107 Domestic Hot Water

The creation of domestic hot water (DHW) represents about 4 percent of the annual energy consumption in typical non-residential buildings. In buildings where sleeping or food preparation occur, including hotels, restaurants, and hospitals, DHW may account for as much as thirty percent of total energy consumption.

Some older lavatory faucets provide a flow of 4 to 6 gal/min (0.25 to 0.38 l/s). Since hand washing is a function more of time than water use, substantial savings can be achieved by reducing water flow. Reduced-flow faucets which produce an adequate spray pattern can reduce water consumption to less than 1 gal/min (0.06 l/s). Flow reducing aerator replacements are also available.

Reducing DHW temperature has also been shown to save energy in non-residential buildings. Since most building users accept water at the available temperature, regardless of what it is, water temperature can be reduced from the prevailing standard of 140°F (60°C) to a 105°F (40°C) utilization temperature saving up to one-half of the energy used to heat the water.

Many large non-commercial buildings employ recirculating DHW distribution systems in order to reduce or eliminate the time required and water wasted in flushing cold water from hot water piping. Recirculating distribution is economically attractive only where DHW use is high and/or the cost of water greatly exceeds the cost of water heating. In most cases the energy required to keep water in recirculating DHW systems hot exceeds the energy used to heat the water actually used.

To overcome this waste of energy there is a trend to convert recirculating DHW systems to localized point-of-use hot water heating, particularly in buildings where plumbing facilities are widely separated. In either case insulation of DHW piping is essential in reducing the waste of energy in distribution. One-inch of insulation on DHW pipes will result in a 50% reduction in the distribution heat loss.

One often-overlooked energy conservation opportunity associated with DHW is the use of solar-heated hot water. Unlike space-heating, the need for DHW is relatively constant throughout the year and peaks during hours of sunshine in non-residential buildings. Year-round use amortizes the cost of initial equipment faster than other active-solar options.

Many of the techniques appropriate for reducing energy waste in DHW systems are also appropriate for energy consumption in heated service water systems for industrial buildings or laboratories.

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