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Low Heating Value vs High Heating Value for Thermal Fluid Heaters

Coil Model Heater Cutaway

LHV vs HHV Efficiency

When it comes to defining operating efficiency, Thermal Fluid Heaters are often rated by either Low Heating Value (LHV) or High Heating Value (HHV). The difference in efficiency ratings is determined by whether the water vapor produced during the combustion process is factored into the calculation. This can cause a significant difference between the LHV and HHV efficiencies.

To help explain the differences in how LHV and HHV efficiency calculations, the following points should be considered:

  • The combustion process produces water vapor and condensing this water vapor releases a significant amount of energy.
  • The High Heating Value (HHV) efficiency calculation includes this energy while the Low Heating Value (LHV) calculation does not.
  • Typical calorific values for natural gas in North America are:
    • HHV = 1,020 Btu/ft3
    • LHV = 910 Btu/ft3
  • LHV operating efficiencies are higher than HHV efficiencies due to the lower Btu value being used in the efficiency calculation.


Based on Firing Rates and Fluid Outlet Temperatures

C-Model Opertating Efficiencies

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