What is the enthalpy formula used here?

Total moist-air enthalpy is h = 0.24 × T + W × (1061 + 0.444 × T), where T is dry-bulb in Fahrenheit and W is the humidity ratio in pounds of water per pound of dry air. The first term is sensible heat of the dry air, the second is the latent and sensible heat carried by the water vapor.

How is humidity ratio found from relative humidity?

The tool computes saturation vapor pressure at the dry-bulb temperature, multiplies by the relative humidity to get actual vapor pressure, then applies W = 0.621945 × p / (Patm − p) at standard sea-level pressure of 14.696 psia.

How is humidity ratio found from wet-bulb?

It uses the standard psychrometric relation tying the humidity ratio to the saturation humidity ratio at the wet-bulb temperature and the dry-bulb minus wet-bulb depression, the same equation behind a psychrometric chart at sea level.

Why do economizers use enthalpy instead of temperature?

A temperature economizer ignores moisture, so it can pull in cool but very humid outdoor air that actually adds latent load. An enthalpy economizer compares total heat content, enabling free cooling only when outdoor air is both cool and dry enough to reduce the coil load.

Is this accurate at altitude?

The humidity-ratio step assumes sea-level pressure of 14.696 psia. At higher elevations the atmospheric pressure is lower, which raises the humidity ratio for the same conditions. Adjust the barometric pressure input for accurate enthalpy above a few thousand feet.

What is the Air Enthalpy Calculator (HVAC)?

Computes moist-air enthalpy from dry-bulb temperature and humidity, deriving the humidity ratio from wet-bulb temperature or relative humidity, matching psychrometric-chart values at sea level. For HVAC engineers and controls technicians setting up enthalpy economizer control. It runs free in your browser on Gera Tools, with nothing uploaded.

Air Enthalpy Calculator (HVAC)

Name: Air Enthalpy Calculator (HVAC)
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

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Enthalpy is the total heat content of moist air, sensible plus latent, and it is the quantity an enthalpy economizer compares to decide whether outdoor air can provide free cooling. This calculator computes moist-air enthalpy in BTU per pound of dry air from a dry-bulb temperature and either a wet-bulb temperature or relative humidity.

How it works

The enthalpy equation combines dry-air and water-vapor heat:

h = 0.24 × T + W × (1061 + 0.444 × T)      (BTU per lb dry air)

where T is dry-bulb in °F and W is the humidity ratio. To get W from relative humidity the tool finds the saturation vapor pressure at T, scales it by RH, and applies:

W = 0.621945 × p_vapor / (P_atm − p_vapor)

From a wet-bulb temperature it instead uses the psychrometric balance between the saturation humidity ratio at the wet-bulb and the dry-bulb depression. Both paths match a sea-level psychrometric chart.

Example

At 75°F dry-bulb and 50% RH, the humidity ratio is about 0.0093 lb/lb and the enthalpy is roughly 28.2 BTU/lb. An enthalpy economizer might enable free cooling when outdoor enthalpy is below the return-air enthalpy minus a deadband. Use the wet-bulb input when you have a sling psychrometer reading, and set the barometric pressure correctly at altitude — lower pressure raises the humidity ratio and the computed enthalpy.

Temperature economizer vs enthalpy economizer

A temperature-only economizer enables the outdoor-air damper when the outdoor dry-bulb temperature is below a setpoint (commonly 65–70°F). This is simple but can admit very humid outdoor air in coastal and humid climates — for example, 65°F outdoor air at 90% RH has an enthalpy of about 30 BTU/lb, which may actually be higher than the 72°F, 50% RH return air at roughly 28 BTU/lb. Pulling in that air increases the latent (dehumidification) load on the cooling coil even though the outdoor temperature is “cool.”

An enthalpy economizer measures both temperature and moisture, computes total heat content, and only enables free cooling when outdoor enthalpy is genuinely below return-air enthalpy. This prevents the humid-air problem at the cost of more complex controls and sensor maintenance.

ASHRAE Standard 90.1 requires enthalpy-based or differential dry-bulb control (not simple dry-bulb) in certain climate zones and system sizes. For engineers specifying or commissioning economizers, computing outdoor enthalpy from field measurements is a critical check.

Sensor calibration and the enthalpy calculation

Enthalpy is highly sensitive to humidity measurement errors. A 5% RH error at 75°F dry-bulb shifts the computed enthalpy by roughly 1.5–2 BTU/lb, which is large enough to cause an economizer to make the wrong decision at the changeover setpoint. When commissioning an enthalpy economizer:

Verify the humidity sensor against a calibrated reference
Re-zero after any sensor replacement
Check periodically — humidity sensors drift more than temperature sensors

For field work, a sling psychrometer provides a reliable independent wet-bulb reading that can be entered here to cross-check the enthalpy against the controller’s display.