The recovered heat is calculated by multiplying the unavailable heat (waste heat) by the heat input and heat exchanger efficiency. The unavailable heat is the complement of the available heat fraction.

\begin{equation}\label{eq:water-heating-exhaust-recovered-heat} Q_{rec} = (1 - AH) \cdot Q_{in} \cdot \eta_{hx}\end{equation}

Symbols

\(Q_{rec}\)	Recovered heat \([\unit{ \btu\per\hour}]\)
\(AH\)	Available heat fraction \([\unit{ \unitless}]\)
\(Q_{in}\)	Heat input \([\unit{ \btu\per\hour}]\)
\(\eta_{hx}\)	Heat exchanger efficiency \([\unit{ \unitless}]\)