Domestic Hot Water System Modeling for the Design of Energy-Efficient Systems

This report evaluates the use of demand water heating equipment in conjunction with various hot water piping configurations. These systems are being evaluated as an alternative to a standard tank with a tree delivery system used in most new homes today. Four different domestic hot water heating systems are evaluated for incremental performance changes. Specific performance issues, such as hot water delivery temperatures at the outlet, are used as a basis for understanding the adequacy of the system as well as comparing delivered outlet energy relative to the electric energy required to supply the outlet energy.

Using one-minute hot water flow data, variable interior air temperatures, and monthly variable cold water inlet temperatures, the performance of a hypothetical domestic hot water system is simulated. Both high and low hot water consumption profiles are considered. These profiles were developed through previous testing in U.S. homes supported by NREL. Maximum energy savings resulted from using a combination of a centrally located demand water heater with a parallel piping system supplying individual outlets. For the high consumption home, savings were 17 percent or 920 kWh annually; savings were 35 percent or 817 kWh for the low use home. Savings included an adjustment to the input electric energy if the delivery temperature falls below the set point and an adjustment to water heater system efficiency for higher than necessary delivery temperatures.

For the demand water heating equipment, hot water delivery temperatures show hot water temperature degradation at outlets during periods of high flow rates. This performance issue appears problematic in the high use home but not in the low use home. Performance gains such as higher than necessary delivered outlet temperatures are assumed to be an efficiency gain that results in a decrease of water heating energy.

Other performance issues such as demand heater response time or comfort issues are not evaluated here, but are suggested for further testing and evaluation of an optimal system design. 

Domestic Hot Water System Modeling for the Design of Energy Efficient Systems
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