TrLMTD_VBA

Function TrLMTD(TrGMTD, Ts, Ta, q2, qo, LMTDo, n)
'' Calculates the return temperature from the radiator unit (Space heating in Buildings)
'   emprical equation based on LMTD by Bøhm B.
'   Ref: Phetteplace - Optimal Design of Piping Systems for District Heating
'   prepared by Hakan ibrahim Tol, PhD

'' INPUT
' TrGTMD : [°C] Initial estimate for the return temperature (suggested: use GMTD)
' Ts     : [°C] Supply temperature at the actual condition
' Ta     : [°C] Indoor set temperature at the actual condition
' q2     : [kW] Heat Output at the actual condition
' qo     : [kW] Heat Output at the design condition
' LMTDo  : [°C] Logarithmic mean temperature difference at the original condition
' n      : [-]  Emprical radiator constant

Tr = TrGMTD

fTol = 0.001   ' Termination Tolerance(Iteration)
Err = 10       ' Initial fake iteration error

Do Until Err < fTol
    Trout = Ta + ((Ts - Ta) / Exp((q2 / qo) ^ (-1 / n) * (Ts - Tr) / LMTDo))

    Err = Abs(Trout - Tr)
    Tr = Trout
Loop

If Trout > Ts Then
    TrLMTD = CVErr(xlErrNA)
Else
    TrLMTD = Trout
End If

End Function