The flue gas exit temperature is calculated by subtracting the heat lost (recovered heat) divided by the flue gas heat capacity rate from the inlet temperature. This represents the cooled flue gas temperature leaving the heat exchanger.

\begin{equation}\label{eq:water-heating-flue-gas-exit-temp} T_{flue,out} = T_{flue,in} - \frac{Q_{rec}}{\dot{C}_{flue}}\end{equation}

Symbols

\(T_{flue,out}\)	Flue gas exit temperature \([\unit{ \kelvin}]\)
\(T_{flue,in}\)	Flue gas inlet temperature \([\unit{ \kelvin}]\)
\(Q_{rec}\)	Recovered heat \([\unit{ \kilo\joule\per\hour}]\)
\(\dot{C}_{flue}\)	Flue gas heat capacity rate \([\unit{ \kilo\joule\per\hour\kelvin}]\)