Btu (IT) per Second per Square Foot [Btu(IT)/(s·ft²)]

The unit Btu (IT) per second per square foot, written as Btu(IT)/(s·ft²), measures heat flux density or thermal power per unit area in the Imperial system using the International Table British Thermal Unit (1 Btu(IT) = 1055.06 J). It represents the amount of heat energy transferred through 1 square foot of surface every second.

This unit is commonly used in high-intensity heat transfer applications, solar energy studies, and engineering where energy flux per small surface area is important. Typical applications include:

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  Heat flux from industrial furnaces or heaters

  
  
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  Solar radiation incident on small surfaces

  
  
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  Thermal testing of materials

  
  

Heat flux can be expressed mathematically as:

where q is heat flux in Btu(IT)/(s·ft²), Q is energy in Btu(IT), A is area in ft², and t is time in seconds.

Conversion to SI units:

While W/m² is the SI standard, Btu(IT)/(s·ft²) is convenient in U.S. engineering and historical references, providing an intuitive measure of high-intensity heat flux per square foot per second.

Btu (IT) per Hour per Square Foot [Btu(IT)/(h·ft²)]

The unit Btu (IT) per hour per square foot, written as Btu(IT)/(h·ft²), measures heat flux density or thermal power per unit area using the International Table British Thermal Unit (1 Btu(IT) = 1055.06 J). It represents the amount of heat energy transferred through 1 square foot of surface every hour.

This unit is widely used in U.S. building construction, insulation, HVAC, and solar energy studies to quantify energy transfer through walls, roofs, or floors. Typical applications include:

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  Heat loss or gain through building surfaces

  
  
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  Solar radiation incident on horizontal surfaces

  
  
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  Design and analysis of heating and cooling systems

  
  

Heat flux is mathematically expressed as:

where q is heat flux in Btu(IT)/(h·ft²), Q is energy in Btu(IT), A is area in ft², and t is time in hours.

Conversion to SI units:

Although modern practice uses W/m², Btu(IT)/(h·ft²) remains prevalent in U.S. building codes, insulation tables, and historical thermal data, offering a convenient measure of energy transfer per square foot per hour.