The goal of this project was to simulate and compare the energy and water performance, economics, and barriers to use of various domestic hot water distribution systems in new and existing California residences, and to evaluate the potential statewide impact of the use of more efficient hot water distribution systems.
Methodology
A new numerical model, developed using LabVIEW, was used to estimate the heat loss or gain from insulated and non-insulated hot water pipes. Heat loss from distribution piping affects overall energy use, water consumption, and homeowner waiting time at the end use points. This model permitted the evaluation of a wide range of options and alternatives (>250 scenarios were studied).
Two draw cycles (use patterns) were investigated. The first assumed that each individual draw was a "cold start", i.e. the water had reached the ambient temperature surrounding the pipe before each use. This pattern represents a "worst case" for potential water and energy waste. The second was a "clustered use" which had individual draws clustered in the early morning and late afternoon/evening, thereby retaining some hot water between draws. This pattern represents the likely "best case" regarding water and energy waste. Actual residential water use patterns vary between these extremes.
The economic implications of the various distribution systems and options were based on an analysis of expected utility cost savings. The average utility cost of ten California cities was used in the analysis (Gas: $.638/therm, Electric: $.116/kWh, and Water: $.85/HCF or 100 cu ft). The construction costs of the various distribution systems and options were developed from cost data provided by a major plumbing contractor based in Southern California. The results shown in all tables in this report that reflect costs are based on the utility costs shown above. While these costs change over time, the relative ranking of the distribution system options to each other will not change unless the rate of escalation for utilities varies significantly from the rate of construction cost escalation.
New construction and existing housing were studied. The housing characteristics used for new construction included five examples that ranged from a four bedroom, 2 & ½ bath, 3080 square foot single family detached home down to a one bedroom, one bath, 580 square foot apartment. The existing residences evaluated included a three bedroom, two bath, 1100 square foot single family home and a four bedroom, 2 & ½ bath, 1960 square foot single family home. The characteristics used for new and existing hot water systems were typical of standard California practice.
The following changes to conventional trunk and branch distribution systems were evaluated:
- Compare alternative piping materials used in conventional trunk and branch systems.
- Relocate water heater to a more central location.
- Add insulation to the various piping materials in standard system configurations.
The following alternative new home distribution systems were evaluated:
- Demand-actuated recirculating pump and controls in a conventional trunk and branch system using the cold-water line for the return.
- Continuous recirculating system with a dedicated return line for larger residences.
- A parallel-pipe system with a manifold located near the water heater and ½ inch piping from the manifold to each individual fixture.
The following scenarios were evaluated for existing housing:
- Retrofit existing conventional system with a demand recirculation system and controls, using the cold water line for the return.
- Replace existing conventional system in kind and evaluate the impact of pipe materials and insulation.
- Replace existing conventional system with a parallel-pipe system with a manifold located near the water heater and ½ inch piping from the manifold to the individual fixtures.
Prepared For:
Davis Energy Group, Inc., and California Energy Commission
Prepared By:
Oak Ridge National Laboratory
Robert Wendt, Co-Principal Investigator
Evelyn Baskin, Co-Principal Investigator
David Durfee (University of Tennessee)