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Recirculating Residential Water Heating Systems

Do Such Systems of Potential Savings

September 16, 2003

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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.

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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)

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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- 362 kWh/yr

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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)

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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)

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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%

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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.