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Multiple Hot Water Heaters Bedroom Weighted Average Efficiency Calculation

This document provides the methods needed to satisfy requirement 6.1.3.3 of the Energy Simulation Tool Approval Technical Assistance Manual of the 2010 Florida Building Code, Energy Conservation.  (Edit: the 2014 TAM section is 6.5.4.12 and the  2017 TAM section is 6.6.4.13 and each state the same solution)

6.1.3.3 Multiple Water Heating System 

Where two or more water heating systems are installed with different levels of efficiency, a single efficiency weighted by bedrooms served may be calculated for determining compliance with this code. Use the average capacity for the systems. 

The formula for calculating bedroom weighted average efficiency:

Weighted EF_avg = [(EF_System1*Nbr _{Served by System1})+(EF_System2*_{Nbr Served by System2}] / (Nbr _{Served by System1}+ Nbr_{Served by System2})

Or

Weighted UEF_avg = [(UEF_System1*Nbr _{Served by System1})+(UEF_System2*_{Nbr Served by System2}] / (Nbr _{Served by System1}+ Nbr_{Served by System2})

Where two hot water systems, System1 and System2, are present.

For example, for a project with two hot water systems where System 1 serves 3 bedrooms and has an efficiency of 0.92 and system 2 serves 2 bedrooms and has and efficiency of 0.90:

Apply the formula for calculating the bedroom weighted average efficiency.

Weighted EF_avg = [(0.92 x 3) + (0.90 x 2)] / (3 + 2) = 0.912

Or

Weighted UEF_avg = [(0.92 x 3) + (0.90 x 2)] / (3 + 2) = 0.912