Subsequent to space conditioning, domestic hot water is the second largest consumer of energy in American homes. Improvements have been made in the energy efficiency of water heating equipment, but few gains have occurred in the distribution system’s energy performance. Energy and water waste associated with distribution system performance can be a significant fraction of the total expenditure of delivering and generating hot water. Improving hot water delivery systems will reduce the energy usage and system performance. Results from numerically simulating four different hot water distribution systems in three different system locations in a conventional house reveal that system configuration and location as well as pipe material and insulation noticeably impact the overall performance of the distribution system. Model results based on two usage profiles reveal that:

• In most CPVC distribution systems, significantly more energy is wasted from previously heated water remaining in the pipe than heat loss through the pipe walls while hot water is flowing. Changing the assumed hot water use patterns can dramatically change the performance of an individual system and its performance compared to copper systems.

• Demand recirculation systems reduce the waiting time for hot water as well as the energy and water waste, while continuous recirculation systems also reduce waiting and water consumption but increase energy consumption about 600%.

• Parallel pipe systems made of PEX reduce the waiting as well as energy and water waste compared with conventional branch and trunk systems.

• Adding pipe insulation does not have a significant impact on heated water energy waste or piping heat loss for hot water delivery initiated from a “cold start” but can be effective if hot water uses are clustered in a short time frame.

• Distribution systems located in the attic (buried in attic insulation) should not be insulated because it looses additional energy when standard pipe insulation is added.

• Copper pipes have ~10 times more heat loss when they are located below the floor slab in the soil (clay) for “cold start” compared with a clustered use hot water delivery.

Units: Dual