According to E Source in Boulder, Colo., optimal setpoint for residential water heaters is 111deg.F. Allowing for heat loss through pipes and an appropriate margin of error due to inaccurate aquastats prevalent in most water heaters, a setpoint of 111deg.F will provide an adequate temperature for most household tasks and, at the same time, conserve energy by reducing standby losses experienced with higher settings. The recommendation comes from Optimal Setpoints in The State of the Art:Water Heating, which is part of E Source's comprehensive State of the Art Technology Atlas Series, formerly published as Rocky Mountain Institute's COMPETITEK reports.

In addition to providing the ASHRAE-recommended temperature of 105deg.F for baths and showers, a lower setpoint of about 115deg.F or slightly lower--well below the manufacturers' setpoints--allows unchanged, and even improved dishwashing through the use to today's enzymatic detergents and booster heaters. E Source notes that the average Veterans Administration hospital laundry uses a maximum of 110deg.F water, and it has found 72deg.F provides sufficient disinfection, whiteness, and stain removal of laundry.

For those situations that may warrant a higher setpoint temperature, such as households with occupants at risk of Legionella infection, the optimal setpoint should be determined on a case-by-case basis, taking all options into consideration.

An important proviso: the 111deg.F setpoint is optimal if there are minimal heat losses through pipes, pipe runs are insulated and relatively short, and the heater is a state-of-the-art appliance with a new aquastat with only +/-2deg.F accuracy swing. Otherwise, co-author Brady Bancroft recommends a setpoint of 120deg.F: At 120deg.F, the variables allow a temperature swing of 10deg.F, which allows 100deg.F at the lowest point. This will cover, for example, the optimal user temperature for showers of 105deg.F.

--Cathlene Casebolt

Cathlene Casebolt is a researcher for the National Center for Appropriate Technology in Butte, Mont.