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

The unit Btu (th) per second per square foot, written as Btu(th)/(s·ft²), measures heat flux density or thermal power per unit area using the thermochemical British Thermal Unit (1 Btu(th) = 1054.35 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, industrial processes, solar energy studies, and material testing, especially where energy flux over relatively large surfaces is significant. Applications include:

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  Heat flux from furnaces, radiators, or heaters

  
  
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  Solar radiation received by panels or flat surfaces

  
  
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  Thermal testing of building or industrial materials

  
  

Mathematically, heat flux is expressed as:

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

Conversion to SI units:

Although W/m² is the SI standard, Btu(th)/(s·ft²) remains useful in U.S. engineering, thermal studies, and historical literature, providing an intuitive measure of high-intensity heat flux per square foot per second.

CHU/Hour/Square Foot

The unit CHU/hour/square foot is used to measure Cooling Heat Units per hour per square foot, commonly applied in air conditioning and refrigeration calculations. It indicates the amount of cooling energy, in CHUs, that passes through or is required for one square foot of a surface area over an hour. This measurement helps engineers and designers assess cooling loads, select appropriate air conditioning systems, and ensure thermal comfort in buildings. A higher CHU/hour/square foot value shows greater heat gain, meaning more cooling is needed, while a lower value indicates better insulation or lower cooling requirements. It is essential in designing energy-efficient HVAC systems and optimizing equipment sizing, helping reduce operational costs and maintain desired indoor temperatures. By using CHU/hour/square foot calculations, building designers can balance comfort, energy efficiency, and sustainability effectively.