Overall Heat Transfer Coefficient
From the previous example, the total thermal resistance can be written as Rtotal=Rsoil+Rinsulator+Rconvection.
The heat transfer can be expressed as: q=DTlm/Rtot=UAs DTlm
by defining the overall heat transfer coefficient UAs=1/Rtot. (Consider U as an equivalent heat transfer coefficient taking into consideration of all heat transfer modes between two constant temperature sources.)
We can replace the convection coefficient h by U in the temperature distribution equation derived earlier: