The annual energy savings is calculated by multiplying the operating hours by the heat recovered, then dividing by the heater efficiency and converting from Btu to MMBtu. The heater efficiency accounts for the fact that the recovered heat reduces the fuel input needed by the heating system.

\begin{equation}\label{eq:air-heating-exhaust-energy-savings} E_{savings} = \frac{t_{op} \cdot Q_{recovered}}{\eta_{heater} \cdot 10^6}\end{equation}

Symbols

\(E_{savings}\)	Annual energy savings \([\unit{ \mega\btu\per\year}]\)
\(t_{op}\)	Annual operating hours \([\unit{ \hour\per\year}]\)
\(Q_{recovered}\)	Heat recovered by cold air \([\unit{ \btu\per\hour}]\)
\(\eta_{heater}\)	Heater efficiency \([\unit{ \unitless}]\)
\(10^6\)	Conversion factor from Btu to MMBtu - see physics::conversions::kMMBtuToBtu \([\unit{ \btu\per\mega\btu}]\)