What is Enthalpy in an HVAC system?

Enthalpy is a measure of the total heat content in a sample of air. It is typically used to determine the amount of fresh outside air that can be added to recirculated air for the lowest cooling cost. Enthalpy takes into account both the dry air and water vapor content within the air sample.

  • Higher enthalpy values indicate more heat energy in the air. When outside air with a higher enthalpy is mixed with inside air, it requires more energy to cool the mixture back down to the desired temperature. This increased energy consumption results in higher cooling costs.
  • Conversely, lower enthalpy values mean less heat energy and require less energy to cool. Mixing outside air with lower enthalpy with inside air can help reduce cooling costs.

What is the air enthalpy?

The air enthalpy is the sum of the heat energy of the dry air and water vapor within a sample of air. It is typically expressed in British thermal units (Btu) per pound of dry air.

Air enthalpy formula: Enthalpy = Heat content of dry air + Heat content of water vapor.

What does enthalpy mean in refrigeration?

In refrigeration, enthalpy is used to calculate the heat load on the system. The heat load is the amount of heat that must be removed from the space being cooled in order to maintain the desired temperature.

Enthalpy is also used to determine the efficiency of a heat pump or air conditioner. The higher the enthalpy of the air being cooled, the more work the system has to do and the lower the efficiency.

What is the function of an enthalpy control?

An enthalpy control is a device that is used to regulate the amount of fresh outside air being brought into space. The control brings in just enough outside air to meet the cooling needs of the space while minimizing the energy consumption of the system.

Enthalpy controls are typically used in commercial buildings with large heating, ventilation, and air conditioning (HVAC) systems. By optimizing the mixing of inside and outside air, enthalpy controls can help reduce energy costs while maintaining occupant comfort.

How is enthalpy calculated in HVAC?

The enthalpy of air can be calculated using the following equation:

  • Enthalpy (BTU/lb of dry air) = 0.24 * Dry Bulb Temperature(°F) + Wet Bulb Temperature(°F).
  • For example, if the dry bulb temperature is 70°F and the wet-bulb temperature is 60°F, the enthalpy would be: Enthalpy (BTU/lb of dry air) = 0.24 * 70 + 60 = 66 BTU/lb of dry air.

Outdoor enthalpy calculator

The outdoor enthalpy calculator can be used to calculate the enthalpy of air at different temperature and humidity conditions.


Enthalpy is just one factor to consider when determining the ideal mix of outside and inside air for your HVAC system. Other factors such as humidity, temperature, and airflow also play a role in the equation. Working with a qualified HVAC contractor can help you determine the best way to manage enthalpy in your system for optimal efficiency and cost savings.

Related Links


Enthalpy – Chemistry LibreTexts


Enthalpy – Definition, Equation, & Units

What is enthalpy of a system?

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