Measured Residential Hot Water End Use

Water heating accounts for 24% of residential natural gas use in the U.S. Yet very few data are available on how hot water is actually used in homes. Better data on hot water usage can help manufacturers, energy program designers, and policy makers design better products, programs, and policies to reduce energy and water use and improve water heating performance.

This paper provides high-resolution data on draw volume, length, flow rate, and spacing both overall and by hot water end use. It is based on data collected in ten homes in the Minneapolis/St. Paul metro area in 2009-2010. Data were collected in each home using both storage and tankless water heaters. Daily hot water consumption varied by household size but averaged 38 gallons (range of site averages 20-59 gallons). It was not affected by water heater type. The number of draws per day averaged 226 (range 47 to 1080). On average only 26 of these draws lasted 5 seconds or longer (range 8 to 48). The average draw of 5 seconds or more used 1.4 gallons of hot water and lasted 61 seconds. These draw patterns differ significantly from the assumptions implicit in current residential water heater rating methods.

For 91% of draws, no hot water reached a fixture. These draws account for 11% of total draw volume. Draws for which hot water actually reached a fixture accounted for 9% of all draws, 73% of draws five seconds or greater, and 89% of total hot water volume used. These were broken down into draws where only a single fixture was active (8% of all draws, 71% of draws >= 5 seconds, and 78% of hot water volume) and draws where more than one fixture was active (0.3% of all draws, 2% of draws >=5 seconds, and 11% of total hot water volume). Showers and baths accounted for 8% of single fixture draws but 41% of total HW volume. Kitchen sinks accounted for 51% of single fixture draws and 18% of volume. Bathroom sinks accounted for 28% of single fixture draws and 11% of volume. Clothes washing accounted for12% of single fixture events and 9% of total hot water volume. Dishwashers accounted for 4% of the events and 1% of hot water volume in sites that used dishwashers. 11% of hot water volume went to multiple fixtures at once, and the remaining 11% (or 7% in sites with dishwashers) of hot water volume never reached a hot water fixture.

Hot water use showed morning and evening peaks with the morning peak later on weekends than weekdays.

Published in and available for download at: ASHRAE Transactions, Volume 118, Part 1, Chicago, IL