The available heat represents the fraction of input fuel energy that remains for useful work after accounting for sensible and latent heat losses in the flue gas. It includes contributions from fuel sensible heat, preheated air, and moisture, minus the enthalpy carried away by flue gas constituents.

\begin{equation}\label{eq:gas-flue-gas-material-available-heat-cpp} Q_{avail} = \frac{100 \cdot (Q_{sens} + H_{fuel} + Q_{preheat,air} + Q_{preheat,moisture}) - (H_{H_2O} + H_{CO_2} + H_{N_2} + H_{O_2})}{100 \cdot H_{fuel}}\end{equation}

Symbols

\(Q_{avail}\)	Available heat percentage \([\unit{ \percent}]\)
\(Q_{sens}\)	Sensible heat of fuel \([\unit{ \btu\per\hour}]\)
\(H_{fuel}\)	Heating value of fuel \([\unit{ \btu\per\cubicFoot}]\)
\(Q_{preheat,air}\)	Sensible heat of preheated combustion air \([\unit{ \btu\per\hour}]\)
\(Q_{preheat,moisture}\)	Sensible heat of moisture in combustion air \([\unit{ \btu\per\hour}]\)
\(H_{H_2O}\)	Heat content of water vapor in flue gas \([\unit{ \btu\per\hour}]\)
\(H_{CO_2}\)	Heat content of CO2 in flue gas \([\unit{ \btu\per\hour}]\)
\(H_{N_2}\)	Heat content of N2 in flue gas \([\unit{ \btu\per\hour}]\)
\(H_{O_2}\)	Heat content of O2 in flue gas \([\unit{ \btu\per\hour}]\)
\(100\)	Percentage conversion factor \([\unit{ \unitless}]\)