASHRAE summer comfort75°F50%28.118.010.1
Hot humid summer90°F70%42.221.620.6
Outdoor Miami summer91°F75%45.421.823.6
Very hot dry desert110°F15%43.726.417.3
Saturated air (fog)60°F100%26.414.412.0

Frequently Asked Questions

Enthalpy in HVAC is the total heat content of moist air, measured in BTU per pound of dry air. It has two parts: sensible heat (from temperature) and latent heat (from moisture). HVAC engineers use enthalpy because it captures both components in one number. The difference in enthalpy between supply and return air, multiplied by the airflow rate, gives you the total cooling or heating load — both sensible and latent combined.
Use the ASHRAE formula: h = 0.240 × T + W × (1061 + 0.444 × T), where h is enthalpy in BTU/lb, T is dry bulb temperature in °F, and W is humidity ratio in lb water per lb dry air. To find W from relative humidity: first find the saturation pressure at your temperature, multiply by RH to get actual vapor pressure, then W = 0.62198 × Pw ÷ (P − Pw). This calculator does all these steps automatically.
Sensible heat changes the air temperature — you can measure it with a thermometer. Latent heat is stored in water vapor — it doesn’t change the temperature reading but represents energy that must be removed to dehumidify the air. When you run an AC, it removes both types of heat. The ratio of sensible to total heat removal is called the Sensible Heat Ratio (SHR). Equipment must be matched to the SHR of the space — in humid climates, more latent capacity is needed.
Total cooling load (BTU/hr) = 4.5 × CFM × Δh. Where CFM is the airflow in cubic feet per minute and Δh is the enthalpy difference between return and supply air in BTU/lb. The factor 4.5 comes from standard air density (0.075 lb/ft³) × 60 minutes per hour. At higher elevations, air is less dense — use a corrected density factor for accuracy. This formula captures both sensible and latent cooling simultaneously.
At higher altitudes, atmospheric pressure is lower. This reduces air density, which means: (1) The same CFM of airflow carries less mass — less cooling or heating capacity. (2) The load factor 4.5 must be corrected downward. At Denver (5,280 ft), the correction is about 0.83× — meaning a system sized for sea level delivers about 17% less capacity at Denver. This calculator adjusts the atmospheric pressure and density for your elevation automatically.
Typical AC supply air conditions are 55°F dry bulb at 85–95% relative humidity. At these conditions, enthalpy is approximately 22–24 BTU/lb dry air. Return air in a typical office at 75°F and 50% RH has an enthalpy of about 28 BTU/lb. The enthalpy difference (Δh ≈ 5–6 BTU/lb) × airflow gives the total cooling capacity. A system moving 1,200 CFM with a 5 BTU/lb enthalpy drop delivers about 27,000 BTU/hr — roughly 2.25 tons of cooling.
Bottom Ad (970×250 Billboard)