The annual energy savings is calculated by dividing the recovered heat by the available heat fraction and multiplying by the operating hours. This is then converted to GJ/yr by dividing by 1,000,000. The division by available heat fraction accounts for the boiler efficiency to determine the fuel energy savings.

\begin{equation}\label{eq:water-heating-flue-annual-energy-savings} E_{annual} = \frac{Q_{rec}}{AH} \cdot t_{op} \cdot \frac{1}{10^{6}}\end{equation}

Symbols

\(E_{annual}\)	Annual energy savings \([\unit{ \giga\joule\per\year}]\)
\(Q_{rec}\)	Recovered heat \([\unit{ \kilo\joule\per\hour}]\)
\(AH\)	Available heat fraction \([\unit{ \unitless}]\)
\(t_{op}\)	Operating hours \([\unit{ \hour\per\year}]\)