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Recirculating Residential Water Heating Systems

Do Such Systems of Potential Savings September 16, 2003. Recirculating Residential Water Heating Systems. What Is The Issue?. City of Port Angeles requested that the RTF review the a recirculating hot water system for use in residential buildings to determine:

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Recirculating Residential Water Heating Systems

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  1. Do Such Systems of Potential Savings September 16, 2003 Recirculating Residential Water Heating Systems

  2. What Is The Issue? • City of Port Angeles requested that the RTF review the a recirculating hot water system for use in residential buildings to determine: • 1) Whether it could qualify for C&RD and • 2) What the potential savings might be from such a system.

  3. Available Data • “Economic Operating Cost Analysis Summary for Laing Instant Hot Water Recirculating Systems (October 2001) • Sub-metered water use from AWWA • Sub-meter water heating use from multiple utility studies ( ELCAP, RSDP, RCPD, etc.) • LBNL Water and Water Heating Use Study (LBL-35475)

  4. Laing Report Assumptions/Assertions • Calculations based on 4 persons/household • Hot Water wasted without recirculating systems = 14,087 gals/yr (38 gals/day) • Hot Water savings with recirculating = 12,320 gals/yr (33.8 gals/day) • Gross Electricity savings = 3,007 kWh/yr – • Pumping energy use @ 289 kWh/yr • Piping losses @ 3080 kWh/yr • Net Electricity savings = - 362 kWh/yr

  5. Other Reports • Average hot water use for US (1993 data) = 60 gal/day/household with 2.6 persons/household - (LBNL) • 57.4 gal/day/household for 2.8 person/household) of which 48.2 was used in sinks, bath or showers – Seattle Water Department sub-metering study • Average “baseline” DHW use in PNW is 3,800 kWh/yr/household (2.6 occupants)

  6. Breakeven Case Assumptions • Total hot water use = 55.4 gal/day/household • With EF 90 water heater (Fed. Std Jan ‘04) annual DHW = 3,604 kWh/yr • Pumping use w/autostat = 1 hr/day * 33 watts = 12 kWh/yr (pump only runs when returning hot water temp drops below 120 F) • Piping losses = 60 ft*10 Btu/hr/ft*16hrs/day/350 days/3414 Btu/kWh = 985 kWh/yr (R3 insulation)

  7. Conclusions • Baseline DHW electricity use for typical homes is significantly lower than Laing assumptions • Piping losses increase as a percent of total water heating use as household hot water use decreases (I.e., things get worse as the number of occupants decrease) • In order to break-even, the recirculating system must reduce daily total hot water use by 17 gals (30%) and average faucet and shower use by 45%

  8. Recommendation • Decline request to make recirculating hot water systems in single family buildings eligible for C&RD due to lack of demonstrable savings as well as high probability that such systems would actually increased energy use.

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