Daily Hot Water Use - Scoring Tool

The Home Energy Scoring tool uses a different hot water use model from that in HES professional/consumer. Total Hot water used remains the sum of clothes washer, dishwasher  and fixture related usage, but the individual factors are calculated differently. Rather than take inputs for the number of occupants or frequency of cleaning appliance use, the Scoring Tool takes the number of bedrooms as a proxy of occupancy, using the Building America calculation method (Hendron, R. and Engebrecht 2010). More details on our methodology in this background report.  The calculation of hot water use is given in equation 1:

HWgpd = Fixturegpd + CWgpd + DWgpd                                                                            - Equation 1 


HWgpd = Total hot water use in gallons per day

CWgpd = hot water use by clothes washers in gallons per day

DWgpd = hot water use by dishwashers in gallons per day

Fixturegpd = hot water use by fixtures in gallons per day

Water use by fixtures depends upon two factors; the base water consumption, which is calculated from the number of bedrooms, and the proportion of hot water used. This is shown in equations 2-4:

Fixturegpd = Basegpd * fmix                                                                                              -  Equation 2:

Basegpd  = Base water consumption in gallons per day = 30 + (10 * Nbed)                       -  Equation 3:

fmix   = 1 - (Tempset - Tempuse) / (Tempset - Tempin)                                                      -  Equation 4:


Fixturegpd = hot water use by fixtures in gallons per day

fmix = the fraction of total water use that is hot water

Basegpd  = Base water consumption in gallons per day, both hot and cold

Nbed = user entered number of bedrooms

Tempset  = water heater set point_temp, degrees F.  = 120

Tempuse = hot water use temperature, degrees F. = 105

Tempin  = water heater inlet temp (from weather file)

Calculation of the hot water use by appliances is described on separate pages: Clothes Washers  Dishwashers.