The water mass flow method estimates the heat transferred to a water stream from the flow rate and temperature differential between inlet and outlet conditions. The heat flow rate is then scaled to annual energy use using operating hours, the quantity multiplier, and system efficiency.

Heat flow from the water stream:

\begin{equation}\label{eq:natural-gas-reduction-water-heat-flow} \dot{Q}_{water} = \frac{500 \cdot \dot{V}_{water} \cdot (T_{out} - T_{in})}{1{,}000{,}000}\end{equation}

Annual energy use:

\begin{equation}\label{eq:natural-gas-reduction-water-energy-use} Q_{total} = \frac{\dot{Q}_{water} \cdot t_{op} \cdot k_{units}}{\eta}\end{equation}

Symbols

\(\dot{Q}_{water}\)	Heat flow rate from the water stream \([\unit{ \MMBtu\per\hour}]\)
\(500\)	Sensible heat factor for water (8.33 lb/gal × 1 Btu/lb·°F × 60 min/hr) \([\unit{ \unitless}]\)
\(\dot{V}_{water}\)	Water flow rate \([\unit{ \gallon\per\minute}]\)
\(T_{out}\)	Outlet water temperature \([\unit{ \degreeF}]\)
\(T_{in}\)	Inlet water temperature \([\unit{ \degreeF}]\)
\(Q_{total}\)	Annual natural gas energy use \([\unit{ \MMBtu\per\year}]\)
\(t_{op}\)	Annual operating hours \([\unit{ \hour\per\year}]\)
\(k_{units}\)	Quantity multiplier (number of identical units) \([\unit{ \unitless}]\)
\(\eta\)	System efficiency \([\unit{ \unitless}]\)