To use this equation effectively, several physical parameters must be defined:
Where:
If you have landed on this article, you are likely an HVAC design engineer, an architectural engineering student, or a energy modeler looking for in the 2009 ASHRAE Handbook—Fundamentals (HOF), Chapter 4 ("Load Calculation Principles"), page 4.21. 2009 ashrae hof chapter 4 equation 48 -pg 4.21--
The answer lies in legacy referencing. Many university heat transfer courses and older software documentation (DOE-2, BLAST, early EnergyPlus) referred to in the 1989, 1993, or 1997 ASHRAE Fundamentals — specifically the equation for outside surface heat balance used in the Response Factor Method . By 2009, ASHRAE had fully migrated to the Heat Balance (HB) Method and Radiant Time Series (RTS) Method , renumbering all equations. By 2009, ASHRAE had fully migrated to the
Chapter 4 of the 2009 ASHRAE Fundamentals Handbook serves as the theoretical foundation for heat transfer in HVAC&R. It covers conduction, convection, and radiation, with Equation 48 acting as a bridge between pure physics and applied mechanical design. Since the 2009 edition does not have Equation
Since the 2009 edition does not have Equation 48, you will find its functional equivalent in (still on page 4.21 in some printings). The modern equation is:
The 2009 ASHRAE HOF Chapter 4 page 4.21 does not contain an Equation 48. This is a legacy reference from the 1997 edition (Chapter 29) or 2005 edition (Chapter 31). The engineering principle——remains vital, but it is now embedded in the Heat Balance Method and solved using Conduction Transfer Functions (Equation 40 or 41 on page 4.21).